2.2: Darwinian Theory

THE TRANSMUTATION HYPOTHESIS

The publication of The Origin of Species by Charles Darwin in 1859 became an intellectual flash point in European intellectual life (Darwin 1859). It was focused on a significantly narrow point: Where do new species, adapted to their surroundings, come from? The Bible says God made all species. However, the Bible also says that God made all languages at the foot of the Tower of Babel; and yet, half a century of historical linguistics had showed clearly that such was not the case (French and Spanish had only been different languages, having diverged from Vulgar Latin, for a matter of a few centuries), and nobody seemed to get too upset about it.

Tower of Babel

According to the Bible (Genesis 11), all people once spoke a single language and decided to cooperate to build a giant tower that would stretch into the heavens. For this arrogance, they are made to speak different languages and must give up building the tower. The story’s setting is generally thought to refer to the ancient ziggurats of Babylonia.

Moreover, the suggestion that species came from other species was not all that radical. The celebrated French naturalist Lamarck had said as much in 1809 and an anonymous 1844 English bestseller called Vestiges of the Natural History of Creation had sensationalized it—to the consternation of both theologians and naturalists. Indeed, by the 1850s European biologists were very confident that cells were fundamental units of life and that the only way you could get new cells was from old cells. While this begged the question of where the first cell came from, it nevertheless was not too much of a stretch to see species as fundamental units of life as well and to ask whether new ones arose miraculously, or just from older species. The idea that species had their beginnings in other, older, similar species was known as “the transmutation hypothesis.”

Definition: transmutation hypothesis

 The idea that species had their beginnings in other, older, similar species.

Charles Darwin had come to think about the origin of species upon returning from a long voyage around the world in the early 1830s on the H.M.S. Beagle. In South America, Darwin had observed that the unusual species he saw alive there were very similar to the unusual extinct animals in the same area. This suggested some sort of historical continuity between them—descent with modification, he called it. The problem was how to make sense historically, rather than miraculously, of the particular adaptations that differentiate species. The engine of adaptation, Darwin realized, was competition. This did not necessarily entail face-to-face competition but simply the fact that not all members of a species are equally likely to survive and breed. Which ones are more likely? The ones that randomly are a bit more in sync with their environment. Those creatures will disproportionately thrive and breed, and the next generation of the species will come to look just a bit more like them, on the average. The core of Darwin’s thought is thus a two-step process: the random generation of variation, and the nonrandom process by which the environment subtly favors organisms with certain features to thrive and breed.

Definition: descent with modification

Darwin’s term for what we now call “evolution,” in which animals and plants look different from their ancestors.

The biology that Darwin learned in college had invoked a famous simile: a species is like a watch, meticulously crafted by a wise watchmaker, implying a heavenly species-maker. Darwin substituted a more powerful simile, arguing that a species is actually like a breed or strain of animals, rather than like a watch. But we know that a breed or strain of animals arises naturally, historically, by the actions of breeders who select certain features to characterize populations.

Whether dogs, pigeons, or roses, the properties of living beings can, and have, changed in quite dramatic ways by virtue of human activity in rather short periods of time. If people could make beagles and greyhounds and bulldogs by selecting the progenitors of particular stocks, then maybe nature could work to select progenitors as well, although more subtly and over vastly longer periods of time (see Figure 2.2.1).

Figure \(\PageIndex{1}\): A Beagle

Figure \(\PageIndex{2}\): Bulldog

Figure \(\PageIndex{3}\): Greyhound

Darwin called this principle “natural selection” and planned to write a long book about it someday. But in 1858 he received a manuscript from a fellow naturalist, Alfred Russel Wallace, who had come up with quite similar ideas to his own while working in the Malay archipelago. Darwin’s friend, the geologist Charles Lyell, had papers by Darwin and Wallace read into the record, The Transactions of the Linnaean Society, July 1, 1858, so they could share credit for the discovery, and Darwin set about to publish the work he had done on natural selection. The result was called On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, published on November 24, 1859.

Definition: natural selection

The process whereby organisms that are better adapted to their environment tend to survive longer and transmit more of their genetic characteristics to succeeding generations than do those that are less well adapted.

Darwin’s central thesis was that the differences among breeds or strains or varieties of animals and plants were the same kinds of differences that exist between species, only smaller and formed over short periods of time. The origin of new species lay in the long-term biases of survival and reproduction in older species. The result was a convincing naturalistic explanation for adaptation. Moreover, it finally explained the nested pattern of similarities among species that Linnaeus had discovered a century earlier but couldn’t explain. Those nested patterns were the legacy of common ancestries; they were literally family resemblances.

Darwin was especially careful to omit any discussion of people from his book. He wanted the discussion to be about the general process; consequently, he wrote just a single line, near the end, about people: “Light will be thrown on the origin of man and his history” (Darwin 1859, 488). He was willing to acknowledge the possibility that life had “been originally breathed into a few forms or into one,” but he was satisfied with having described the mechanism by which adaptive change has taken place in the organic world since then—in parallel with Isaac Newton, who famously refused to speculate on where gravity came from, focusing instead only on how it works (Darwin 1859, 490).

People, however, were bound to be the central issue. A British scholar named Herbert Spencer had also come up with a similar idea, which he called “survival of the fittest” and he convinced Darwin that his phrase was synonymous with “natural selection.” This confusion of human history (that is, the construction of social and political hierarchies) for evolutionary biology would prove to be a consistent irritation for students of human diversity and ancestry. Indeed, this issue eventually led Darwin and Wallace to part ways. Wallace asked: if natural selection does not produce useless organs, then why does the “savage” have a brain as big as a civilized European’s, if the savage doesn’t use it? This seeming paradox led Wallace into spiritualism and the possibility that all species of organisms had evolved…but human intelligence had had a little divine help. Darwin wrote him, “I hope you have not murdered too completely your own and my child” (Darwin, 1869). In 1871, the early British anthropologist Edward Tylor formally separated the evolution and study of “culture” from the biological properties of people. Of course, the so-called “savage’s” brain was as good as the European’s, and he does use it fully, but it was filled with different information—“knowledge, belief, art, morals, law, custom, and any other capabilities and habits acquired by man as a member of society” or culture (Tylor 1871, 1). Furthermore, this cultural information was the product of historical process, not miracle. This understanding marks the beginning of modern anthropology.

Definition: culture

As defined by Edward Tylor:  That complex whole which includes knowledge, belief, art, morals, law, custom, and any other capabilities and habits acquired by man as a member of society (Tylor 1871, 1).

Within the academy, there was not too much reaction against the proposition that humans had descended with modification from an ape stock, and had then differentiated from that stock over the eons as a result of the differential preservation of favorable variations. The heart of Darwinism as applied to humans is simply ape ancestry and adaptive divergence.

Figure \(\PageIndex{4}\): The frontispiece to Ernst Haeckel’s (1868) popular German book on Darwinism. The English translation lacked this illustration.

But the early Darwinians were faced with a dilemma—in 1860, there was no fossil evidence linking humans to apes. The German biologist Ernst Haeckel solved this problem by fatefully arguing that we don’t need a fossil record to link us to the apes, because Europeans are linked to the apes through the nonwhite peoples of the world. He envisioned 12 different species of living peoples, each at different distances from the apes, thus sacrificing the full humanity of most people on the altar of Darwinism (see Figure 2.2.4). Scientists of the 1860s thought the full humanity of Africans was less important than evolution. Today that is morally repugnant. While Darwin and his English colleagues did not agree with these details, they nevertheless saw Haeckel as an ally in the broader struggle to get evolution accepted. With hindsight, we can judge this to be a morally questionable decision. Today we would hopefully, universally, consider the full humanity of Africans to be more important than whether humans are descended from apes, and thoroughly repudiate anyone who denied it.

POST-DARWINIAN THEORIES AND DISPUTES

The immediate theoretical weakness of Darwinism lay in its reliance upon a pool of undirected variation for nature to select from. The dominant theory of heredity at the time was known as blending inheritance, in which a child is a blend of the parents—like paint, if mom is red and dad is blue, then the child is purple (see Figure 2.2.5). The problem is that any descendants of purple child will never be as different as blue mom and red dad. You can’t recover the original blue and red from purple paint—which simply means that for people, variation is lost every generation. How can natural selection work if you lose variation every generation?

Figure \(\PageIndex{5}\): Blending inheritance in color.

Definition: blending inheritance

An obsolete theory where the offspring of two parents will have characteristics that are intermediate between the two parents.

Darwin fell back on a principle developed by Lamarck known as the inheritance of acquired characteristics or “use and disuse of organs.” Here, whatever attributes you develop over the course of your life—muscles, a tan, compassion, bad breath—can be stably passed on to your children, somehow. That way, variation can be reintroduced every generation, by virtue of this new pool of acquired characters. Unfortunately, an influential school of German biologists in the 1880s, led by August Weismann, had identified just two types of cells in bodies: reproductive or germ cells, and somatic or body cells. It was the germ cells that formed the next generation; the somatic cells, which form the body, comprise merely an evolutionary dead-end to aid in the transmission of the germ-line. Life could thus be seen as a continuous series of germ-cells, with adult bodies as transient receptacles grown up around them every generation. On this basis, the English writer Samuel Butler quipped that a hen is just an egg’s way of making another egg. But how, then, could information about your elbow or your cerebral cortex during the course of your life get into your germ cells? There didn’t seem to be a way, so that generation called themselves “neoDarwinians” to express their belief in natural selection minus the inheritance of acquired characteristics.

Definition: inheritance of acquired characteristics

The idea that you pass on the traits that you have acquired during your lifetime, not just your genes; also known as Lamarckian inheritance.

The entire problem was rendered moot with the discovery in 1900 of Gregor Mendel’s work on heredity in peas from 35 years earlier. Mendel showed that heredity didn’t actually work like the blending of paints at all. When you isolated particular traits, you saw that offspring were not midway between their parents; rather, they were like one or the other parent. The offspring of a plant with green peas and one with yellow peas was green, not chartreuse. The offspring of a plant with wrinkled peas and one with round peas was round, not wrinkly-round. This suggested, rather, that heredity worked like interacting particles that came into new combinations but fundamentally retained their structural integrity every generation. Unlike paints, you could indeed recover the original variants under this model; variation wasn’t lost every generation.

Mendelian genetics soon created new problems for Darwinism, however. The new geneticists were focused on discrete binary states of existence, like Mendel’s peas: green/yellow, wrinkled/round, tall/short, in experimental populations. But the old Darwinian naturalists were working with quantitative variations in real populations—many of them intermediate, not extreme, in form. So, the Mendelians had a robust theory of heredity that had difficulty explaining natural patterns of variation, and the Darwinians had a robust theory of biological change that had difficulty accommodating discontinuous variation. One solution might be to reconceptualize all variation as fundamentally binary; the American geneticist Charles Davenport, for example, argued with considerable success that there were two kinds of people—smart and stupid—and that the stupid people simply had the allele for “feeblemindedness.” This actually had a major and regrettable impact on American science and social policy in the 1920s.

Definition: allele

A genetic variant. A non-identical DNA sequence found in the same gene location on a homologous chromosome, or gene copy, that codes for the same trait but produces a different phenotype.

A better solution came with the invention of population genetics, in works published around 1930 by the British geneticists Ronald Fisher and J. B. S. Haldane and the American geneticist Sewall Wright. In this model, a gene has small but cumulative effects. If we reduce a body to its genetic composition or genotype, and we reduce a species to its cumulative genetic composition, or gene pool, we can mathematically model the ways in which the gene pool can be transformed. There are rather few ways to accomplish it, and each has characteristic and predictable effects.

Definition: gene

Segment of DNA that contains protein-coding information and various regulatory (e.g., promoter) and noncoding (e.g., introns) regions.

Definition: genotype

The entire set of genes of an individual organism; the combination of two alleles that code for or are associated with the same gene.

Definition: gene pool

 The entire collection of genetic material in a breeding community that can be passed on from one generation to the next.

Figure \(\PageIndex{6}\): George Gaylord Simpson (1983). Photo courtesy of Jonathan Marks.

This became the first part of the Synthetic Theory of Evolution, the extension of Mendelian genetics to population genetics and the formal mathematical study of how gene pools may be transformed through time. The second part involved the study of how species diversify in addition to simply changing, and it entailed integrating speciation and geography in the story of how animal species have come to be. The primary scholars involved were the Russian-American fruit fly geneticist Theodosius Dobzhansky, the German-American ornithologist Ernst Mayr, and the American paleontologist George Gaylord Simpson (see Figure 2.2.6).

Definition: Synthetic Theory of Evolution

Explanation of the evolution of life in terms of genetic changes occurring in the population that leads to the formation of new species.

By the 1960s, biologists had a robust theory to explain the history of life. Genetic or genotypic changes (known to be encoded in molecules of DNA) cause changes in the physical appearance or phenotype. The environment sorts out these changes, and their proportion within a species rises or falls with the nature and stringency of the environment. Selection could now be reduced to the favoring of certain genotypes over alternatives, which can make populations genetically and adaptively different from one another. Genetic drift, or stochastic (random) changes to the gene pool, makes populations genetically different from one another nonadaptively—that is to say, in ways that don’t track the environment. The genetic contact of populations, or gene flow, makes populations more similar to one another. Disrupting gene flow acts to divide gene pools, which is in turn stabilized by the development of reproductive barriers between the populations. These processes can be directly studied within living species and can be extrapolated and can adequately explain the differences we find among species.

Definition: phenotype

Observable physical or biochemical traits of an organism that are produced through the interaction of genotype and environment.

Definition: genetic drift

Random changes in allele frequencies within a population from one generation to the next with nonadaptive effects.

Definition: gene flow

Geographical movement of genes due to the transfer of alleles from one population to another through the contact of different populations.

Figure \(\PageIndex{7}\): Gene flow

REFERENCES

Darwin, Charles. 1859. On the origin of species by means of natural selection, or, the preservation of favoured races in the struggle for life. London: J. Murray.

Darwin, Charles. 1869. Correspondence Project. “Letter no. 6684,” accessed on 11 July 2019, http://www.darwinproject.ac.uk/DCP-LETT-6684.

Tylor, Edward B. 1871. Primitive Culture. London: John Murray.

FIGURE ATTRIBUTIONS

Figure 2.2.1 Pet (pet-dog-animals-cute-animal-dogs-3635986/) by Somo_Photography has been designated to the public domain (CC0).

Figure 2.2.2 English bulldog (2705136) by mk817 has been designated to the public domain (CC0) .

Figure 2.2.3 ItalianGreyhound by Just chaos is used under a CC BY 2.0 license.

Figure 2.2.4 Ernst Haeckel – Natürliche Schöpfungsgeschichte, 1868 by Foto H.-P.Haack Das Foto darf gebührenfrei verwendet werden, sofern der Urheber mit “Foto H.-P.Haack” vermerkt wird. is used under a CC BY-SA 3.0 DE license.

Figure 2.2.5 Blending inheritance in color original to Explorations: An Open Invitation to Biological Anthropology by Mary Nelson is under a CC BY-NC 4.0 License.

Figure 2.2.6 George Gaylord Simpson (1983) photo courtesy of Jonathan Mark is under a CC BY-NC 4.0 License.

Figure 2.2.7 Gene Flow (2012) by Jessica Krueger, CC BY-SA 3.0, via Wikimedia Commons.