In the previous post in this series, we discussed how scientific theories—broad, well-tested explanatory frameworks—get their start as hypotheses. As a hypothesis is used to make predictions, and those predictions are supported by experimentation, over time, scientists come to have more and more confidence in that hypothesis as a reliable guide for making predictions about the natural world. This means any current theory in science has gone through this transition, and its history can be traced.
Like any theory, Darwin’s idea that evolution proceeds through natural selection was once merely a hypothesis. In this post, we’ll look at some of the early observations Darwin made on biogeography: the study of where species are distributed across the globe. These lines of evidence would later prod him to consider the possibility that species arise through a natural process of gradual change over time, rather than being independently created in each location where they are found.
The curious case of the missing mammals
As a widely-travelled naturalist on the HMS Beagle, Darwin studied a large number of different environments and documented the species he found in each. The Beagle, engaged as it was in an effort to map the coastline of South America, naturally paid call to numerous island groups along the way, including islands at a great distance from a continent (i.e. oceanic islands). One observation that Darwin made about oceanic islands is that none that he studied had terrestrial mammals on them. Later work, after his voyage, would confirm that this was a general rule. Oceanic islands lack terrestrial mammal species, except for small species that were introduced by humans. In contrast, flying mammals (i.e. bats) were found on oceanic islands, and often these species were endemic (i.e. found nowhere else in the world but the island in question).
Darwin found these observations difficult to square with his (then) working assumption that species were independently created in (and specifically created for) the locations in which they are found across the globe. He discusses these observations, and the questions they raised in his mind, in two chapters entitled “Geographical Distribution” in his Origin of Species. After discussing the similar case that amphibians (such as frogs, newts, and so on) are also not to be found on oceanic islands, he turns his attention to the missing mammals:
Mammals offer another and similar case. I have carefully searched the oldest voyages, but have not finished my search; as yet I have not found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above 300 miles from a continent or great continental island…. It cannot be said, on the ordinary view of creation, that there has not been time for the creation of mammals; many volcanic islands are sufficiently ancient, as shown by the stupendous degradation which they have suffered and by their tertiary strata: there has also been time for the production of endemic species belonging to other classes; and on continents it is thought that mammals appear and disappear at a quicker rate than other and lower animals. Though terrestrial mammals do not occur on oceanic islands, aërial mammals do occur on almost every island. New Zealand possesses two bats found nowhere else in the world: Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across. Bats have been seen wandering by day far over the Atlantic Ocean; and two North American species either regularly or occasionally visit Bermuda, at the distance of 600 miles from the mainland. I hear from Mr. Tomes, who has specially studied this family, that many of the same species have enormous ranges, and are found on continents and on far distant islands. Hence we have only to suppose that such wandering species have been modified through natural selection in their new homes in relation to their new position, and we can understand the presence of endemic bats on islands, with the absence of all terrestrial mammals.
(As an aside, it’s important to note that Darwin, when he discusses the “supposed creative force” is not here arguing against the existence of God as creator in general, but rather against the “ordinary view of creation” common at the time: that God had episodically created species at specific geographical locations (what were called “centers of creation”) and that biogeographical patterns could be explained with limited dispersal from those centers. Darwin himself held to this common view at the start of his voyage on the Beagle, and that is the model he is attempting to refute in Origin, since it was a prevailing view among scientists at the time. Darwin and many of his scientific contemporaries also had no difficulty viewing natural processes as part of God’s regular action in the world, as is evident in Darwin’s correspondence with American botanist Asa Gray, among others.)
So, for Darwin, his biogeographical observations sat at ease with his (later) ideas of colonization and subsequent species change through natural selection, but made no sense to him if one held to an independent creation model. Many oceanic islands were very old, yet no mammals had been created there. Many oceanic islands had habitat suitable for mammals (or, indeed, for amphibians, as he notes) yet no such species had been created for that suitable habitat.
Island endemics and their continental “allied species”
Darwin noticed more than the absence of certain species groups on oceanic islands. He also noticed an interesting feature of the species that were present: an endemic species on an oceanic island would often have strong similarities with a species on the mainland closest to the island in question. Additionally, the pairing of oceanic endemic species with continental species often seemed to override expectations that species found in similar environments would be more similar to each other. These observations prompted him to reflect further on the possible means by which these “closely allied species” arose. As Darwin would write in his Origin this repeated pattern made a significant impression on him, and further caused him to doubt that endemic species had been individually created for each oceanic island. His visit to the Galapagos would prove instrumental on this point:
The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. Numerous instances could be given of this fact. I will give only one, that of the Galapagos Archipelago, situated under the equator, between 500 and 600 miles from the shores of South America. Here almost every product of the land and water bears the unmistakeable stamp of the American continent. There are twenty-six land birds, and twenty-five of these are ranked by Mr. Gould as distinct species, supposed to have been created here; yet the close affinity of most of these birds to American species in every character, in their habits, gestures, and tones of voice, was manifest. So it is with the other animals, and with nearly all the plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, yet feels that he is standing on American land. Why should this be so? why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which resembles closely the conditions of the South American coast: in fact there is a considerable dissimilarity in all these respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagos: but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, from America; and the Cape de Verde Islands from Africa; and that such colonists would be liable to modification;—the principle of inheritance still betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal rule that the endemic productions of islands are related to those of the nearest continent, or of other near islands.
Rethinking independent creation
For Darwin, both of these observations (that oceanic islands lacked terrestrial mammals, and that endemic species on islands were most similar to a species on the closest mainland) had the same explanation: his hypothesis that endemic, oceanic species were the modified descendants of a colonizing species from the nearest continent. This also explained the lack of amphibians and terrestrial mammals (but allowed for bats) – simply based on the ability of these classes of life to disperse across large expanses of ocean. Those that could disperse and colonize oceanic islands would experience modification in the new environment, and species unable to colonize these islands would never appear. To Darwin’s thinking, this explanation was wholly more satisfactory than the assumption that God had independently created every endemic species in its place, and arbitrarily chosen that oceanic islands did not need terrestrial mammals and amphibians.
Despite Darwin’s musing on the biogeographical patterns he observed, and the strong suggestion these patterns made of species change over time, a mechanism for that change would take some time for him to imagine. In our next post, we’ll look at that mechanism: Darwin’s idea of natural selection, and the evidence he assembled in its support prior to publishing the Origin.
Earlier, we described how Darwin’s studies on biogeography were important in prompting him to consider that species were not fixed, but rather could change over time to become new species. Darwin was at a loss, however, to explain the mechanism by which that gradual change might take place. Since his observations (a) documented variation, or differences, between closely-related species, and (b) suggested that this variation arose through the gradual modification of two separated populations over time, Darwin correctly surmised that understanding variation itself might shed light on the formation of new species. Accordingly, Darwin would undertake a major investigation of heritable variation within domesticated animals. His studies led him to note the importance of selection in forming new domestic breeds, and later he would realize that nature could act as a selective force.
These two concepts, heritable variation and natural selection, remain core ideas within modern evolutionary biology. Given how important these concepts are even today, we’ll examine them briefly before describing how Darwin came to hypothesize about their role in forming new species.
Evolution made simple: heritable variation and differential reproduction
Within any population of organisms, whether domesticated or in the wild, heritable differences exist. We now understand that these heritable differences arise from differences in genetic information (i.e. variation in DNA sequences), but this insight was unknown in Darwin’s time. What Darwin did appreciate, without knowing its molecular basis, was that offspring on average tend to resemble their parents more so than other members of the population at large. From this he inferred, correctly, that much variation was heritable: it was passed down from parent to offspring.
As we will see below, Darwin would also note that if variation is subjected to selection, that average character traits of a population could shift over time. Selection is simply the observation that not all variants within a population reproduce at the same rate. In the case of artificial selection, a human agent decides which variants reproduce at a greater rate than others by enforcing selective breeding. If selection is consistent from generation to generation, over time certain variants will increase in frequency in a population, and other variants will decrease in frequency.
In an nutshell, this is the core of evolutionary theory: that changes in heritable variation over time can shift average characteristics of a population, and that differential reproductive success (selection) is a major mechanism for driving changes in heritable variation from one generation to another. Though our understanding of evolution has greatly increased since Darwin’s time (since evolution is a theory in the scientific sense), these basic principles remain fundamental components of evolutionary biology. Despite their simplicity, it took a significant amount of work over several years for Darwin to piece these ideas together into a coherent framework.
Darwin’s studies on variation, 1837 – 1838
As Darwin recounts in his autobiography, upon his return to England from his voyage on the Beagle, he undertook a systematic effort to accumulate information on variation in “races” of domesticated plants and animals as well as variation in natural populations. (As an aside, in the 1800s, the term “race” was the accepted scientific term for what we could today call breeds or subspecies of plants and animals. “Race” would later come to have its present-day, human-specific connotation, but that was not the case in Darwin’s day, as even a brief perusal of literature of the period will demonstrate. As such, attempts to portray Darwin’s works as racist are unfounded, but perennially tempting for those not knowledgeable of Victorian English, given that the full title of his treatise is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.) As Darwin conversed at length with plant and animal breeders (for example, he would spend a great deal of time learning from pigeon fanciers and document their ability to create numerous exotic breeds of pigeons), he was impressed with the power of artificial selection to effect change within a breed over time. Impressed as he was with this power, however, he was yet to conceive of how nature might act as a selective force:
After my return to England it appeared to me that by following the example of Lyell in Geology, and by collecting all facts which bore in any way on the variation of animals and plants under domestication and nature, some light might perhaps be thrown on the whole subject. My first note-book was opened in July 1837. I worked on true Baconian principles, and without any theory collected facts on a wholesale scale, more especially with respect to domesticated productions, by printed enquiries, by conversation with skilful breeders and gardeners, and by extensive reading. When I see the list of books of all kinds which I read and abstracted, including whole series of Journals and Transactions, I am surprised at my industry. I soon perceived that selection was the keystone of man’s success in making useful races of animals and plants. But how selection could be applied to organisms living in a state of nature remained for some time a mystery to me. (Autobiography, pp. 119 -120)
Later he would hit upon his key idea: that nature might apply selection in much the same way that humans could. Rather than a breeder choosing which individuals to mate, the ability of different variants to reproduce in a given natural setting would allow some to reproduce at a greater rate than others. Since their traits would be heritable, this would drive changes in traits over time in the population experiencing “natural selection”, a term Darwin coined as an analogy to human, or artificial, selection. Darwin’s realization was that, in the wild, organisms generate more offspring than are capable of surviving, and this was the last piece of the puzzle (famously supplied by Thomas Malthus) to fall into place for him. As such, there would be competition among the members of a species for limited resources. Competition would also take place between members of different species. These sources of competition would create a “struggle for existence” that would act to select certain variants and promote their reproduction over others:
In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. (Autobiography p. 120)
Later, in Origin, he would spell out his ideas in great detail. In the opening chapter, Darwin discusses numerous examples of heritable variation in domestic animals. He then turns to heritable variation in natural populations in the second chapter, again citing numerous examples, and comparing his findings to variation in domestic stocks. In the third chapter he describes the “struggle for existence” in nature, and then ties the three ideas together: if human breeders can use artificial selection to “accumulate” variation over time, then so, too, can natural selection:
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, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally 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 each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art. (Origin, p. 61)
Artificial selection, then and now
Having found his “theory by which to work” , Darwin would spend decades accumulating evidence for his ideas prior to publishing his Origin in 1859. Documenting evidence for natural and artificial selection would be a major focus of his efforts. In the next post in this series, we’ll discuss some observations Darwin made on artificial selection and the domestication of dogs, and examine some recent genomics studies that reveal the molecular details of how artificial selection shaped the dog genome over time.
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Artificial Selection and the Origins of the Domestic Dog
We are beginning to see the genetic underpinnings of artificial selection at a genome-wide level, and the results are absolutely in keeping with Darwin’s ideas...