Author: Larry A. Moran (firstname.lastname@example.org)
Title: The Modern Synthesis of Genetics and Evolution
THE MODERN SYNTHESIS
A Theory of the Mechanism of Evolution
(version 1.5, January 22, 1993)
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 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 beleive 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view has been replaced by a new concept which includes several other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
(ie. 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 warrent 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.