16 CHAPTER 1Mutationist theories were advanced by some geneticists who observed that
discretely different new phenotypes can arise by a process of mutation. They sup-
posed that such mutant forms constituted new species and thus believed that
natural selection was not necessary to account for the origin of species. The last
influential mutationist was Richard Goldschmidt (1940, [8]), an accomplished
geneticist who nevertheless erroneously argued that the origin of new species
and higher taxa is entirely different in kind from evolutionary change within
species. New species or genera, he said, originate by sudden, drastic changes
that reorganize the whole genome. Although most such reorganizations would
be deleterious, a few “hopeful monsters” would be the progenitors of new forms
of life.The evolutionary synthesis
These anti-Darwinian ideas were refuted in the 1930s and 1940s by the geneti-
cists, systematists, and paleontologists who reconciled Darwin’s theory with the
facts of genetics [19, 28]. The consensus they forged is known as the evolutionary
synthesis, or modern synthesis, and its chief principle, that adaptive evolution is
caused by natural selection acting on particulate (Mendelian) genetic variation, is
often referred to as neo-Darwinism.^1 Ronald A. Fisher and John B. S. Haldane in
England and Sewall Wright in the United States developed a mathematical theory
of population genetics, which showed that mutation and natural selection together
cause adaptive evolution: mutation is not an alternative to natural selection, but is
rather its raw material. The study of genetic variation and change in natural popu-
lations was pioneered in Russia by Sergei Chetverikov and continued by Theodo-
sius Dobzhansky, who moved from Russia to the United States. In his influential
book Genetics and the Origin of Species (1937, [4]), Dobzhansky conveyed the ideas
of the population geneticists to other biologists, thus influencing their apprecia-
tion of the genetic basis of evolution. Other major contributors to the synthesis
included the zoologists Ernst Mayr, in Systematics and the Origin of Species (1942,
[17]), and Bernhard Rensch, in Evolution Above the Species Level (1959, [24]); the
botanist G. Ledyard Stebbins, in Variation and Evolution in Plants (1950, [29]); and
the paleontologist George Gaylord Simpson, in Tempo and Mode in Evolution (1944,
[26]) and its successor, The Major Features of Evolution (1953, [27]). These authors
argued persuasively that mutation, gene flow or migration, natural selection, and
genetic drift are the major causes of evolution within species (which Dobzhansky
called microevolution)—and that continued over long periods of time, these same
causes account for the origin of new species and for macroevolution: the evolution
of the major alterations that distinguish higher taxa (genera, families, orders, and
classes). The principal claims of the evolutionary synthesis are the foundations of
modern evolutionary biology.
Although some of these principles have been extended, clarified, or modified
since the 1940s, most evolutionary biologists today accept them as substantially
valid. They are summarized in BOX 1A.Evolutionary biology since the synthesis
Since the evolutionary synthesis, a great deal of research has tested and elaborated
its basic principles. These principles have largely been supported. Progress in evo-
lutionary biology has modified some of these ideas and many extensions of these
ideas, and it has spurred additional theory to account for new phenomena as theyRonald A. FisherJ. B. S. HaldaneSewall Wright(^1) “Neo-Darwinism” properly refers to Weismann’s strict version of Darwin’s theory of evolution
by natural selection. Darwin had admitted a role for inheritance of acquired characteristics, but
Weismann rejected this. Today, “neo-Darwinism” is often used to mean the theory articulated in
the evolutionary synthesis.
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