By Jane Eastwood, XRI Lead Exobiologist
If you've ever tried to identify mosses, little gray birds, or bacteria, you'll know that telling species apart can be nearly impossible. The greatest diversity of species isn't present in things that look wildly different from each other — it's between several species that appear more or less identical, species that differ only in location or habitat and tiny, nearly invisible characters. It's not distinguishing between peacocks and hummingbirds, it's distinguishing between sparrows with white head stripes and sparrows with pale yellow head stripes. I hope that what we're seeing with the bristletongues is similar; closely related species that provide evidence that species and speciation works the way it does on Earth.
One of the first pieces of evidence for natural selection were the finches of the Galapagos islands. Darwin collected birds from different islands; he thought that they were a mix of grosbeaks, blackbirds, and finches. An expert in ornithology examined them and revealed that they were all closely related finches; finches with beaks you'd expect to see on grosbeaks and blackbirds. (Current DNA evidence states that they are actually tanagers — for convenience I will be referring to them as "finches" throughout this document. Convenience and the fact that I didn't learn that until the article was nearly done.) Darwin realized that there were no grosbeaks or blackbirds on the Galapagos — and thus that the enormous seeds grosbeaks usually eat with their enormous beaks were going un-nibbled. He made the inductive leap that a finch that could eat those un-nibbled seeds would get very fat and have lots of chicks — and that presumably, some of those chicks would also have enormous beaks. Thus, he got all of the premises for natural selection in one go; there is variation in the population, that variation is heritable, things with one kind of variation are better at surviving than their neighbors, and there is a finite population size; thus do beneficial mutations spread through the population. It was his "Eureka!" moment, except he kept his clothes on, and spent the next twenty years amassing data and a solid reputation before presenting his monumental theory.
Now that these poor birds have been nearly studied to death, we know that 2-odd million years ago, a very few members of a species of South American finch were blown to the Galapagos Islands where they did very well and had many offspring — there were almost no other perching birds to compete with them, just shorebirds. Some of those offspring spread to adjacent islands — islands that were just far away enough to prevent frequent travel for finches. On each island, adaptation to eating local food sources selected for a specific beak shape — some birds eat large seeds, some eat small seeds, some eat insects, some eat fruit, and some drink the blood of other birds (Not really. Just making sure you're still paying attention). The selection on each island proceeded more or less as Darwin had described. What throws a wrench in the process is finches migrating back to the island their ancestors were from; they are now hanging out with closely related species, but since they use different food sources, they aren't competing. This — and the isolation of the Galapagos that prevents most other perching birds from showing up in numbers large enough to sustain a gene pool — leads to pretty much every niche a bird can fill being filled by one species of finch or another. The finches show no evidence of sympatric speciation; species never split without being separated by stretches of ocean that a typical finch could not easily traverse.
What is essential to this rapid and variable speciation is the island archipelago; the habitats that are separate enough to prevent interbreeding but close enough to have some migration prompt rapid speciation. I am excited that we started on an island; it's likely we'll see the results of rapid diversification because of the isolated habitats.
Or the bristletongue could be the result of that most important feature of natural selection; variation within a population. I guess I'll just have to keep watching.