Evolution__3rd_Edition

18 PART 1 / Introduction
. . . and biological classification ...
. . . and research on fossils
The modern synthesis was
established by the 1940s
make of variation between individuals within a species. Species, in the typological conception,
had been defined as a set of more or less similar-looking organisms, where
similarity was measured relative to a standard (or “type”) form for the species. A
species then contains some individuals of the standard type, and other individuals who
deviate from that type. The type individuals are conceptually privileged, whereas the
deviants show some sort of error.
However, the concept of a species as type plus deviants was inappropriate in the
theory of population genetics. The changes in gene frequencies analyzed by population
geneticists take place within a “gene pool” a that is, a group of interbreeding
organisms, who exchange genes when they reproduce. The crucial unit is now the set of
interbreeding forms, regardless of how similar looking they are to each other. The idea
of a “type” for a species is meaningless in a gene pool containing many genotypes. One
genotype is no more of a standard form for the species than any other genotype. A gene
pool does not contain one, or a few, “type” genotypes that are the standard forms for a
species, with other genotypes being deviants from that “type.” No type form exists that
could be used as a reference point for defining the species. Population geneticists therefore
came to define the members of a species by the ability to interbreed rather than
by their morphological similarity to a type form. The modern synthesis had spread to
systematics.
A similar treatment was given to paleontology by George Gaylord Simpson (Figure
1.11) in Tempo and Mode in Evolution (1944). Many paleontologists in the 1930s
still persisted in explaining evolution in fossils by what are called orthogenetic
processes a that is, some inherent (and unexplained) tendency of a species to evolve
in a certain direction. Orthogenesis is an idea related to the pre-Mendelian concept
of directed mutation, and the more mystical internal forces we saw in the work
of Lamarck. Simpson argued that no observations in the fossil record required these
processes. All the evidence was perfectly compatible with the population genetic
mechanisms discussed by Fisher, Haldane, and Wright. He also showed how such
topics as rates of evolution and the origin of major new groups could be analyzed by
techniques derived from the assumptions of the modern synthesis (Chapters 18–23).
By the mid-1940s, therefore, the modern synthesis had penetrated all areas of biology.
The 30 members of a “committee on common problems of genetics, systematics, and
paleontology” who met (with some other experts) at Princeton in 1947 represented all
areas of biology. But they shared a common viewpoint, the viewpoint of Mendelism and
neo-Darwinism. A similar unanimity of 30 leading figures in genetics, morphology,
systematics, and paleontology would have been difficult to achieve before that date.
The Princeton symposium was published as Genetics, Paleontology, and Evolution
(Jepsen et al. 1949) and is now as good a symbol as any for the point at which the
synthesis had spread throughout biology. Of course, there remained controversy
within the synthesis, and a counterculture outside. In 1959, two eminent evolutionary
biologists a the geneticist Muller and the paleontologist Simpson a could still both
celebrate the centenary of The Origin of Species with essays bearing (almost) the same
memorable title: “One hundred years without Darwinism are enough” (Muller 1959;
Simpson 1961a).
In this book, we shall look in detail at the main ideas of the modern synthesis, and see
how they are developing in recent research.
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