Life Glossary

Evolution : Glossary

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Abiogenesis The development of life from non-living systems via natural mechanisms. cf. creation. (W. R. Elsberry talk.origins via W.J. Hudson)

Abiotic factors The non-biological environmental influences that affect organisms; for example, temperature, rainfall, and humidity. (Wikipedia glossary)

Acquired trait A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weightlifter). (PBS evolution Glossary)

Adaptation the evolutionary process whereby a population becomes better suited to its habitat. Can also refer to a feature which is especially important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass, or their ability to run fast and escape predators. Such adaptations are produced in a variable population by the better suited forms reproducing more successfully, that is, by natural selection. (Wikipedia)

Adaptationism or panselectionism a set of methods in the evolutionary sciences for distinguishing the products of adaptation from traits that arise through other processes. It is employed in fields such as ethology and evolutionary psychology that are concerned with identifying adaptations. Critics (most notably Richard Lewontin and Stephen Jay Gould) contend that the adaptationists (John Maynard Smith, W.D. Hamilton and Richard Dawkins being frequent examples) have over-emphasized the power of natural selection to shape individual traits to an evolutionary optimum, and ignored the role of developmental constraints, and other factors to explain extant morphological and behavioural traits. (Wikipedia)

Adaptive radiation the rapid expansion and diversification of a group of organisms as they fill unoccupied ecological niches, evolving into new species or sub-species; the classic example being Darwin's finches. This occurs as a result of different populations becoming reproductively isolated from each other, usually by adapting to different environments. Radiations specifically to increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace, may affect one clade or many, and be rapid or gradual The term can also be applied to larger groups of organisms, as in "the adaptive radiation of mammals" (see diagram below), although in this context it is perhaps better referred to as evolutionary radiation. Evolutionary radiation in this context refers to a larger scale radiation; whereas rapid radiation driven by a single lineage's adaptation to their environment is adaptive radiation proper. Adaptive and evolutionary radiations in this latter context follow mass-extinctions, as when during the early Cenozoic mammals and large flightless birds filled ecological roles previously occupied in the Mesozoic by dinosaurs.

Adaptive radiation of Mammals

Spindle diagram showing the adaptive radiation of placental mammals in the Cenozoic (Geological timeline at top of diagram). Placentals radiated rapidly after the extinction of the dinosaurs, and the modern diversity of form was established within the first 10 million years of the Tertiary (during the Paleocene). (Based on Gingerich 1984.)

Advanced some evolutionary scientists and systematists reject terms like "primitive" or "advanced" when discussing fossil or recent organisms. It is felt that these terms imply ascent or teleology, and that terms like primitive and advanced terms suggest some degree of "improvement" or superiority in the case of organisms considered advanced in relation to those considered primitive. Such associations are of especial concern in cladistics, where an emphasis is on only verifiable empirical methodology. Hence value-neutral words like "derived" are used as an alternative. However, it could be argued that evolution can indeed refer to an increase in complexity and emergence of new characteristics. This being so, there is no reason why these terms cannot be used. (MAK)

Allele Different versions of the same gene. For example, humans can have A, B or O blood type alleles. (W. R. Elsberry talk.origins via W.J. Hudson)

Allometry The relation between the size of an organism and the size of any of its parts, first outlined by Otto Snell in 1892 and Julian Huxley in 1932. Allometric growth is the phenomenon where parts of the same organism grow at different rates. For example in various insect species (e.g., the Hercules Beetle), where a small change in overall body size can lead to an enormous and disproportionate increase in the dimensions of appendages such as legs, antennae, or horns. Allometric relations can be studied during the growth of a single organism, between different organisms within a species, or between organisms in different species. Contrast with isometric growth.

Amino acid The molecular building blocks of proteins. The properties of a protein are determined by its particular amino acid sequence. There are 20 amino acids in the proteins of life on Earth.

Anagenesis the evolutionary transformation of one species over time into another, or in other words , the emergence of a new character or attribute (which in in this case a new species) from an older one. One of the two main parameters of evolutionary change, the other being branching (either cladogenesis or budding). O'Keefe & Sander 1999 provide a case study of among mid Triassic pachypleurosaurs, and its interpretation using phenetic, cladistic, and stratigraphic methodologies. The diagram at the right by Paul Olsen, Lecture 5 Evolution, showing the relation between anagenesis and cladogenesis. See also fig. 1 at Talk Origins: Macroevolution showing anagenesis and cladogenesis as complementary parameters (see also ancestor, descendant). (MAK)

Analogy/analogous structure Structures having similar function or superficial appearance, but have a different evolutionary origin. For example the wings of insects and the wings of birds. Contrast with homologous structures.

The Ancestor's Tale popular science book written by Richard Dawkins. The book charts the evolutionary history of life, which is illustrated as a pilgrimage backward in time heading towards the origin of life. This creates of series of 40 "rendezvous" by following man, as the selected currently existing creature, through the most recent common ancestors (called 'concestor'). The basic structure of the book is modeled after Chaucer's Canterbury Tales. (EvoWiki)

1880 photo of the Berlin Archaeopteryx specimen - Wikipedia

1880 photo of the Berlin Archaeopteryx specimen

From Vogt, C. 1880. "Archaeopteryx macrura, an Intermediate Form between Birds and Reptiles". Ibis 4:434-456.

Archaeopteryx arguably the most famous of all transitional forms, Archaeopteryx is the earliest and most primitive known bird, most of whose fossil remains were recovered in the 19th century, from the Jurassic Solnhofen limestone in Bavaria. Perfectly intermediate between reptile (or more correctly, theropod dinosaur) and modern bird, its discovery was powerful evidence for Darwinian evolution. (MAK). Wikipedia page (detailed coverage)

Arms race in evolutionary biology just as is the case between two rival nations, positive feedback between two or more evolutionary lineages coevolving in such a way that each, in turn, develops more and more extreme/efficient defenses and weapons in response to the others' attributes. For example, a predator may evolve larger teeth or claws, resulting in the prey species developing faster speed, larger size or protective armour, requiring the predator lineage itself to develop further to be able to capture its prey. In addition to predator and prey, can also occur with the co-evolution of a parasite and its host. Alternatively, the arms race may be between members of the same species, as in sexual selection or Red Queen effects. See also escalation hypothesis. (MAK, Wikipedia)

Artificial selection Selectively breeding animals and cultivate crops to select the most desirable traits in a plant or animal population. Most domesticated and agricultural species have been produced by artificial selection. It was Darwin's observations in this area that inspired the idea of natural selection (without human intervention)

Ascent The premise that evolution directional, moving from primitive and less perfect to more complex and perfect forms, the whole constituting a sort of hierarchical gradation, usually with man at the top. The progression from (what is anthropocentrically considered) a lower to a higher form of life. Zallinger's iconic and often misinterpreted (it was never intended to portray a strictly linear model of evolution) March of Progress gives the classic representation of the layman's conception of evolution, showing man's progression from an ape-like ancestor through various intervening stages of ape-men, to modern human. . According to popular science writers like Stephen Jay Gould, thes idea of evolution as a straight-line from the slime to man and beyond is a concept that really has very little to do with true Darwinism, despite superficial appearances to the contrary. On the other hand, modern fields such as systems theory and the study of biodiversity through time shows that evolution is indeed directional in that it does progress to more complex forms (while simpler organisms such as bacteria continue alongside, it is a misinterpretation to assume that Darwinian thought and evolutionary theory in general support a naive anthropocentric hierarchy of being.

The Evolution as Progress meme is however immensely influential in human thinking. It appears in Marxism, in Theosophy, in Humanism, in Transhumanism, and elsewhere besides. It is criticized and rebuked by anti-evolutionist religious creationists, who think they are opposing Darwinism, when they are actually opposing something that has nothing to do with Darwinism. Some popular thinkers, such as Teilhard de Chardin, have argued for an anthropocentric cosmology, culminating in a future omega point. (MAK)

Asexual reproduction (also called Vegetative Reproduction) A form of duplication using only mitosis. Example, a new plant grows out of the root or a shoot from an existing plant. This process produces only genetically identical offspring since all divisions are by mitosis. 1. offspring called clones meaning that each is an exact copy of the original organism 2. this method of reproduction is rapid and effective allowing the spread of an organism 3. Since the offspring are identical, the only mechanism for introducing genetic diversity is mutation. (W.J. Hudson)


Base The information coding part of DNA, the letters of the genetic code. The DNA molecule is a chain of nucleotides; each consisting of a backbone made of a sugar and a phosphate group, with a nitrogenous base attached. There are four bases ("letters" so to speak) in the DNA "language": adenine (A), guanine (G), cytosine (C), or thymine (T). In RNA, uracil (U) is used instead of thymine. A and G belong to the chemical class called purines; C, T, and U are pyrimidines. In a strand of DNA, bases are paired and are lined up across from one another: A pairs with T and G pairs with C. The sequence of bases along the DNA molecule determines what the DNA codes for (such as making a protein, or turning on or off a gene). In protein-coding regions, three base pairs code for a single amino acid. For example, the base pair sequence ATG codes for the amino acid methionine. (adapted from UCMP Understanding Evolution Glossary, and PBS evolution Glossary)

Batesian mimicry A form of mimicry in which one non-poisonous species (the Batesian mimic) has evolved to imitate the warning signals of a harmful or poisonous species, to deter a predator. It is named after the English naturalist Henry Walter Bates, after his work in the rainforests of Brazil. Contrasted with Müllerian mimicry, a form of mutually beneficial convergence between two or more harmful species. (adapted from Wikipedia)

Biological species concept An integral part of the modern evolutionary synthesis, defines a species as "a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature." BSC applies well to sexually reproducing animals, but not as well to plant life because there is greater gene flow between plant species. It is also difficult if not impossible to apply to the fossil record. Fossils are divided into species based on taxonomic classification (similarity of physical characteristics—see morphological species concept. See also cladistic species concept, ecological species concept, phenetic species concept, and recognition species concept. (W.J. Hudson. Kutschera & Niklas 2004 p.263).

Bottleneck, bottleneck effect A form of genetic drift that occurs when a population's size is greatly reduced. Gene frequencies in the population are likely to change just by random chance and many genes may be lost from the population, reducing the population's genetic variation. When the population later expands in numbers, the resulting gene frequencies may be distinctly different from those before the bottleneck. (See also Founder effect.) (UCMP Understanding Evolution Glossary, M. W. Strickberger)

Branching for the sake of convenience I use this term as the counterpole to anagenesis. See also Multiplication of species.

Budding in a phylogenetic context, the origin of a new taxon (population group, species, or group of species), that does not affect the existence and attributes of the parental taxon (stem population group, or stem group of species). (Horandl & Stuessy 2010, p.1643). Mayr & Bock (2002) coined this term for divergence of a small group of populations, while the rest of the populations remain unchanged. Most obvious are cases of peripatric speciation after geographical isolation of a small group of populations. This is expected to happen mostly after colonizing events by a few individuals, then followed by rapid speciation and adaptation to new environments. Recent evidence from biogeographical studies on both animals and plants suggests that peripatric speciation may be more common than previously thought, since dispersal, even transoceanic dispersal, explains many disjunct distributional patterns. Buddings of this kind are often connected to a high amount of phenotypic change in the derivative species, which undergoes drift and adaptive change in the new ecological situation. In contrast, the source populations are neither in any novel environment, nor under any novel selective pressure." Contrast anagenesis, cladogenesis. (Horandl & Stuessy 2010, p.1644)


Cladogenesis, according to Moore, Lalicker, & Fischer


according to Moore, Lalicker, & Fischer

Cambrian explosion The sudden appearance of all current animal phyla during the Early and Middle Cambrian.

Cladogenesis (also called Splitting): The division of an ancestral parental lineage into two or more daughter lineages or species, rather than the transformation of the ancestral species in toto (anagenesis). In contrast to budding, splitting leads to extinction of the parental lineage. (W. R. Elsberry talk.origins via W.J. Hudson; Horandl & Stuessy 2010, p.1643). As shown by the diagram (right) from Moore, Lalicker, & Fischer 1952 cladogenesis was recognised, along with anagenesis, as one of the two types of gradual evolution. This evolutionary paradigm was replaced in the 1970s and 80s by cladistics. The highly formalised trees that cladistics rely on do not allow for anagenesis, as a result cladogenesis (and then only a division into two daughter species) becomes the standard form of speciation . However, according to Horandl & Stuessy 2010, p.1644 (who argue for recognition of paraphyly): "only a portion of known speciation processes can be categorized as a split of a species in two or more isolated population groups. Allopatric speciation, whereby, e.g., a geographic barrier isolates population groups, does result in a complete disappearance of the original species. Allopatric speciation has been long advocated as the main speciation mechanism, especially in the zoological literature (Coyne & Orr, 2004). This mode of speciation occurs over longer time dimensions, and it divides the ancestral species into more or less equal portions. Allopatric speciation, therefore, fits well the cladistic model of symmetrical divergence, but this is no longer regarded as the predominant mode of speciation, especially in plants (e.g., Rieseberg & Brouillet, 1994). Other evolutionary processes, especially budding and merging, enhance asymmetrical divergence and therefore occurrence of paraphyly." . See also multiplication of species, adaptive radiation. (MAK)

Codon a three base unit of DNA that specifies an amino acid or the end of a protein.

Co-evolution Evolution in two or more species, such as predator and its prey or a parasite and its host, in which evolutionary changes in one species influence the evolution of the other species. See also evolutionary arms race. (PBS evolution Glossary)

Co-extinction the loss of one species due to the extinction of another; for example, the extinction of parasitic insects following the loss of their hosts. Co-extinction can also occur when a flowering plant loses its pollinator, or through the disruption of a food chain. (Wikipedia)

Common ancestor The ancestral species that gave rise to two or more descendant lineages, and thus represents the ancestor they have in common.

Common descent the premise that every group of organisms descended from a common ancestor, and that all groups of organisms, including animals, plants, and microorganisms, ultimately go back to a single origin of life on earth. (W.J. Hudson)

Convergence of forms between placentals (left) and marsupials (right).

Convergent evolution, Convergence process in which two or more distinct lineages independently evolve similar characteristics of one another. In other words, there is an evolutionary convergence between two unrelated or only distantly related types. This often occurs because both lineages face similar environmental challenges and selective pressures. A form of homoplasy. Compare Parallel Evolution: e.g. the shark, tuna, ichthyosaur, and dolphin all evolved a similar streamlined shape as large aquatic fast-swimming predators. (adapted from UCMP Understanding Evolution Glossary) Two types of convergence that could be distinguished are analogy (convergent modifications of a non-homologous structure or behaviour, e.g. the wings of insects (presumably derived from tergal paranota) and the wings of birds (derived from the vertebrate fore legs)) and homoiology (convergent modifications of a homologous structure or behaviour—e.g. The wings of pterosaurs, birds, and bats represent such a homoiology, since they are homologous as tetrapod fore leg, but were convergently modificated to flight devices (wings)). (Glossary of Phylogenetic Systematics by Günter Bechly)

Creation The bringing forth of matter from nothing, or the development of life from non-living systems. cf. abiogenesis. (W. R. Elsberry in talk.origins via W.J. Hudson)

Crossover The exchange of nucleotides between pairs of homologous chromosomes during mitosis or especially meiosis. (W. R. Elsberry in talk.origins)


Charles Darwin aged 51

Charles Darwin aged 51

Darwin, Charles 19th-century naturalist considered the father of the science of evolution. His landmark work, On the Origin of Species, published in 1859, presented a wealth of facts supporting the idea of evolution and proposed a viable theory for how evolution occurs, via the mechanism he called "natural selection" (as a natural process analogous to artificial selection) Also published important works on coral reefs and on the geology of the Andes, and a popular travelogue of his five-year voyage aboard HMS Beagle, and a comprehensive scientific study of barnacles. (adapted from PBS evolution Glossary)

Darwin's theory of evolution through natural selection can be summarised by means of three principles:

  1. Principle of variation. Among individuals within any population, there is variation in morphology, physiology, and behavior.
  2. Principle of heredity. Offspring resemble their parents more than they resemble unrelated individuals.
  3. Principle of selection. Some forms are more successful at surviving and reproducing than other forms in a given environment.

(Griffiths AJF, Miller JH, Suzuki DT, et al. "Introduction", in An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000)

Darwinian Of or pertaining to natural selection, or Darwin's theory of evolution in general. Sometimes taken to mean natural selection with gradualist assumptions, although it is now considered doubtful that Darwin was a uniformitarian to this degree. (modified from W. R. Elsberry in talk.origins)

Darwinian classification see Evolutionary systematics.

Darwinian evolution See Darwinism.

Darwinism In 1859 Charles Darwin supplied a mechanism, namely natural selection, that could explain how evolution occurs. Darwin's theory of natural selection helped to convince most people that life has evolved and this point has not been seriously challenged in the past one hundred and forty years. It is important to note that Darwin's book "The Origin of Species by Means of Natural Selection" did two things. It summarized all of the evidence in favor of the idea that all organisms have descended with modification from a common ancestor, and thus built a strong case for evolution. In addition Darwin advocated natural selection as a mechanism of evolution. Biologists no longer question whether evolution has occurred or is occurring. That part of Darwin's book is now considered to be so overwhelmingly demonstrated that is is often referred to as the fact of evolution. However, the mechanism of evolution is still debated. cf. Modern Synthesis. (W.J. Hudson). Historically, Darwinism represented the stage in the development of evolutionary thought that began with the 1859 publication of On the Origin of Species. "Specifically, it refers to the Darwin/Wallace principle of natural selection as the major driving force in evolution. Since Darwin (1859, 1872) accepted Lamarck's principle of the inheritance of acquired characteristics as a source of biological variability, it is equally fair to call this the "Lamarck/Darwin/Wallace" period of evolutionary thought. (Kutschera & Niklas 2004, p.259–260)

Descendent in this context, a population, lineage, or species, that arises through evolution from an ancestor (an earlier species or taxon). Where a number of descendants share the same ancestor (cladogenesis), the ancestor is called a common ancestor. (MAK)

Diploid Having two alleles for every gene at every locus, one from the mother and one from the father. Most animals, including humans, are diploid. (W.J. Hudson)

Directionality (in evolution) as here defined, the premise that evolution begins with simple or primitive structures or forms of life and moves to greater complexity or perfection; hence some forms of life are more complex, advanced, or evolved relative to others; see Systems Theory's definition of evolution. Results in a tree or hierarchy in which—depending on your perspective—multicellular life, vertebrate animals, or human beings, or self-consciousness, culture, or omega point are at its apex. Whilst the emergence of complexity is a self-evident fact, philosophers and scientists are divided over whether evolution itself is directional. See also complexification, emergence, great story. (MAK)

Diversity the variation of genomes, populations, species, families, or whatever, within a lineage.

DNA Deoxyribonucleic acid, the molecule that contains genetic information.


Escalation hypothesis a hypotheses put forward by Geerat J. Vermeij. It states that organisms are in constant conflict with one another and therefore devote a lot of resources to thwarting the adaptations evolution brings to all competing organisms as time advances. This is in contrast to adaptations evolution may bring that are unrelated to competition with other organisms such as adapting to ecological niches based upon other factors such as geology and climate. Vermeij's extensive work with the characteristics of marine gastropod fossils informed his development of thoughts on escalation. One prediction of the Escalation Hypothesis is that individual species having fewer adaptations that enable them to compete with other life forms are more likely to survive a mass extinction event such as one of The Big Five. This is because there is more flexibility to fit into new ecological niches that arduous adaptations such as heavy shells or energy consuming venom production would hinder. (Wikipedia)

Evolution (Biology) A change in the gene pool of a population over time. The process of evolution can be summarized in three sentences: Genes mutate. Individuals are selected. Populations evolve. (W.J. Hudson) A subset of Evolution (Systems Theory). See also Darwinism, Modern Synthesis . Note that in the biological context, evolution does not apply to individuals (in contrast with the premises of Conscious evolution). (MAK)

Evolutionary game theory (EGT) is the application of game theory to interaction dependent strategy evolution in populations. EGT is useful in a biological context by defining a framework of strategies in which adaptive features can be modeled. It originated in 1973 with John Maynard Smith and George R. Price's formalization of evolutionarily stable strategies as an application of the mathematical theory of games to biological contexts, arising from the realization that frequency dependent fitness introduces a strategic aspect to evolution. EGT differs from classical game theory by focusing on the dynamics of strategy change more than the properties of strategy equilibria. Despite its name, evolutionary game theory has become of increasing interest to economists, sociologists, anthropologists, and philosophers.

In Smith's and Price's paper, "The Logic of Animal Conflict", a computer model was used to show why animals had not adapted a "total war" strategy. Adaptations for males focused on maximizing their ability to compete with each other in order to maximize their dominance over a territory and better compete for mates. Using game theory, they were able to test a variety of evolutionary strategies to see which one emerged with the highest average payoff, explaining why animals have only evolved limited war strategy, in which risk of serious injury is low. (Wikipedia)

Evolutionary psychology branch of psychology or evolutionary science that examines psychological traits—such as memory, perception, or language—from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations , that is, the functional products of natural selection or sexual selection. Evolutionary psychology has its historical roots in Charles Darwin's theory of natural selection.[4] Darwin's theory inspired William James's functionalist approach to psychology. Along with W.D. Hamilton's (1964) seminal papers on inclusive fitness, E. O. Wilson's Sociobiology (1975) helped to establish evolutionary thinking in psychology and the other social sciences. (Wikipedia)

Evolutionary radiation see Adaptive radiation.

Evolutionary synthesis see Modern Synthesis.

Evolutionary Theory (or Evolutionary Mechanism Theory) Any one of several theories in biology dealing explicitly with some aspect of evolution or cumulative evolution. Examples include Sewall Wright's "shifting-balance theory", Eldredge and Gould's "punctuated equilibrium theory", the theory of common descent, Darwin's "descent with modification", Henry Fairfield Osborn's "orthogenesis", and "Gene Flow". While "evolutionary theory" is equivalent, the point that mechanisms are proposed and tested in evolutionary mechanism theories is worthy of stress and repetition. Some mechanisms increase genetic variation ( cf. mutation, recombination, gene flow ) and some decrease genetic variation ( cf. natural selection, genetic drift). (W. R. Elsberry talk.origins via W.J. Hudson)


Fitness the ability of an individual organism to both survive and reproduce; a central element of evolutionary theory. Fitness is equal to the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype. If differences between alleles at a given gene affect fitness, then the frequencies of the alleles will change over generations; the alleles with higher fitness become more common (in other words, natural selection). (Wikipedia)

Fitness landscape Sewall Wright proposed that populations occupy adaptive peaks on a fitness landscape. In order to evolve to another, higher peak, a population would first have to pass through a valley of maladaptive intermediate stages. A given population might be "trapped" on a peak that is not optimally adapted. (Wikipedia)

Founder effect Changes in gene frequencies that usually accompany starting a new population from a small number of individuals. The newly founded population is likely to have quite different gene frequencies than the source population because of sampling error (i.e., genetic drift). The newly founded population is also likely to have a less genetic variation than the source population. For a more detailed explanation, see our resource on adaptation in Evolution 101. (UCMP Understanding Evolution Glossary)


Gene The fundamental physical and functional unit of heredity which carries information from generation to the next. (W. R. Elsberry talk.origins via W.J. Hudson)

Gene family A set of related genes occupying various loci in the DNA, almost certainly formed by duplication of an ancestral gene and having a recognizably similar sequence. Members of a gene family may be functionally very similar or differ widely. The globin gene family is an example. (PBS evolution Glossary)

Gene flow An evolutionary mechanism theory. Gene Flow states that new organisms may enter a population by migration from another population. If they mate within the population, they can bring new alleles to the local gene pool. In some closely related species, fertile hybrids can result from interspecific matings. These hybrids can vector genes from species to species. (W.J. Hudson)

Gene frequency The frequency in the population of a particular gene relative to other genes at its locus. Expressed as a proportion (between 0 and 1) or percentage (between 0 and 100 percent). (PBS evolution Glossary)

Gene pool The set of all genes in a species or population. (W.J. Hudson)

Gene selection, "selfish gene" theory, or gene-centered view of evolution theory that genes are the unit of selection. The theory states that although individuals are the object of selection, because of crossing over and recombination which shuffles genes around, it is the genes which are selected for over time. The alternatives to gene selection are group selection and individual selection.

Gene selection theory is central to the understanding of contemporary evolutionary theory, and has developed from population genetics and the modern synthesis, and was established as the leading theory of natural selection during the Williams revolution. The revolution was based on the findings of population genetics, and other principal architects of the revolution include W.D. Hamilton, John Maynard Smith, Robert Trivers and Richard Dawkins, who popularised the revolution in The Selfish Gene.

There is still some scientific debate about gene selection, which leading biologists such as Ernst Mayr rejecting the theory. Mayr (2000) states that the gene can not be the object of selection because it is the whole organism that lives, reproduces and dies, not individual genes. This, however, is not a problem for gene selectionism, which has always maintained that part of the environment in which genes are selected includes the other genes in the population, but because of recombination no combination of genes exist more than once, so although individuals may be the object of selection, genes are the units, and evolution consists of a change in independent allele frequencies in populations. (EvoWiki)

Genetic diversity resulting from sources of genetic variation, it is the variety of alleles and genotypes within a population or species.

Genetic drift Random changes in the frequency of genes in the population that are not due to selective pressure. This may occur because the different genotypes do not have a noticeable effect on the relative fitness of individuals (such as different mitochondrial haplotypes), or selection may not be strong enough to affect transmission of the genotype (for instance, on a recently-colonised island without predators). Genetic drift is a factor in neutral evolution. The significance of genetic drift in evolution is uncertain. In a large population, most of the factors affected by genetic drift will be minor, and drift is probably not significant over the population as a whole. However, in a small, isolated population drift may have a significant effect on the makeup of the population. CKT061201

Genetic engineering Removing genes from the DNA of one species and splicing them into the DNA of another species using the techniques of molecular biology. (PBS evolution Glossary)

Genetic recombination see Recombination.

Genetics The branch of science which deals with elucidating the attributes and mechanisms of heredity in living systems. On Earth, this involves research into RNA and DNA. (W. R. Elsberry in talk.origins)

Genome complete haploid complement of DNA (including all genes) from the chromosomes of the nucleus of an organism. (Developmental Biology 376 Glossary)

Genotype The heritable information contained in an individual. (W. R. Elsberry in talk.origins); The set of two genes possessed by an individual at a given locus. More generally, the genetic profile of an individual. (PBS evolution Glossary)

Phyletic gradualism, according to Moore, Lalicker, & Fischer

Phyletic gradualism

according to Moore, Lalicker, & Fischer

Gradualism or phyletic gradualism evolutionary mechanism theory, based on the premise that evolutionary change takes place through the gradual change of populations and not by the sudden (saltational) production of new individuals that represent a new type. New species evolve through the steady and gradual transformation of the entire population. The standard evolutionary paradigm prior to the early 1970s, as shown by the diagram (right) from Moore, Lalicker, & Fischer 1952. This view is usually attributed to Darwin because of his being influenced by uniformitarian geology by Eldredge and Gould, who instead argued for Punctuated Equilibria. But Richard Dawkins explained that such constant-rate gradualism is not present in the professional literature, thereby the term only serves as a straw-man for punctuated equilibrium advocates. In his book The Blind Watchmaker, Dawkins argues against the idea that Charles Darwin himself was a constant-rate gradualist, as suggested by Niles Eldredge and Stephen Jay Gould. See also comments by John Wilkins and Larry Moran. (MAK; W.J. Hudson, Wikipedia)

Group selection theory that alleles can become fixed or spread because of the benefits they bestow on groups, regardless of the fitness of individuals within that group. Group selectionist ideas have been around since Darwin mentioned it in the Descent of Man as a possible mechanism of evolution of human altruism but were further elaborated by V.C. Wynne-Edwards in the 1960s.

More correctly, group selection is defined as the differential survival and reproduction of groups (Wade 1977). A response to group selection occurs when the differences among groups has a heritable basis. For group selection this means not only single locus allelic differences, but also epistatic genetic differences, differences in genetically based interactions among individuals, and even potentially cultural differences. Thus, it is simplistic to speak of group selection simply in terms of the spread of an altruistic allele.

Critiques, particularly by George C. Williams (1966), John Maynard Smith (1964) and C.M. Perrins (1964) cast serious doubt on group selection as a major mechanism in evolutionary history. These responses were part of the lead up to the Williams revolution in which gene selection theory became the prominent paradigm. (EvoWiki)


Haploid having only half the normal complement of chromosomes. (W. J. Hudson)

Heterozygous Having two different alleles at a given locus. (W. J. Hudson)

Heredity the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve. Evolution in organisms occurs through changes in heritable traits—particular characteristics of an organism. In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype. The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment. As a result, many aspects of an organism's phenotype are not inherited. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. The study of heredity in biology is called genetics. See also Modern Synthesis, Mendelian inheritance. (Wikipedia)

Homoiology Convergent modifications of a homologous structure (or behaviour). The wings of pterosaurs, birds, and bats represent such a homoiology, since they are homologous as tetrapod fore leg, but were convergently modificated to flight devices (wings). (Glossary of Phylogenetic Systematics by Günter Bechly

Homologous chromosomes chromosome pairs of the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father. (Wikipedia)

Homology/homologous structure coined by Richard Owen to refer to essential similarity, rather than analogy. With the rise of evolutionary theory, came to mean similarity due to sharing a common evolutionary origin (Rieppel, 1993, pp.2–3). In this definition, which is still the one used, homology refers to a character shared by a set of species and present in and inherited, with or without modification, from their common ancestor. For example, the bones in a bat's wing, a dog's front leg, and a human arm, are the same, although modified to serve different functions (see following diagram). Contrast with homoplasious and analogous.

The principle of homology - diagram from Wikimedia

The principle of homology illustrated by the evolutionary radiation of the forelimb of mammals. All conform to the basic pentadactyl pattern but are modified for different usages. The third metacarpal is shaded throughout; the shoulder is crossed-hatched.

Diagram by Jerry Crimson Mann via Wikipedia..

Homoplasy in relation to apomorphy, autapomorphy, synapomorphy, plesiomorphy and symplesiomorphy - diagram from Wikimedia

Homoplasy in relation to apomorphy, autapomorphy, synapomorphy, plesiomorphy and symplesiomorphy

Homoplasy having an independent evolutionary origin. Features that are similar but not the result of inheritance from a common ancestor. The two main causes of homoplasious characters are convergent evolution (appearance of the same character in at least two distinct lineages) and character reversion (the return to an ancestral character). Use of homoplasies when building a cladogram is sometimes unavoidable but is to be avoided when possible. (modified from Wikipedia and UCMP Understanding Evolution Glossary)

Homozygous Having two identical alleles at a given locus. (W.J. Hudson)

Hopeful monster termed coined by the German-born geneticist Richard Goldschmidt, who thought that small gradual changes could not bridge the divide between microevolution and macroevolution. In The Material Basis of Evolution (1940), Goldschmidt wrote "the change from species to species is not a change involving more and more additional atomistic changes, but a complete change of the primary pattern or reaction system into a new one, which afterwards may again produce intraspecific variation by micromutation." His thesis however was universally rejected and has been widely ridiculed within the biological community, which favored the neo-Darwinian explanations of R.A. Fisher, J. B. S. Haldane and Sewall Wright. (Wikipedia)

Horizontal gene transfer (HGT) or Lateral gene transfer (LGT) any process in which an organism incorporates or transfers genetic material to or from another organism, without being the offspring of that organism. Often, the transference is between members of different species. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g., its parent or a species from which it has evolved. Most thinking in genetics has focused upon vertical transfer, but there is a growing awareness that horizontal gene transfer is a highly significant phenomenon and amongst single-celled organisms perhaps the dominant form of genetic transfer. Bacteria, for example, frequently pass copies of particular genes to one another and pick up foreign genetic material from their environment, resulting in horizontal transfer. Mechanisms include Transformation, the genetic alteration of a cell resulting from the introduction, uptake and expression of foreign genetic material (DNA or RNA), a process relatively common in bacteria, less so in eukaryotes, and used in laboratories to insert novel genes into bacteria for experiments or for industrial or medical applications (genetic engineering); Transduction, the process in which bacterial DNA is moved from one bacterium to another by a virus; Bacterial conjugation, a process in which a bacterial cell transfers genetic material to another cell by cell-to-cell contact; and Gene transfer agents, virus-like elements encoded by the host that are found in the alphaproteobacteria order Rhodobacterales. (Wikipedia, UCMP Understanding Evolution Glossary) "has had an important role in eukaryotic genome evolution, but its importance is often overshadowed by the greater prevalence and our more advanced understanding of gene transfer in prokaryotes. Recurrent endosymbioses and the generally poor sampling of most nuclear genes from diverse lineages have also complicated the search for transferred genes. Nevertheless, the number of well-supported cases of transfer from both prokaryotes and eukaryotes, many with significant functional implications, is now expanding rapidly." (Keeling & Palmer 2008, abstract)

Hybrid an offspring resulting from cross-breeding between two different species. Animal hybrids are often infertile. The mule for example is a cross of female horse and a male donkey. The hinny, a cross between a female donkey and a male horse (mule and hinny are reciprocal hybrids). However there are also fertile hybrids, e.g. between coyotes, wolves, dingoes, jackals and domestic dogs. Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. (Wikipedia)


Inclusive fitness theory in evolutionary biology and evolutionary psychology it holds that an organism can improve its overall genetic success by cooperative, social behavior. The theory defines the inclusive fitness of an organism as the sum of its classical fitness (how many of its own offspring it produces and supports) and the number of equivalents of its own offspring it can add to the population by supporting others. (PBS evolution Glossary)

Inheritance of acquired characteristics theory proposed by Jean Baptiste Lamarck, according to whom evolution occurs through the inheritance of traits or abilities an organism acquires in life. For example, the ancestral giraffe stretched its neck to reach the leaves of trees, and as a result passed on a slightly longer neck and legs to its offspring. Also referred to as the "use–disuse theory." Despited being rejected by Weismannian Neo-Darwinism, Lamarckism remained popular well into the early twentieth century, especially in France, but was supplanted by the synthesis of Darwinian and Mendellian theory.

Intron "intervening sequence," a stretch of nucleic acid sequence spliced out from the primary RNA transcript before the RNA is transported to the cytoplasm as a mature mRNA; can refer either to the RNA sequence or the DNA sequence that from which the RNA is transcribed. See also exon. (Developmental Biology 376 Glossary)




Lineage in this context, an evolutionary lineage, a sequence of ancestors and descendants (which may be cells, genes, populations, species) that evolve through time.

Locus The location of a gene on a chromosome. At any locus there can be many different alleles in a population, more alleles than any single organism can possess. For example, no single human can simultaneously carry the A, B and an O blood-type allele. (W.J. Hudson)


Macroevolution Evolution at or above the species level. The boundary between macro- and micro- is fuzzy, as some researchers prefer to include speciation in micro- and others reason that the only macro-process that gives distinctive events is speciation. Speciation events are thus, to many scientists, examples of macroevolution. Another definition is evolution too imperceptible to be observed within the lifetime of one researcher . (W. R. Elsberry in talk.origins via W. J. Hudson) link: Macroevolution Its Definition, Philosophy and History by John Wilkins

Sexual cycle

Sexual cycle

By Wikipedia users Seb951 and Stannered.

Mass extinction Event involving higher extinction rates than the usual degree of background extinction. See Big Five for diagram of extinction rates, and synopsis of five major extinctions.

Meiosis A process which converts a diploid cell to a haploid gamete, and cause a change in the genetic information to increase diversity in the offspring. (W.J. Hudson). In the first stage of sexual reproduction, which is meiosis, the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During fertilization, haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored. (Wikipedia graphic by Stannered)

Meme controversial concept proposed by Richard Dawkins. A meme is a "a unit of cultural inheritance, hypothesized as analogous to the particulate gene and as naturally selected by virtue of its 'phenotypic' consequences on its own survival and replication in the cultural environment." A meme can be an idea, skill, story, or custom, which is passed from one person to another by imitation or teaching. Some theorists argue that memes are the cultural equivalent of genes, and reproduce, mutate, are selected, and evolve in a similar way. The study of memes is called memetics. (Mavericks of the Mind; PBS evolution Glossary)

Mendelian inheritance The mode of genetic inheritance of all diploid species, and therefore of nearly all multicellular organisms. Inheritance is controlled by genes, which are passed on to the offspring in the same form as they were inherited from the previous generation. At each locus an individual has two genes—one inherited from its father and the other from its mother. The two genes are represented in equal proportions in its gametes. (PBS evolution Glossary) For quite some time, the rediscovery of Mendel's work was considered to be the conclusive nail in the Darwinian coffin, killing off the idea of natural selection as Darwin proposed it. Since by the publication of the sixth edition of Darwin's "Origin of Species," Darwin had almost inextricably bound natural selection with his hypothesis on the mechanism of heredity, "pangenesis," this view was quite understandable. However, by the early 1940's, the neo-Darwinian synthesis had met and addressed the criticisms of the Mendelists. (Peter J. Bowler. 1984. Evolution: the history of an idea. University of California Press. Review by W. R. Elsberry link)

Table showing how genes exchange according to segregation or independent assortment during meiosis and how this translates into the Mendel's Laws.

Microevolution Evolution within the species level, or a change in allele frequency in a population over time. Note that this connotation is equivalent to evolution. All "Scientific Creationists" so far admit that microevolution is observed. Some Theistic Anti-Evolutionists may not. (W. R. Elsberry in talk.origins via W. J. Hudson)

Mimicry imitative behavior, one species resembling one another, and gaining advantages as a result. For example harmless flies that have the same colouration as bees and wasps. Because predators know that wasps sting they tend to avoid anything that looks like them. See Batesian mimicry and Müllerian mimicry. (Wikipedia glossary)

Mitochondria (sing. mitochondrion): A a small round organelle found in most cells in nearly all eukaryotes. They are surrounded by two membranes, the inner of which is folded into invaginations called cristae, where aerobic respiration takes place. Mitochondria produce enzymes that convert food to energy. They contain DNA that codes for some mitochondrial proteins. Because mitochondria are generally carried in egg cells but not in sperm, mitochondrial DNA is inherited from mothers but not fathers. Hence it is possible to trace ancestry through the mother's line (see also mitochondrial Eve). (PBS evolution Glossary, Wikipedia)

Mitosis Cell division. All cell division in multicellular organisms occurs by mitosis except for the special division called meiosis that generates the gametes. (PBS evolution Glossary)

Modern Synthesis Also referred to as "evolutionary synthesis", "synthetic theory", and especially modern evolutionary synthesis. The 1920s saw the emergence of an expanded version of Darwinism, which was founded by Ronald Fisher, J. B. S. Haldane and Sewall Wright. They reconciled the idea of evolution by natural selection with the discontinuous, particulate nature of genes. This was the essence the modern synthesis of Darwin's theory and Mendelian genetics. The new synthesis continued to develop in the 1940s, notably with Julian Huxley's, Evolution: The Modern Synthesis (1942) and Bernhard Rensch's, Evolution Above the Species Level (1947). Natural selection was seen as the dominant force shaping evolutionary change. Rensch expressed the view that nothing in biological nature suggests that any evolutionary processes other than natural selection work on the natural genetics of variation within populations. The Great Debate: Darwinism Today. The synthetic paradigm revolution was much broader than the neo-Darwinian concept of Weismann and Wallace, incorporating facts from such fields as genetics, systematics, and paleontology. (Kutschera & Niklas 2004, p.256)

The Modern Synthesis is a theory about how evolution works at the level of genes, phenotypes, and populations whereas Darwinism was concerned mainly with organisms, speciation and individuals. Modern Synthesis differs from Darwinism in three important aspects: 1. It recognizes several mechanisms of evolution in addition to natural selection. One of these, random genetic drift, may be as important as natural selection. 2. It recognizes that characteristics are inherited as discrete entities called genes. Variation within a population is due to the presence of multiple alleles of a gene. 3. It postulates that speciation is (usually) due to the gradual accumulation of small genetic changes. This is equivalent to saying that macroevolution is simply a lot of microevolution. (W. R. Elsberry in talk.origins via W. J. Hudson)

Mosaic evolution Because evolution does not occur uniformly, but rather different characteristics evolve at different rates, transitional organisms tend to have a mosaic of characteristics of both ancestral/primitive and more specialised descendants. So for example early tetrapods had both fish-like and amphibian features, and Archaeopteryx possessed both dinosaur and bird-like features. (MAK)

Multiplication of species The theory that species multiply, either by splitting into daughter species or by "budding", that is, by the establishment of geographically isolated founder populations that evolve into new species. (W. R. Elsberry in talk.origins via W. J. Hudson)

Mutation An error in duplication of genetic material which results in a different sequence of and/or a different number of base pairs in the copy than were in the original. Mutation creates new alleles. (W. R. Elsberry in talk.origins via W. J. Hudson)

Morphology The study of the form and structure of organisms, such as animals and plants and their fossil remains. For example, comparing the shape of the femur in different grazing mammals is a morphological study. (UCMP Understanding Evolution Glossary). Gross morphology refers to the collective structures or an organism as a whole as a general description of the form and structure of an organism, taking into account all of its structures without specifying an individual structure. Anatomy is the study of the form and structure of internal features of an organism. Comparative Morphology is analysis of the patterns of the locus of structures within the body plan of an organism, and forms the basis of taxonomical categorization. Functional Morphology: the study of the relationship between the structure and function of morphological features. Experimental Morphology is study of the effects of external factors upon the morphology of organisms under experimental conditions, such as the effect of genetic mutation. (Wikipedia Morphology pertains to the phenotype rather than the genome ("molecular morphology" has been used for some time for describing the structure of compound molecules, such as polymers and RNA, is a distinct field).


Natural selection The differential reproduction and, thereby, transmission of alleles between generations, of individuals in a population, due to heritable variation in a trait or traits which they possess. This is one mechanism by which evolution can occur. (W. R. Elsberry in talk.origins via W. J. Hudson). Conceived independently and then jointly published by Darwin and Wallace, and substantially elaborated upon in the early part of the twentieth century with the rediscovery of Mendelian genetics and then advances in population genetics. (Kutschera & Niklas 2004, p.256)

Neo-Darwinism historically, term coined by Romanes (1895) to refer to the incorporation of Weismann's ideas on heredity into Darwin's theory of natural selection, showing how biological variation is generated and rejecting the Lamarckian inheritance of the earlier Darwinism. (Kutschera & Niklas 2004, p.260). The term is also used as a synonym for Modern Synthesis, or even any modern approach to evolutionary theory

Neutral theory of molecular evolution The neutral theory of molecular evolution was first formally suggested by Motoo Kimura in 1968, and maintains that the majority of mutations occurring within a population are selectively neutral (i.e. have neither a positive or negative effect), and that genetic drift rather than natural selection is a major factor in differences between populations. While debate still occurs about the relative importance of these two processes, the neutral theory has become the null hypothesis for tests of whether natural selection has occurred in a given lineage. One major implication of this theory is that mutations should accumulate at a fairly constant rate, and therefore the divergence times between lineages should be calculable from the degree of divergence—the so-called molecular clock. The usefulness and correct application of molecular clocks remains a highly contentious subject in studies of evolution. References: Kimura, M. 1968. Evolutionary rate at the molecular level. Nature 217: 624–626. pdf Wikipedia: Neutral theory of molecular evolution. CKT070830

Neo-Lamarckism Popular alternative to Darwinism during the late 19th and early 20th centuries, based on Lamarck's idea of acquired characteristics. Neo-Lamarckism was supported by natural theology, popular in America at the turn of the century. Spencer supported neo-Lamarckism. "Against selection itself Spencer [1893] used an argument that had considerable force when measured against the pregenetical selection theory (Ridley, 1982a). He pointed out that when a new structure evolved, all the rest of the body would have to accommodate the new development. Thus a series of variations would be required to adjust the overall structure in a manner correlated to the new organ. What would be the chance of all these variations appearing together at the right time, if the species had to depend on random variation? Selection might explain the changes in a single organ, but not an integrated transmutation of the whole body." Lamarckism, as Spencer pointed out, could provide an explanation for the integrated development or elimination of organs. This was seen to be a weakness of natural selection. "The law of "acceleration of growth" was first published in Cope's "On the origin of genera" of 1867 (reprinted in Cope, 1887) and in Hyatt (1868). According to this law, evolution progresses by a series of sudden additions to the growth of the individual. At certain points in time, every individual in a species begins to exhibit a new phase of growth that advances all to the form of a new species. To make room for this addition, the old adult form is compressed back to an earlier phase of growth, hence the "acceleration" of growth to accommodate an extra stage before maturity. Cope denied that evolution on a small scale is a branching process, claiming instead that each genus represents a group of species that have reached the same point in the historical development of their group. Their close relationship is not a sign of common descent but of identical position in the scheme of development." "Cope postulated a growth-force named "bathmism;" concentrated in those parts of the body most in use, it developed them at the expense of other areas. By the last decade of the century, this Lamarckism had been developed to considerable depth (Cope, 1887, 1896; Hyatt, 1880, 1884, 1889)." Referring to the case of the midwife toad: "Was the india ink added by someone wishing to preserve the original marks, or was it deliberate sabotage, perhaps a Nazi plot to discredit evidence hostile to their racial theories? Koestler certainly has suggested that Kammerer's experiments may have been genuinely successful, although others think he was simply dishonest. (Aronson, 1975)." (Peter J. Bowler. 1984. Evolution: the history of an idea. University of California Press. Review by W. R. Elsberrylink)

Nondirectionality (in evolution) as here defined, the premise that evolution does not have a direction, that nature does not tend towards greater complexity, that it is misleading to speak of "lower", "simpler", or "primitive", and that all attempts to impose a narrative are hold-overs of Victorian ideas such as ascent. Non-directionality is favoured by some evolutionists such as Steven Jay Gould. See also cosmicism, reductionism. Contrast with anthropocentrism, ascent, directionality, Evolution (Systems Theory) and teleology. (MAK)

Non-missing link Although creationists often claim that no transitional forms are known in the fossil record, in fact the reverse is the case. (see Link). As it would be oxymoronic to refer to these intermediate species by their popular moniker as "missing link" (e.g. link link) I have coined the informal term "non-missing link". See also anagenesis, ancestor, common ancestor, basal taxon, stem group. Note that even though, in view of the vagaries of the fossil record, the non-missing link may not necessarily be the actual, literal, common ancestor of all later species in that lineage (although in some cases where stratigraphic preservation is very good it might), but it would certainly be a closely related form. (MAK)


Ontogeny The process of the development and growth of an individual from zygote to adult. (W. R. Elsberry in talk.origins). See also developmental biology, evo-devo, morphogenesis.

Ontogeny recapitulates phylogeny See Recapitulation.

Orthogenesis a conjecture related to Lamarckism. "The crucial difference is that the trends of orthogenesis are not adaptive. Far from being a positive response to the environment, they represent a nonutilitarian force that can in some cases drive the species to extinction. In this there is a similarity to Hyatt's concept of racial senility." "A famous case was that of the recently extinct "Irish elk", thought to have died out because its antlers became too large as a result of an internal trend (Gould, 1974b). It seemed as though the trend that produced the antlers, perhaps originally for some useful purpose, had acquired a momentum of its own that had carried it far beyond the point of utility. This "overdevelopment" theory of extinction became widely popular among non-Darwinian paleontologists in the early twentieth century." "Strong support for orthogenesis came from the Russian biologist Leo S. Berg (translation 1926), but perhaps its best known exponent was the American paleontologist Henry Fairfield Osborn." Aristogenesis—Osborn's own term for orthogenesis. Mendelism was originally viewed as an alternative to selection. (Peter J. Bowler. 1984. Evolution: the history of an idea. University of California Press. Review by W. R. Elsberry link)

Organism individual member of a species, that is, a single biological entity, either unicellular (single-celled) or multicellular (many-celled). A living system such as animal, plant, fungus, or eukaryote or prokaryote micro-organism, capable of response to stimuli, reproduction, growth, and maintenance of homeostasis as a stable whole. Colloquially and informally, the term might also be used in evolutionary narratives to refer to a species or population, rather than just an individual. (from Wikipedia, MAK, Fossil Mall)


Parallel evolution the development of a similar trait or traits in related, but distinct, species descending from the same ancestor, but from different clades or lineages. For example:

One of the most spectacular examples of parallel evolution is provided by the two main branches of the mammals, the placentals and marsupials, which have followed independent evolutionary pathways following the break-up of land-masses such as Gondwana roughly 100 million years ago. In South America, marsupials and placentals shared the ecosystem (prior to the Great American Interchange); in Australia, marsupials prevailed; and in the Old World the placentals won out. However, in all these localities mammals were small and filled only limited places in the ecosystem until the mass extinction of dinosaurs sixty-five million years later. At this time, mammals on all three landmasses began to take on a much wider variety of forms and roles. While some forms were unique to each environment, surprisingly similar animals have often emerged in two or three of the separated continents. Examples of these include the litopterns and horses, whose legs are difficult to distinguish; the European sabre-toothed cat (Machairodontinae) and the South American marsupial sabre-tooth (Thylacosmilus); the Tasmanian wolf and the European wolf; likewise marsupial and placental moles, flying squirrels, and (arguably) mice. (modified from Wikipedia)

Phenotype The set of measurable or detectable physical or behavioral features of an individual. The phenotype represents the expression of the genotype of the individual as modified by environmental conditions during the individual's ontogeny. (W. R. Elsberry in talk.origins)

Phylogeny term coined by Haeckel (Haeckel 1866): the study of the family history of life, the evolutionary relationships among groups of organisms, often illustrated with a branching evolution tree. More

Piltdown Man famous 1912 hoax of early fossil man, consisting of a human skull, ape jaw, and filed down teeth. Had a significant detrimental impact on early research on human evolution: discoveries of Australopithecine fossils found in the 1920s in South Africa were ignored and instead the popular (but erroneous) theory argued that the human brain expanded in size before the jaw adapted to new types of food. rather than the reverse. Definitively exposed as a forgery by scientists back in 1953. (MAK, Wikipedia)

Polyploidy containing more than two paired (homologous) sets of chromosomes. (Wikipedia)

Population A group of potentially inter-breeding individuals of the same species found in the same place at the same time (Booth et al. 2003). A group of organisms, typically a single species, and typically isolated from other members of its species in some manner. (W.J. Hudson)

Plasmid A genetic element that exists (or can exist) independently of the main DNA in the cell. In bacteria, plasmids can exist as small loops of DNA and be passed between cells independently. (PBS evolution Glossary)

Primitive ancestral, similar or identical to the original forms, basal or stem member of a lineage, tends to be a generalist, lacks the specialised features of its descendants. Cladistics rejects terms like "primitive", instead using the more technical and (to outsiders and non-paleo geeks) obscure plesiomorphy. Nevertheless "primitive" does not have to equate anthropomorphically with advancement, technology, etc, compare "primeval" or "primordial". See also my comments re "advanced". (MAK)

Gradual and Punctuated evolution - diagram by Paul Olsen

Gradual and Punctuated evolution

diagram by Paul Olsen

Punctuated equilibria (More popularly known as punctuated evolution): an evolutionary theory that argues that new species evolve suddenly and in geographically isolated areas. Most speciation involves cladogenesis rather than anagenesis, and occurs via peripatric speciation. Hence speciation is rarely found in the fossil record, because established, populous and widespread species (the sort that are most likely simply through greater numbers to leave fossil remains) usually change slowly, if at all, during their time of residence. New species tend to develop in a geographically limited region and stratigraphically limited extent, which is small in relation to the overall time and distribution of the species. Sampling of the fossil record will reveal a pattern of most species in stasis, with abrupt appearance of newly derived species being a consequence of ecological succession and dispersion. Adaptive change in lineages occurs mostly during periods of speciation, and trends in adaptation occur mostly through the mechanism of species selection. See punctuated equilibria FAQ on the talk.origins archive site. (W. R. Elsberry in talk.origins via W. J. Hudson, modified).
Right: Gradual and Punctuated evolution. Gradual evolution (or phyletic gradualism) occurs where change is small and constant; punctuated evolution where change is very rapid, while most of the time there is virtually no change. (Diagram by Paul Olsen, Lecture 5 Evolutionurl; main reference: Eldredge & Gould 1972)

Protein the building blocks of cells; large molecules made up of a sequence of amino acids. Many of the important large molecules in living organisms—for example, enzymes—are proteins. (Fossil Mall glossary, MAK)


Quasispecies Darwinian evolution of self-replicating entities within the framework of physical chemistry. Put simply, a quasispecies is a large group or cloud of related genotypes that exist in an environment of high mutation rate, where a large fraction of offspring are expected to contain one or more mutations relative to the parent. This is in contrast to a species, which from an evolutionary perspective is a more-or-less stable single genotype, most of the offspring of which will be genetically accurate copies.

The quasispecies model is useful in providing a qualitative understanding of the evolutionary processes of self-replicating macromolecules such as RNA or DNA or simple asexual organisms such as bacteria or viruses (viral quasispecies), and is helpful in explaining something of the early stages of the origin of life. Quantitative predictions based on this model are difficult because the parameters that serve as its input are hard to obtain from actual biological systems. The quasispecies model was put forward by Manfred Eigen and Peter Schuster based on initial work done by Eigen. (Wikipedia)


r-selection, r-selected species A species that produces a large number of off-spring, each of which receives little care (quantity rather than quality). R-selected species are better suited for variable or unpredictable environments. (Wikipedia glossary)

Racial senility intriguing but long refuted theory that certain long-lived lineages became old and "senile", by analogy with individual development, as their evolutionary novelty is used up. Developed by Alpheus Hyatt to explain the exotic shapes of some Cretaceous ammonite shells, horns and plates on dinosaurs, and so on. (MAK)

Random Unpredictable in some way. Mutations are "random" in the sense that the sort of mutation that occurs cannot generally be predicted based upon the needs of the organism. However, this does not imply that all mutations are equally likely to occur or that mutations happen without any physical cause. Indeed, some regions of the genome are more likely to sustain mutations than others, and various physical causes (e.g., radiation) are known to cause particular types of mutations. (UCMP Understanding Evolution Glossary)

Random drift See genetic drift.

Recapitulation The theory of recapitulation, also called the biogenetic law or Embryological parallelism, and often expressed as the phrase "ontogeny recapitulates phylogeny". The hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages in the evolution of their remote ancestors. Therefore, each phase of the ontogeny of an individual directly represents the adult phase of some ancestor species in the phylogeny of the species to which the individual belongs. With different formulations, such ideas have been applied to several fields, including biology, anthropology and education theory. In biology, there are several examples of embryonic stages showing features of ancestral organisms, but a "strong" formulation of the concept has been discredited. The concept originated in the 1790s among the German Natural philosophers and, as proposed by Étienne Serres in 1824–26, became known as the "Meckel–Serres Law". In 1866, the German zoologist Ernst Haeckel proposed that the embryonic development of an individual organism (its ontogeny) followed the same path as the evolutionary history of its species (its phylogeny). This principle is recognized to be inaccurate in several respects, and its use is now generally deprecated. The turn of phrase is attributed to Ernst Haeckel, while the "biogenetic law" upon which it was based can be traced back to von Baer. (W. R. Elsberry in talk.origins, Wikipedia)

Recognition species concept A definition of a species as a set of organisms that recognize one another as potential mates; they have a shared mate recognition system. Compare with biological species concept, cladistic species concept, ecological species concept, and phenetic species concept. (Fossil Mall glossary) See other species definitions.

Recombination Recombination creates new combinations of alleles. Recombination primarily occurs through sexual reproduction, where diploid cells form haploid gametes. The organism inherits one gamete each from the mother and the father, and the gametes are 'recombined' to form a new diploid chromosome. Recombination can occur not only between genes, but within genes as well. Recombination within a gene can form a new allele. (cf. mutation ) Recombination is a mechanism of evolution because it adds new alleles and combinations of alleles to the gene pool. (W.J. Hudson)

Red Queen's Hypothesis or Red Queen Effect is an evolutionary hypothesis. The term is taken from the Red Queen's race in Lewis Carroll's Through the Looking-Glass. The Red Queen said, "It takes all the running you can do, to keep in the same place." The Red Queen Principle can be stated thus: In reference to an evolutionary system, continuing adaptation is needed in order for a species to maintain its relative fitness amongst the systems being co-evolved with. The hypothesis is intended to explain two different phenomena: the advantage of sexual reproduction at the level of individuals, and the constant evolutionary arms race between competing species. In the first (microevolutionary) version, by making every individual an experiment when mixing mother's and father's genes, sexual reproduction may allow a species to evolve quickly just to hold onto the ecological niche that it already occupies in the ecosystem. In the second (macroevolutionary) version, the probability of extinction for groups of organisms is hypothesized to be constant within the group and random among groups. It's counterpart is the Court Jester Hypothesis, which proposes that macroevolution is driven mostly by abiotic events and forces. (Wikipedia)

Reproductive isolation Isolation of one species or population from another species or population by differences in reproductive traits or habits. The two species or populations may or may not share the same environmental range. An example of two species being reproductively isolated are similar species of animals that breed at different times of the year. (W.J. Hudson)

Ring species A situation in which two reproductively isolated populations living in the same region are connected by a geographic ring of populations that can interbreed. (PBS evolution Glossary)

Ribonucleic acid (RNA): A molecule similar to DNA, but with only a single strand, by which the genetic code of DNA is converted into proteins in cells. It has three forms: Messenger RNA, ribosomal RNA, and transfer RNA. Some viruses carry RNA as their genetic material instead of DNA. There has been speculation that an "RNA world" preceded current life on Earth.


Selection see natural selection.

Selective pressure any environmental factors such as scarcity of food or extreme temperatures that favour the survival of only those organisms with characteristics that provide resistance or adaptability. (based on PBS evolution Glossary)

Sexual selection a trait that makes an individual more likely to find a mate than others. A microevolutionary process. (Wikipedia: Glossary of ecology). This process may produce traits that seem to decrease an organism's chance of survival, while increasing its chances of mating. (UCMP Understanding Evolution Glossary)

Shifting Balance Theory Sewall Wright's 'Shifting Balance' theory argues that populations are often divided into smaller subpopulations. Drift could cause allele frequency differences between subpopulations if gene flow was small enough. If a subpopulation was small enough, the population could even drift through fitness valleys in the adaptive landscape. Then, the subpopulation could climb a larger fitness hill. Gene flow out of this subpopulation could contribute to the population as a whole adapting. (W.J. Hudson)

Social Darwinism a 19th century political philosophy which attempted to explain differences in social status (particularly class and racial differences) on the basis of evolutionary fitness. Based on the misinterpretation of Darwinian theory, Social Darwinism is generally considered unscientific by modern philosophers of science. (Wikipedia glossary)

Species Highly controversial term given a variety of definitions by biologists. Currently, the Biological Species Concept (BSC) is widely popular: Groups of actually or potentially interbreeding populations, which are reproductively isolated from other such groups (Mayr, 1963, Animal Species and Evolution). More (W. R. Elsberry in talk.origins via W. J. Hudson) See also cladistic species concept, ecological species concept, phenetic species concept, and recognition species concept. See other species definitions. (Fossil Mall glossary)

Speciation The the basic process of evolution by which new species appear. Although the theory of evolution is a century and a half old the precise mechanism by which new species make their appearance in the biosphere is still a field of active research, with all the disagreements and debates that go with it. A number of types of speciation have been proposed:

Allopatric speciation is supposed to be caused by the physical separation of specimens of what was one and the same species. The classical example is Darwin's work on the finches of the Galapagos Islands. The presumed scenario is that an ancestral species of finch reached the various islands and evolved in about as many different species as there are islands. The critical factor causing the speciation is usually assumed to be the severing of the gene flow between the population on an island and the mother population on the mainland. Mayr also stressed the small size of the new population and contended that e.g. the emergence of the Isthmus of Panama did not lead to much speciation of biota of the shallow seas at either side because both represented a far too large gene pool to allow allopatric speciation to occur. Jcwf100131

Peripatric speciation is taken to occur in the same geographic area—without severance of the gene flow—due to ecological differences, e.g. the existence of two different ecological niches into which an existing species can specialize. Jcwf100131. Alternatively, a population of an ancestral species in a geographically peripheral part of the ancestral range is modified over time until even when the ancestral and daughter populations come into contact, there is reproductive isolation. See also cladogenesis, anagenesis, punctuated equilibria. (W. J. Hudson)

Splitting see cladogenesis.


Tierra Artificial life simulation of Tom Ray's which demonstrates the utility of natural selection in computer implementations for finding novel approaches to difficult problems. This is prima facie evidence that A. E. Wilder-Smith was premature in declaring "simulations of natural selection 'jam' the best computers". (W. R. Elsberry in talk.origins)

Transitional form, or transitional fossil A fossil or group of organisms that are intermediate and a link between a more primitive or ancestral group and a more advanced or specialised one, possessing characteristics or traits of both (see Mosaic evolution). Generally any evolutionary lineage constitutes a series of transitional forms; for example in the evolution of birds from dinosaurs, or whales from terrestrial ancestors, there are a number of intermediate forms or non-missing links.An important aspect of evolutionary systematics, see also anagenesis. Note that strict application of cladistics rejects the possibility of identifying transitional forms (it doesn't deny the reality of evolution of course, just that it is possible to know for sure which fossils represent transitional forms) (Systematics and Biogeography: Paraphyly Watch 3: Transitional Fossils, Microbes & Patrocladistics). An alternative approach (given in Wikipedia) would be to make a distinction between "transitional" and "intermediate". Transitional forms do not have a significant number of unique derived traits, so it is morphologically close to the actual common ancestor it shares with its more derived relative (see also basal taxon and stem group). Intermediate can be used for those forms with a larger number of uniquely derived traits. According to this definition, Archaeopteryx is transitional whereas the platypus (an specialised egg laying mammal, descended from very primitive mammals) is intermediate. But rather than multiply terminology, it would be better to retain intermediate in the informal but more grammatically correct sense of meaning the same as "transitional". Some intermediate/transitional forms linking major groups of vertebrates include the fish/amphibian sequence from Eusthenopteron (fish) to Panderichthys to Tiktaalik to Acanthostega to fully developed amphibians (Devonian period), transitional reptile/mammal forms such as the cynodont Thrinaxodon and other mammal-like reptiles that show a blend of mammalian and reptilian characteristics (Triassic), Velociraptor and relatives, and even more so Microraptor, a four-winged gliding dromaeosaurid, and even more advanced forms such as Anchiornis and Scansoriopteryx, representing an intermediate stage between the flightless theropods and primitive birds such as Archaeopteryx (Jurassic); Pezosiren, an intermediate form of a primitive seacow with both terrestrial (land mammal) and aquatic adaptations (Eocene); Pakicetus, Ambulocetus, Rodhocetus and similar forms constitute links between amphibious and terrestrial artiodacyl (even-toed) ungulates and aquatic whales (Eocene); and Sahelanthropus, indicating it is close to the common ancestor of chimpanzees and modern humans) the most basal ape-like African hominid. mosaic of primitive (chimpanzee-like) and derived hominid features (Miocene) See Transitional vertebrate fossils FAQ, at the TalkOrigins Archive, and Wikipedia: List of transitional fossils for a much more detailed lists. (MAK; Kutschera & Niklas 2004, p.259).


Unicellular organism a living system consisting of only a single cell. May be simple, as with bacteria, or complex, as with protists. In the case of protists, different parts of the cell takes on the functions that organs and other systems fulfill in multicellular (many-celled) organisms. (MAK)

Uniformitarianism Assumption that processes acting in the past are the same as those acting in the present. proposed the late 18th century theory of James Hutton that the natural forces now changing the shape of the earth's surface have been operating in the past much in the same way. The most important implication is that the earth is very old (deep time) and that the present is the key to understanding the past. Developed by Charles Lyell in the 19th century, who in turn influenced Darwin. Contrast with catastrophism, punctuated equilibrium.

Universal tree of life See tree of life.


Variation differences between individual organisms, or populations. An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. A substantial part of the variation in phenotypes in a population is caused by the differences between their genotypes. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will fluctuate, becoming more or less prevalent relative to other forms of that gene. Evolutionary forces act by driving these changes in allele frequency in one direction or another. Variation disappears when a new allele reaches the point of fixation, when it either disappears from the population or replaces the ancestral allele entirely. Variation comes from mutations in genetic material, migration between populations (gene flow), and the reshuffling of genes through sexual reproduction. Variation also comes from exchanges of genes between different species; for example, through horizontal gene transfer in bacteria, and hybridisation in plants. Despite the constant introduction of variation through these processes, most of the genome of a species is identical in all individuals of that species. However, even relatively small changes in genotype can lead to dramatic changes in phenotype: for example, chimpanzees and humans differ in only about 5% of their genomes. (Wikipedia)

Vestigial, vestigial structure A non-functional anatomical component retained merely as a matter of contingent history. (W. R. Elsberry in talk.origins) Usually, vestigial structures are formed when a lineage experiences a different set of selective pressures than its ancestors, and selection to maintain the elaboration and function of the feature ends or is greatly reduced. UCMP Understanding Evolution Glossary, Many organisms have vestigial organs, which are the remnants of fully functional structures in their ancestors. As a result of changes in lifestyle the organs became redundant, and are either not functional or reduced in functionality. With the loss of function goes the loss of positive selection, and the subsequent accumulation of deleterious mutations. Since any structure represents some kind of cost to the general economy of the body, an advantage may accrue from their elimination once they are not functional. Examples: wisdom teeth in humans; the loss of pigment and functional eyes in cave fauna; the loss of structure in endoparasites. (Wikipedia)

Vicariance a process in which a species' range is divided even though the species has remained in place. This might happen through tectonic action, geologic activity (like the rise of a mountain range or shift in the course of a river), or other processes. Vicariance is usually contrasted with dispersal as a biogeographic mechanism. (UCMP Understanding Evolution Glossary)

Virus infectious agent that can replicate only inside the living cells of organisms, and infect all types of organisms, from animals and plants to bacteria. Most viruses are too small to be seen directly with a light microscope. Since the initial discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. An enormous variety of genomic structures can be seen among viral species; as a group they contain more structural genomic diversity than plants, animals, archaea, or bacteria. A virus has either DNA or RNA genes and is called a DNA virus or a RNA virus respectively. The vast majority of viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes. Viruses are not typically considered to be organisms because they are incapable of "independent" or autonomous reproduction or metabolism. Their origins are unclear: some may have evolved from plasmids, others from bacteria. Viruses are an important means of horizontal gene transfer, which increases genetic diversity. The study of viruses is known as virology, a sub-speciality of microbiology. (Wikipedia)


Web of life conventionally refers to the food chain or trophic network, describes the feeding relationships between different species in an ecosystem. However, in reference to horizontal gene transfer can also refer to genetic transfer and evolution by non-hereditary means; especially common among bacteria.

Williams revolution paradigm shift of the 1960s which saw the gene become the focus of evolutionary thinking, which saw evolutionary biology united with genetics. The revolution is named after George C. Williams, whose 1966 book Adaptation and Natural Selection popularised the theory. Previously most evolutionary thinkers considered selection to favour individuals, groups (group selection) and species, such as individuals acting "for the good of the species". The Williams revolution, however, established gene selection as the principal process of selection, and showed that because genes were the units of selection, selection would favour genes which maximised their own survival, not that of the group or species. (EvoWiki)




Zygote The cell formed by the fertilization of male and female gametes. (PBS evolution Glossary)


Some Links to other glossaries: (some of which have been used here) Evolution: Glossary, very detailed general glossary, Synthetic Theory of Evolution: Glossary of Terms, includes pronunciations; Phylogenetics Glossary, various technical, mostly phylogenetic and cladistic terms. By the Evolution discussion group (fall 1996); Modified from: Hillis, D.M., C. Moritz and B.K. Mable (eds). 1996. Molecular Systematics, Second Edition. Sinnauer Assoc., Massachusetts.; Talk.Origins Jargon, includes not only technical terms but also a list of who's who, slanted to the Creation–Evolution debate, but also of general value.

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