The paper is:
Unpredictable evolution in a 30-year study of Darwin's finches.
Grant PR, Grant BR.
Science. 296:707-11 (2002)
This may qualify as the Cool Science of the past thirty years, because that's how long the Grants have been monitoring Darwin's finches on the Galapagos, and they just published the overall findings.
Basically, every year since 1973 they've been measuring two species of ground finch on the island of Daphne Major. They record 6 traits, which reduce to three overall traits: body size, beak size, and beak shape. They also record other variables on the island, such as weather and abundance of various food items. They can track reproduction of the birds they measure, and the reproductive success of their offspring and their offspring's offspring.
The end result is an evolutionary motion picture, putting the snapshots together like one of those flip-the-pages cartoons. Both populations of finches changed over the thirty years:
Mean body and beak size of G. fortis initially decreased, then increased sharply, and decreased again more slowly. Beak shape abruptly became more pointed in the mid-1980s and remained so for the next 15 years. G. scandens, a larger species, displayed more gradual and uniform trends toward smaller size and blunter beaks, thereby converging toward G. fortis in morphology.
More than that, they were able to correlate at least some of these changes with the environmental changes that drove them; for example, "in the late 1970s, large size was selectively favored in both species during a drought".
Oh, but there's more. What mechanism drives the changes? You can imagine at least two (not mutually exclusive) mechanisms. The simplest version of natural selection has selective loss driving it. If your beak isn't quite right, you don't get as much food, and your offspring don't do as well, and your smaller-beaked neighbour outcompetes you. You lose.
But you can also, with some effort, imagine the reverse: Selective gains. Recruitment, that is, to the adult population--pulling in traits from outside the population. Surprisingly, selective gain was a significant contributor to evolution in these finches.
What's "outside the population"? It could be immigrants--same species flying in from different islands. This turns out to be very rare. What "outside the population" means in this case is actually interspecies breeding.
Hybridization does occur rarely between resident G. fortis and G. scandens, and G. fortis also breeds with a rare immigrant species, G. fuliginosa (small ground finch). In both cases there is generally little or no fitness loss. After the dry period of the late 1970s, and beginning in the extraordinarily prolonged wet season of 1983 (El Nino year), successful breeding of F1 hybrids and backcrosses was documented.
The hybrid were much more variable than either initial population, as you'd expect, and the variability from the hybrids greatly increased the overall population variability.
They were able to show that the gene flow was mainly in one direction, and why:
The proportionally greater gene flow from G. fortis to G. scandens than vice versa has an ecological explanation. Adult sex ratios of G. scandens became male biased after 1983 as a result of heavy mortality of the socially subordinate females. High mortality was caused by the decline of their principal dry-season food, Opuntia cactus seeds and flowers; rampantly growing vines smothered the bushes. G. fortis, more dependent on small seeds of several other plant species, retained a sex ratio close to 1:1. Thus, when breeding resumed in 1987 after 2 years of drought, competition among females for mates was greater in G. fortis than in G. scandens. All 23 G. scandens females paired with G. scandens males, but two of 115 G. fortis females paired interspecifically. All their F1 offspring later bred with G. scandens because choice of mates is largely determined by a sexual imprinting-like process on paternal song.
It's like watching dominos fall, click click click, except for one critical point: This was completely unpredictable in the long run.
Mean body size and beak shape of both species at the end of the study could not have been predicted at the beginning. Moreover, sampling at only the beginning and at the end would have missed beak size changes in G. fortis in the middle. The temporal pattern of change shows that reversals in the direction of selection do not necessarily return a population to its earlier phenotypic state. Evolution of a population is contingent upon environmental change, which may be highly irregular, as well as on its demography and genetic architecture.
Here's some of the overall conclusions from the Grants:
Natural selection occurred frequently in our study, occasionally strongly, unidirectionally in one species and oscillating in direction in the other as a result of their dependence on different food supplies. Introgressive hybridization, a phenomenon whose importance has been relatively underappreciated until recently, except in plants, had different effects on the two species for demographic reasons. Hybridization and selection are often connected through the selective disadvantage experienced by hybrids and backcrosses. In the present study they appear to have been connected synergistically in the sense that interbreeding may have been facilitated in part by selection for more pointed, G. scandens-like, beaks in the G. fortis population in the mid-1980s. Choice of mates is partly determined by imprinting on parental beak morphology, as well as on paternal song. The principal causes of selection have been identified as changes in food supply mediated in large part by droughts. The ultimate cause of repeated natural selection and introgressive hybridization may have been a change in the seasonal movement of water masses in the eastern subtropical and tropical Pacific, triggering altered climatic patterns, including the intensification of El Nino and La Nina cycles.
Well, I think it's cool.
Recommended reading:
The Beak of the Finch: A Story of Evolution in Our Time.
Jonathon Weiner, 1996.
Vintage Books; ISBN: 067973337X