Evolution In Detail: The Grants' Study Of Darwin's Finches
Yesterday our department hosted Peter and Rosemary Grant, who spoke about their 30+ years studying natural selection and finches in the Galapagos. (If you're interested in the book version of their work, check out Jonathan Weiner's Pulitzer Prize-winning The Beak of the Finch.)
While the Grants give a great presentation, full of pictures the Galapagos finches in action, my first impression was that none of this was really groundbreaking. As the Grants mentioned multiple times in the talk, Darwin anticipated so much of what they observe in the Galapagos. In an age of molecular genetics, a long-term, non-molecular field study is bound to seem a little old fashioned, although the Grants have recently been taking DNA samples and incorporating the tools of molecular genetics into their work.
In the end, I came away from the talk satisfied. This work may not be conceptually groundbreaking, but I find it important for at least one reason: this is evolution in detail, in the wild. People often unintentionally tend to think of natural selection as a steady pressure that gradually shapes one trait into a more adaptive one. Scientists have long known that evolutionary forces are much more complex than that out in the real world, but when you think about these issues in the abstract, it's easy to let a failure of imagination bury that complexity.
The Grants' work makes it much easier to appreciate how complex the real environment is. Their most famous result, at least in the popular press, is that finches of a given species with bigger beaks that enable them to crack hard nuts tend to survive under drought conditions, while finches with smaller beaks make a comeback during wet years. Their data shows how the population average beak size changes in response to conditions.
Yet this is only the tip of the iceberg: the Grants have observed so many other complex factors coming into play. If the cyclic weather conditions were the only major factor, then natural selection on beak size is simple, and over the long haul the beak size doesn't actually change much. The Grants however found other influences. For example, a different finch species, with a really big beak, colonized one island and was able to out-compete the existing large-beaked finches even during dry years. This meant that, contrary to what had happened before the island was invaded by the new finches, those birds from the original species with the smallest beaks ended up doing better than their larger beaked relatives. A new environmental factor, in the form of the new island invaders, changed the evolutionary pressure on beak size.
You can keep piling on more variables. Sometimes a member of one finch species will take over the nest of another finch species, cleaning house by dumping out the eggs of the hapless, displaced finch. But sometimes one egg will survive the purge, meaning that a hatchling of one species ends up getting raised by parents of another species. That hatchling learns the distinct, species-specific song of its foster parents.
This is important, because song is one way that finches know to mate with the correct species. Sometimes a bird of one species, raised as a member of another species, ends up engaging in cross-species mating, resulting in hybrid finches. This hybridization is rare, but it happens often enough to make a significant impact on the genetics of the two main finch species on the island.
In their talk yesterday, the Grants argued that this gene flow between two species is leading to more genetic convergence - meaning that these two species could be on their way to forming a new mixed species. This is exactly the opposite of what was happening for the last 2-3 million years: the various finch species of the Galapagos have been diverging from each other since they last shared a common ancestor. 2-3 million years of divergence has been enough to create 14 new finch species, but not enough to eliminate all cross-species mating - that kind of sexual isolation takes much, much longer.
The result of the Grants' work is that we can see evolution in flux, not as a linear pathway. The course of evolution ebbs and flows, often reversing itself or, upon the introduction of a new environmental variable, taking a completely orthogonal path. The Grants found two species that hybridize often enough to lead to genetic convergence, but this trend could easily reverse itself, and those two species may never converge. The Grants may have found very little that was not anticipated by Darwin, but they have worked out one of the most detailed pictures we have of evolution in action in the wild, at least the evolution that occurs on the scale of a human lifetime.