The Evolution of Evolution 101still others completely agnostic as to what mechanism drove evolution. Although the fossil
record continued to provide more and more evidence for how life had evolved, paleon-
tologists were not at the forefront for theoretical mechanisms of how evolution occurred.
Meanwhile, systematists (biologists who study the naming and relationships of organisms)
were busy describing new species, but few thought of the evolutionary implications of their
work. There was simply no common thread among them, and there appeared to be no way
to show that Darwinian natural selection was compatible with genetics, paleontology, and
systematics.
The breakthrough occurred in the 1930s, when three scientists introduced a set of
mathematical models known as population genetics. Two were British (Sir Ronald Fisher and
J. B. S. Haldane) and one was an American, Sewall Wright, whom I had the great fortune
to meet in 1983 while he was still active and alert at age 94. These mathematical simula-
tions allowed evolutionists to describe changing gene frequencies through many generations
and simulate the effects of mutation and selection. Population genetics clearly showed
that even slight selection pressure can quickly change gene frequencies and made evolu-
tion by Darwinian natural selection plausible again. By the late 1930s, several books tied
population genetics to the various subdisciplines of evolutionary biology. In 1937, geneticist
Theodosius Dobzhansky published Genetics and the Origin of Species, which updated and
synthesized all the genetics known at the time, and showed how selection in fruit fly experi-
ments gave amazing demonstrations of evolution in action. In 1942, ornithologist Ernst
Mayr published Systematics and the Origin of Species (discussed in chapter 3), which dealt
with the problem of speciation in nature and showed how the allopatric speciation model
was consistent with natural selection. And in 1944 (written in 1941, but delayed by World
War II), paleontologist George Gaylord Simpson published Tempo and Mode in Evolution,
which attempted to show that nothing in the fossil record was inconsistent with Darwinian
natural selection. Together, these works brought the major threads of evolutionary biol-
ogy—genetics, systematics, and paleontology—back into the Darwinian fold. By the late
1940s, biologist Julian Huxley was referring to this new consensus as the “modern synthe-
sis” or the “neo-Darwinian synthesis.” By the 1959 centennial of the publication of On the
Origin of Species, the neo-Darwinian synthesis completely dominated evolutionary biology,
and most biologists thought that all the major problems had been solved and questions had
been answered. After almost 60 years, most current textbooks on evolution still reflect this
dominance of neo-Darwinism.
What are the main ideas of the neo-Darwinian synthesis? Its central core comes from
genetics (with its focus on the genotype of the organism), which has shown just how effective
natural selection can be in changing the frequencies of genes in populations. From this, neo-
Darwinists define evolution in an extremely reductionist manner as a change in gene fre-
quencies through time, without regard to the embryology or development of the organism
or the influence of the body (phenotype). Some extreme neo-Darwinists argue that the body is
simply a device for genes to make more copies of themselves. Population genetics and fruit
fly experiments showed that most variation is due to the recombination of genes from both
parents but that additional variation is the result of slight mutations. These random variants
are then weeded out by natural selection, and the stronger the selection, the more rapid the
genetic change. In some extreme versions of neo-Darwinism, natural selection was treated
as an all-powerful, all-pervasive force that, in Darwin’s words, “is daily and hourly scruti-
nizing throughout the world every variation, even the slightest; rejecting all that which is