This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post, we look at some of the early lines of evidence for evolution from biogeography that caught Darwin’s attention and led him to speculate that species might change over time.
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.