2.9The Reaction to Darwin and Early History of the Modern Synthesis

While various religious leaders challenged almost all the major conclusions that Darwin presented in On the Origin of Species, the scientific community exhibited a more mixed reaction (Mayr 1982, Glick 1974, Numbers and Stenhouse 1999, Jin 2020). For example, as historian of science Gregory Todes shows in his book Darwin without Malthus: The Struggle for Existence in Russian Evolutionary Thought, when Darwin’s work reached Russia, the idea of common descent was accepted in a straightforward manner, as was the idea that traits increase or decrease based on their effects on survival and reproduction. But the tie between Darwin’s theory and Malthus’s overpopulation controlled by famine, war, disease, and so on, was problematic for the Russian scientific community, as well as for the Russian general public.

Russia was on the order of forty times the size of Great Britain in the second half of the nineteenth century; perhaps overpopulation was a problem in Britain, but it was underpopulation that caused more serious problems in Russia. Leo Tolstoy, for example, was merciless when it came to Malthus, whom he considered “a very poor English writer, whose works are all forgotten, and recognized as the most insignificant of the insignificant.” As for Malthus’ ideas, they were “fictitious” and the height of “frivolity” and “stupidity” (Tolstoi, 1887).

Within the Russian scientific community, the solution to the problem of Darwin’s tie to Malthus was to admit that the connection was real, but far overblown—that Darwin’s ideas on natural selection worked just as well without a Malthusian component. For Russian evolutionary biologists like Peter Kropotkin and many others, the way to remove Malthusian ideas from evolutionary biology was to reorient thinking about what sort of “struggle” exists in nature. For this group, natural selection was a powerful force, but Darwin’s ideas had been hijacked and inexorably linked with a kind of vicious individual versus individual struggle. Instead, early Russian evolutionary biologists thought that the sort of metaphorical struggle Darwin described in On the Origin of Species, when he wrote that “a plant on the edge of the desert is said to struggle for life against drought,” was far more important. They proposed that this kind of struggle, particularly when it occurred in animals, would lead to natural selection favoring traits associated with cooperation, not fierce one-on-one competition, as animals would unite to fight their harsh environment and, in the process, aid one another (Dugatkin, 2014; we return to ideas on the evolution of cooperation in Chapter 17).

The reaction to Darwin’s ideas was different in Great Britain. Early on, British scientists almost universally embraced Darwin’s ideas on common ancestry, but many were unconvinced that the primary force generating evolutionary change was natural selection. That is, they accepted that evolutionary change, rather than special acts of creation, explained the world that we see around us, but they rejected the idea that the primary force generating evolutionary change was natural selection. A few British naturalists, including Alfred Russel Wallace, Henry Walter Bates (1825–1892), and Joseph Dalton Hooker (1817–1911), thought that natural selection was important in driving evolutionary change, but many early evolutionary biologists disagreed (Glick 1974).

In the 1880s, experimental work—primarily that of German geneticist and evolutionary biologist August Weismann (1834–1914), who demonstrated that traits acquired during the lifetime of an organism could not be inherited—dealt a death blow to previous theories of Lamarckian inheritance. Scientists were left with only two possible ways that evolution might operate. The processes were either natural selection acting in a slow and methodological way on small genetic differences or saltationism; that is, “evolution via large, sudden changes from the existing norm” (Mayr 1982).

In his now-famous experiments of the 1850s and 1860s, Augustinian monk, plant breeder, and biologist Gregor Mendel (1822–1884) found that inherited factors that form the basis of traits come from both parents. His work on pea plants demonstrated that each parent plant has two copies of what we now call a gene, and that the two gene copies separate with equal probability into gametes (eggs, sperm, pollen, and so on). In Chapter 6, we will discuss Mendel’s experiments in more detail.

Mendel’s results remained virtually unnoticed until 1900, when three scientists (Hugo de Vries, Carl Correns, and Eric von Tschermak) independently rediscovered his work and made it available to the scientific world. Biologists began to explore how natural selection might operate when inheritance operated as Mendel suggested.

At that time, evolutionary biologists fell into one of two camps. On one side were the Mendelians, who viewed evolution as a saltational process. These scientists primarily worked in the lab, were trained more as physical than as biological scientists, and thought that the continuous variation in so many traits seen in nature was not primarily genetic in origin. This was because the Mendelian camp’s original interpretation of Mendel’s work allowed for discrete variation—for example, tall versus short—but not continuous variation in traits. In the other camp were the biometricians, including the English geneticist and statistician Karl Pearson (1857–1936). The biometricians were impressed by the amount of continuous variation—that is, extremely fine gradations of difference—that they saw all around them and thought natural selection was a slow, gradual process.

The differences between the Mendelians and the biometricians began to dissolve with experimental work in the 1930s and 1940s in what came to be called the modern synthesis, or the evolutionary synthesis. This synthesis included experimental work in genetics demonstrating that:

• Genes are passed on from parents to offspring in an intact form, even if they are not expressed in the offspring’s phenotype. That is, genes are particulate: they don’t “blend” with other genes.
• Mutation is an important source of genetic variation.
• Genetic variants that generate large and small phenotypic differences are not qualitatively different from one another—the effects of large differences may be more pronounced, but genetic variation is generated and inherited in similar ways in both cases.
• Not all genetic mutations are harmful, so positive changes can accrue over time—either slowly or in some cases more rapidly.
• Sexual reproduction is an important contributor to the production of massive amounts of genetic variation.
• Some traits are the result of the interaction of numerous genes, while some genes can affect more than one trait. This helps explain the evolution of complex traits without necessarily assuming some saltational (that is, large and sudden) change.
• Many (but not all) changes in the genotype affect the phenotype.
• Variation in the phenotype is the raw material for natural selection.

We discuss each of these points in more depth in later chapters, but for now, what we wish to emphasize is that this work demonstrated that there was no conflict between what was being found in the new, burgeoning field of genetics and Darwin’s idea that evolutionary change was primarily a slow process, driven by natural selection. Another crucial ingredient of the modern synthesis was the work of mathematical population geneticists such as R. A. Fisher (1890–1962), Sewall Wright (1889–1988), and J. B. S. Haldane (1892–1964), who developed mathematically sophisticated models to demonstrate how evolutionary processes lead to changes in gene frequencies and how changes in gene frequencies map onto changes in the phenotypes of organisms (Chapters 7–9).

The modern synthesis represented the collective efforts of systematists, geneticists, paleontologists, population biologists, population geneticists, and naturalists. Although often associated with the publication of British biologist Julian Huxley’s (1887–1975) book, Evolution: The Modern Synthesis, this synthesis was not so much an event per se, but the result of a gradual accumulation of information that melded together to shape biology at the time (Huxley 1942). In addition to the work listed earlier, this synthesis involved a combination of theoretical models and experimental manipulations, like that of German-American evolutionary biologist and ornithologist Ernst Mayr’s (1904–2005) pathbreaking work on the process of speciation and its relationship to systematics (classifying organisms) (Mayr 1942). In essence, the evolutionary approach provided a framework for understanding both the fit of organisms to their environment and the diversity and history of life. We will discuss the major findings of the evolutionary synthesis in many subsequent chapters.

We have seen that midway into the nineteenth century, thinkers began to develop mechanistic, rather than supernatural, explanations for the world around them, and science as a whole began to center on experimentation, data gathering, and hypothesis testing. Theories in geology had created a sense of deep time and gradual, rather than catastrophic, changes. Robert Chambers and others had suggested that new species might arise from existing species, Jean-Baptiste Lamarck had hypothesized that there were generational adaptations to environmental needs, and Patrick Matthew had presented preliminary ideas on natural selection. It was in this context that Charles Darwin developed his ideas. Having laid out both the basic elements of Darwin’s theory and the problems facing that theory, we are now in a good position to examine the components of evolutionary change in subsequent chapters.