charles_darwin

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CHARLES DARWIN AND THE ORIGIN OF SPECIES
done. In 1977, the American paleontologist Stephen Jay Gould
(1941–2002) wrote, ‘‘Our understanding of genetic mutation suggests
that Darwin was right in maintaining that variation is not predirected
in favorable ways. Evolution is a mixture of chance and necessity—
chance at the level of variation, necessity in the working of selection.’’
7 What historians of science call the Neo-Darwinist synthesis
had occurred.
Three major trends contributed to the reemergence of Darwin’s
theories as a major component of the explanation for evolution. The
first of these was population genetics, the study of the spread of species
based on mathematics. The American scientist Sewall Wright
(1889–1988) and the British scientists Ronald Aylmer Fisher (1890–
1962) and J. B. S. Haldane (1892–1964) are the best-known proponents
of this approach to evolution. In his book The Genetical Theory
of Natural Selection (1930), Fisher suggested that genes spread
through a population as discrete units; the more adaptive a gene
became the faster it would increase through a population. As genes
were the source of mutation, a ‘‘good’’ gene, one that adapted the
best, would produce increasingly useful mutations. Haldane demonstrated
that the spread of a ‘‘good’’ gene could occur much more rapidly
than Fisher thought. He drew attention to examples such as the
peppered moth. Between 1850 and 1900, the darker form of the
moth began to dominate the species because it could hide from predators
in the soot, which was a common feature of cities in Britain.
Wright demonstrated that the interaction of genes was much more
complex than Fisher and Haldane suggested. Multiple genes might
affect one characteristic in an organism. Thus, the mutation in one
gene could produce a large range of variations in a species because
of its interaction with other genes. Through natural selection, the
most adaptive of these variations would survive—just as Darwin had
predicted.
Fisher, Haldane, and Wright’s work was highly theoretical. The
second major trend was the ‘‘translation’’ of the theories of population
geneticists into experiments and laws that naturalists could use
in their research. The American zoologist Theodosius Dobzhansky
(1900–1975) was the most important contributor to this trend. In
Genetics and the Origin of Species (1937), Dobzhansky explained that
the application of the laws of genetics to small populations showed
how a mutation could occur and spread. Like Darwin, he argued that
what occurred on a microlevel could occur on a macrolevel. Mutation
ultimately led to transmutation or speciation.
Other scientists had been working on the connection between
genetics and evolution. The Russian population geneticist Sergei