“A Little Bird Told Me” … Happy Birthday Charles Darwin

If you know only a little bit about Charles Darwin, you know that he figured out Evolution via his study of the finches (and other birds) of the Galapagos. If you know a bit more than that about Darwin, you know that he totally messed up his collection of birds from the Galapagos Islands, and didn’t really think up Evolution until much later in time. If you know more than that about Darwin, then you know that neither of these characterizations of the great man’s work is accurate.

Today we view the Galapagos birds, including some finches and some mocking birds, as a great example of evolution. The idea is that some finches from what is now Ecuador ended up on the Galapagos Islands, and subsequently diversified into a number of different forms … they speciated … filling various niches that on the mainland would have been filled by a number of different species. The most commonly cited example is “Darwin’s Finch,” which filled the woodpecker niche by adopting the practice of acquiring a cactus thorn, and using this to access grubs hidden beneath the bark of trees. The other finches differentiated in other ways, having to do with the kinds of food they eat and the part of the landscape they tended to forage on (i.e., ground vs. in bushes, etc.) More recently, the finches of the Galapagos, in particular of one of the Galapagos Islands, have been under intense study by a team lead by the famous Grants (The Beak of the Finch: A Story of Evolution in Our Time). All of the finches of the two species found on this island have been identified as individuals, labeled, and tracked through several generations as local climate has shifted between very very dry and even drier conditions. Food availability has shifted over time, and the two versions of finch do better under different conditions, and their relative abundance reflects this shift. Also, within each species of finch, there have been shifts back and forth in the shape of their beaks, reflecting the differences in the kinds of food available. Variation in finch beak shape is constantly arising due to the usual sources of genetic variation, and that variation is then culled across generational time, but which direction the culling happens in depends on the climate of that particular year or season. Thus, to the Grants, a dead finch is a very important piece of data.

There are things about the Galapagos Islands that many people are not aware of, and there are things about Darwin’s thinking about evolution that are not part of the usual description of Darwin’s life. I’d like to outline what a couple of these things are.

First, about the Galapagos. We think that the original stock from which the Galapagos Finches evolved came from nearby South America. Generally speaking, air and wind currents make it difficult for birds to accidentally end up on the Galapagos, but that may have been different in the past. Regardless, it makes sense that the source of the islands’ birds is the nearest major landmass, though in truth, those finches could have come from anywhere that your basic root stock finch was living. However, this is not the case with the Galapagos Tortoises. They also have an interesting evolutionary story, varying across the different Galapagos Islands, as well as within some of the islands. Some of the tortoises have shells and necks and heads configured for picking leaves off of bushes (involving a lot of reaching upwards) while others have shells, necks and heads that go more with grazing off the surface. The famous Galapagos Iguanas are of two types: The kind that live on land are distinct form the kind that hang out on land but feed off of sea weed underwater. This second adaptation is especially interesting because iguanas generally don’t graze on underwater marine plants; this is a remarkable instance of the rise of a totally new adaptation.

But the Tortoises and Iguanas (and some other animals, probably) did not come from the nearby South American landmass via what is now Ecuador. Rather, they came from the Caribbean region, and prior to that, probably from the eastern or northeastern part of the South American land mass. How did they get from the Caribbean across the Isthmus of Panama, which is mountainous and very much in the way? They did it before the Panama Land Bridge formed. The Panama Land Bridge probably started to form, as a series of islands, about 15 million years ago, and by 3 million years ago it was fully in place. Some estimates put the closure of the passage between the Pacific and Atlantic at about 5 million years ago, which coincides with some major changes in the Earth’s climate systems. Genetic studies seem to show that the Iguanas and Tortoises passed through this region, presumably on rafts of floating vegetation and stuff, via currents running east to west along the northern rim of what was then an “isolated” South America.

Another thing that people often don’t realize about the Galapagos, and this is not something that to my knowledge is totally worked out yet, is that the islands you see there now are not the islands that were first occupied by the various species of plant and animal that gave rise to the species we see today. Among the Galapagos Islands are numerous sea mounts, some of which were above-water islands in the past. Over time, islands in this area have formed from sub oceanic volcanoes, grown in size, stopped growing, and then eroded back into the sea. It is not the case that the islands we see today were populated by various continental forms of animals and plants which then differentiated. Rather, there is a complex history of mainland life forms ending up on various islands, and then colonizing other islands over time as the islands themselves formed, grew, shrank, and went away. Presumably this complex opera of biogeography is still underway today.

Regarding Darwin’s conceptualization of evolution vis-a-vis the Galapagos Islands, the first thing you need to realize is that Darwin’s voyage on the Beagle lasted a very long time. The Beagle left England in December of 1831 and returned in October 1836. The Galapagos was first sighted in September 1835, after four years of voyaging, much of which was spent in other areas of South America where Darwin was focused on the geology of the region as much as he was on the biological wonders he encountered.

Much earlier, the Beagle sailed to the Falkland Islands, arriving there in March of 1833. It is here that Darwin records some observations and thoughts that make me think that thoughts of biogeography and speciation were starting to form in his head. He noted the similarity and differences among animals found on the islands and mainland versions of the same. This included some birds as well as a fox. Today, scholars do not think that Darwin “thought up” evolution at that time because even when he encountered the Galapagos much later, he was still speaking in terms of a creationist (and not a Darwinist!). Nonetheless, he was already beginning to get his mind around the idea that species do not remain the same across vast areas of time and space.

The link between Darwin’s earlier observations on the Falklands and his later observations on the Galapagos is seen in this passage regarding mockingbirds:

These birds are closely allied in appearance to the Thenca of Chile or Callandra of La Plata. In their habits I cannot point out a single difference. They are lively, inquisitive, active, run fast, frequent houses to pick the meat of the Tortoise which is hung up, sing tolerably well; are said to build a simple open nest; are very tame, a character in common with the other birds. I imagined however its note or cry was rather different from the Thenca of Chile? Are very abundant over the whole Island; are chiefly tempted up into the high & damp parts by the houses & cleared land. I have specimens from four of the larger Islands: the two above enumerated [males from Charles and Chatham Islands]; a female from Albemarle Isd. and a male from James Island. The specimens from Chatham & Albemarle Isd appear to be the same; but the other two are different. In each Isld each kind is exclusively found: habits of all are indistinguishable. When I recollect the fact that [from] the form of the body, shape of scales & general size, the Spaniards can at once pronounce from which Island any Tortoise may have been brought. When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds but slightly differing in structure & filling the same place in Nature, I must suspect they are only varieties. The only fact of a similar kind of which I am aware is the constant asserted difference between the wolf-like Fox of East & West Falkland Islds. If there is the slightest foundation for these remarks the zoology of Archipelagoes will be well worth examining; for such facts would undermine the stability of Species.

That is from Darwin’s ornithological notes. Bulletin of the British Museum (Natural History). Historical Series Vol. 2, No. 7, probably penned in 1836, after leaving the Galapagos and on the way back to England.

So the evidence needed to sway Darwin’s thinking about what species are and how they arise congealed in the Galapagos, but he was exposed to it before, and not just in the Falklands. The fact that his evolutionary models that we now know to have been very insightful really came together after the trip was over does not mean that his mind was not turning over these ideas from much earlier on. Darwin needed to go through a transformation in thinking that, once he developed and espoused, took the rest of the scientific world decades to experience. I think that ultimately, if you went back in time to the 1840s and asked Charles Darwin where he came up with the ideas of diversification of species and Natural Selection, he would probably say: “Oh, that … a little bird told me. Or, really, a bunch of little birds.”