Evolution – the Modern Synthesis

The Modern Synthesis

A Theory of the Mechanism of Evolution
version 1.5, January 22, 1993)

Author: Larry A. Moran (from the G.L.K file collection—original url)

Many people do not understand current ideas about evolution. The following is a brief summary of the modern consensus among evolutionary biologists.

The idea that life on Earth has evolved was widely discussed in Europe in the late 1700's and the early part of the last century. 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 thirty 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 often referred to as the fact of evolution. However, the mechanism of evolution is still debated.

We have learned much since Darwin's time and it is no longer appropriate to claim that evolutionary biologists believe that Darwin's theory of Natural Selection is the best theory of the mechanism of evolution. I can understand why this point may not be appreciated by the average non-scientist because natural selection is easy to understand at a superficial level. It has been widely promoted in the popular press and the image of "survival of the fittest" is too powerful and too convenient.

During the first part of this century the incorporation of genetics and population biology into studies of evolution led to a Neo-Darwinian theory of evolution that recognized the importance of mutation and variation within a population. Natural selection then became a process that altered the frequency of genes in a population and this defined evolution. This point of view held sway for many decades but more recently the classic Neo-Darwinian view has been replaced by a new concept which includes several other mechanisms in addition to natural selection. Current ideas on evolution are usually referred to as the Modern Synthesis which is described by Futuyma:

"The major tenets of the evolutionary synthesis, then, were that populations contain genetic variation that arises by random (i.e.. not adaptively directed) mutation and recombination; that populations evolve by changes in gene frequency brought about by random genetic drift, gene flow, and especially natural selection; that most adaptive genetic variants have individually slight phenotypic effects so that phenotypic changes are gradual although some alleles with discrete effects may be advantageous, as in certain color polymorphisms); that diversification comes about by speciation, which normally entails the gradual evolution of reproductive isolation among populations; and that these processes, continued for sufficiently long, give rise to changes of such great magnitude as to warrant the designation of higher taxonomic levels (genera, families, and so forth)."

Futuyma, D.J. in Evolutionary Biology, Sinauer Associates, 1986; p.12

This description would be incomprehensible to Darwin since he was unaware of genes and genetic drift. The modern theory of the mechanism of evolution differs from Darwinism in three important respects:

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.

In other words, 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. This is a major paradigm shift and those who fail to appreciate it find themselves out of step with the thinking of evolutionary biologists. Many instances of such confusion can be seen here in the newsgroups, in the popular press, and in the writings of anti-evolutionists.

The major controversy among evolutionists today concerns the validity of point #3 (above). The are many who believe that the fossil record at any one site does not show gradual change but instead long periods of stasis followed by rapid speciation. This model is referred to as Punctuated Equilibrium and it is widely accepted as true, at least in some cases. The debate is over the relative contributions of gradual versus punctuated change, the average size of the punctuations, and the mechanism. To a large extent the debate is over the use of terms and definitions, not over fundamentals. No new mechanisms of evolution are needed to explain the model.

Some scientists continue to refer to modern thought in evolution as Neo-Darwinian. In some cases these scientists do not understand that the field has changed but in other cases they are referring to what I have called the Modern Synthesis, only they have retained the old name.