In Western European thought, the individual most closely associated with evolution is Charles Darwin (1809–1882). However, as one can see from the individuals and ideas presented in the prior section, he was not the first person to explore evolution and how it might work. In fact, Darwin built upon and synthesized many of the ideas—from geology to biology, ecology, and economy—discussed above. He was simply in the right place at the right time. If he had not worked out his ideas when he did, someone else would have. As a matter of fact, as noted below, someone else did, forcing Darwin to publicly reveal his theory.
Darwin’s journey to the discovery of natural selection began during a childhood spent being curious, experimenting, and collecting natural specimens. When Darwin was 12 years old, his nickname was “Gas” because of the foul-smelling chemistry experiments he and his older brother, Erasmus, performed late into the evenings in their makeshift laboratory in the garden of their parent’s home (Costa 2017). Darwin was also a lifelong collector of biological and geological specimens, most famously beetles, at times going to great lengths in pursuit of a new specimen, as one of his personal letters relates,
I will give a proof of my zeal: one day, on tearing off some old bark, I saw two rare beetles, and seized one in each hand; then I saw a third and new kind, which I could not bear to lose, so that I popped the one which I held in my right hand into my mouth. Alas! it ejected some intensely acrid fluid, which burnt my tongue so that I was forced to spit the beetle out, which was lost, as was the third one. [Darwin 2001 (1897), 50].
Darwin continued his observations and experiments during his formal education, culminating in his graduation from Cambridge in 1831, at which point he was invited to become a gentleman naturalist for a British Royal Navy surveying mission of the globe aboard the H.M.S. Beagle. It is worth noting that Darwin was only 22 years old and the captain’s third choice for the position (Costa 2017), but he proved extremely curious and methodical. The mission departed in December of 1831 and returned five years later (Figure 2.11). During this time, Darwin produced copious notebooks, observations, drawings, and reflections on the natural phenomena he encountered and the experiments he performed.
Discussing all of Darwin’s work aboard the Beagle is beyond the scope of this chapter, but his primary interests were in cataloging new varieties of plant and animal life and examining the geology of the places the ship made landfall. Part of Darwin’s success with regard to both ventures was due to his extreme seasickness, which began before the ship even left Plymouth Harbor. It never let up, encouraging Darwin to go ashore at every available opportunity. “In fact, of the nearly five years of the voyage, Darwin was actually on board the ship for just a year and a half altogether” (Costa 2017, 18).
During the voyage, the young Darwin tried to make sense of what he saw through the lens of the scientific paradigms he held when he left England, but he continually made observations that challenged these paradigms. For example, while the Beagle crewmen were charting the coast of Argentina, Darwin conducted fieldwork on land. There he observed species that were new to him, like armadillos. He also collected fossils, including those of extinct armadillos. Meaning, he had found both extant and extinct members of the same species in the same geographic location, which challenged the theory of catastrophism put forth by Cuvier, who argued that each variant of an animal, living or extinct, was its own distinct species (Moore 1993, 144). Darwin also observed geographic variation in the same species all along the east coast of South America, from Brazil to the southern tip of Argentina. He noted that some species were found in multiple localities and differed from place to place. Those living closer to each other exhibited only slight variations, while those living further apart might be cataloged as entirely different species if one did not know better.
He made similar observations in the Galapagos Islands located off the northwest coast of Ecuador, with regard to giant tortoises and finches (Figure 2.12). A local resident of the islands explained to Darwin that each island had its own variety of tortoise and that locals could discern which island a tortoise came from simply by looking at it. Darwin noted other such examples in both plants and animals, meaning geographic variation was occurring on separate, neighboring islands.
Prevailing views of time argued that variations in living species, and even the fossil armadillos and the living armadillos, were the result of separate creation events. According to this view, each variation, no matter how slight, was a different species. Challenging these ideas would mean challenging not only catastrophism, but the Fixity of Species and other well-accepted ideas of the time. Darwin was aware that he was a young, unestablished naturalist. He was also aware of the ruin that befell Lamarck when his theories were rejected. Lastly, Lyell, who was a good friend of Darwin’s, rejected evolution altogether. It is no wonder that Darwin published a great deal about the geological and fossil data he collected when he returned from the voyage, but not his early hypotheses about evolution.
Upon Darwin’s return to England, it took another twenty years of data collection and experimentation before he was ready to share his conclusions about evolution. Much of this work was conducted at Down House, his home of forty years, where he performed all sorts of experiments that laid the groundwork for his ideas about evolution. Darwin’s home was his laboratory, and he engaged the help of his children, neighbors, friends, and servants in collecting, dissecting, and experimenting. At one point in the 1850s, sheets of moistened paper covered with frogs eggs lined the hallways of the house, while flocks of sixteen different pigeon breeds cooed outside, glass jars filled with salt water and floating seeds filled the cellar, and the smell of dissected pigeon skeletons pervaded the air inside the house. There were also ongoing experiments in the yard, including piles of dissected flowers, beekeeping, and fenced-off plots of land where seedlings were under study. Darwin was a keen experimental scientist, observer, and a prolific writer and presenter of scientific papers. He regarded his work as “one long argument” that never really ended. In fact, Darwin published ten books after On the Origin of Species, addressing such far-ranging topics as animal behavior, orchids, and domestication, among others (Costa 2017).
Darwin may not have published Origins in 1859 had it not been for receiving a paper in June of 1858 from Alfred Russel Wallace, an English naturalist working in Malaysia, espousing the same ideas. Wallace had sent the paper to Darwin asking if it was worthy of publication and requesting he forward it to Lyell and the English botanist, Joseph Hooker. Darwin wrote to Lyell and Hooker about Wallace’s paper, entitled On the Tendency of Varieties to Depart Indefinitely from the Original Type. In recognition that both Wallace and Darwin had arrived at the same discovery, a “joint” paper composed of four parts (none of them actually coauthored) was read to the Linnaean Society by Lyell, then secretary of the Society, on July 1, 1858, and published on August 20. Darwin published On the Origin of Species 15 months later. (The original composite paper read before the Linnaean society is available to read for free from the Alfred Russell Wallace Website, on the 1858 Darwin-Wallace paper page.)
The Mechanism of Natural Selection
On the subject of natural selection and how it works, let’s hear from Darwin himself from the original publication of On the Origin of Species (1859):
A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometric increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence…It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms…There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. …Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species…will tend to the preservation of that individual, and will be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which slight variation, if useful, is preserved, by the term Natural Selection. [Darwin 1859, 61–62]
Let us take a moment here to explore the mechanism of natural selection in more detail. Before we begin, it is important to recognize that Darwin defined evolution as descent with modification, by which he meant that species share a common ancestor yet change over time, giving rise to new species. Descent with modification refers to the fact that offspring from two parents look different from each of their parents, and from each other, meaning they descend with slight differences (“modifications”). If you have ever observed a litter of puppies or a field of flowers and stopped to examine each individual closely, you have seen that each differs from the next, and none look exactly like their parents. These variations are random, not specific, and may or may not be present in the following generations.
Darwin struggled to explain why some slight differences were preserved over time, while others were not. He turned to what he knew of animal breeding (artificial selection) for an explanation (Richards 1998). Darwin bred different breeds of pigeons at Down House, carefully documenting phenotypic differences across generations, including slight anatomical variations he observed through dissection. He also grew and crossbred species of flowers and dissected those too. Darwin was also very fond of hunting and of hunting dogs. In an early draft of his theory on speciation, he used greyhounds as an example of adaptation and selection, “noting how its every bone and muscle, instinct and habit, were fitted to run down hare (rabbits) (University of Cambridge n.d.).” In each case of plant and animal breeding Darwin observed, he noted that humans were selecting variants in each generation that had characteristics humans desired (i.e., sweetness of fruits, colors of flowers, fur type and color of animals). Breeders then continually bred plants and animals with the desired variants, over and over again. These small changes added up over time to create new species of plants and breeds of animals. Darwin also noted that artificial selection does not necessarily render plants or animals better adapted to their original environments. The characteristics humans desire often result in plants less likely to survive in the wild and animals more likely to suffer from certain behavioral or health problems. One has only to examine high rates of hip dysplasia in several modern breeds of dogs to observe what Darwin was referring to.
From his studies of artificial selection, Darwin drew the conclusion that nature also acts upon variations among members of the same species. Instead of human intervention, the forces of nature, such as heat, cold, predation, disease, and now climate change, determine which offspring, with which variants, survive and reproduce. These individuals then pass down these favorable variants to their own offspring. In this way, nature selects for traits that are beneficial within a particular environment and selects against traits that are disadvantageous within a particular environment. Over many generations, populations of a species become more and more adapted (or, in evolutionary terms, “fit”) for their specific environments. Darwin named this process natural selection.
This theory explained the variations in tortoises Darwin had observed years earlier in the Galapagos Islands (see Figure 2.12). Tortoises who lived on larger islands with lush vegetation to feed on were larger than those on smaller islands. They also had shorter necks and dome-shaped shells as their food was close to the ground. Tortoises on smaller, drier islands fed on cacti, which grew much taller. These tortoises had longer necks, longer front legs, and saddle-shaped shells, which allowed them to successfully stretch to reach the edible cactus pads that grew on the tops of the plants. How did these observable differences in the two tortoise populations emerge? Darwin would argue that, over time, small, random variations in the tortoises were differentially selected for by the distinct natural environments on different islands.
In addition to the biogeographical evidence Darwin offered from his research aboard the Beagle, as well as the evidence he documented from the artificial selection of plants and animals, he also relied, where possible, on fossil evidence. One example, mentioned above, were the fossil findings of extinct armadillos in Argentina in the same locations as living armadillos. Unfortunately, as Darwin himself noted, the geological record was incomplete, most often missing the transitional fossil forms that bridge extinct and living species. That issue has since been resolved with scientific research in geochronology and paleontology, among other fields. It is now well-established that life is far more ancient than was believed in Darwin’s time and that these ancient forms of life were the ancestors to all life on this planet (Kutschera and Niklas 2004).