wells again – this time, darwin’s finches

Naturally, Wells had to include ‘Darwin’s finches’ in his list of evolutionary icons. He asks:

DARWIN’S FINCHES. Why do textbooks claim that beak changes in Galapagos finches during a severe drought can explain the origin of species by natural selection — even though the changes were reversed after the drought ended, and no net evolution occurred? 

I find the idea that there must be ‘net evolution’ a strange one, although I suppose it fits with the common misconception that evolution is progressive, moving ever onwards to ‘higher’ & ‘better’ forms of life. In fact, of course, this isn’t the case – a population evolves as it adapts (driven by natural selection) to the current environmental conditions. This doesn’t presuppose a higher level of ‘complexity’ or some sort of progression from simple to complex, & what natural selection does in one environment, it may undo in another. As we shall see, with the finches…

The Galapagos finches have been studied in exquisite detail for more than 30 years by the husband-&-wife team of Peter & Rosemary Grant. (For an excellent account of their work, you can’t go past Jonathan Weiner’s book The beak of the finch.)  Every bird in their study population on the island of Daphne Major has been banded; its parentage is known, & its physical measurement, survival, and reproductive success recorded. Individuals in the population vary in their physical features, and the Grants’ work clearly demonstrates that much of this variation has a genetic component.

And they found that the population’s characteristics could change very rapidly in the face of environmental change: average beak size, for example, changed in the space of a couple of years when a severe drought meant that the seed supply declined rapidly, and those seeds that were available tended to be the larger, harder seeds. Only larger-billed finches could crack and eat these seeds, so they tended to survive & produced a disproportionate number of offspring, many of which also had larger bills – the average bill size increased, compared to what was observed in the years preceding the drought. The population had evolved – and more quickly than scientists had previously thought possible.

Then the rains returned. A wider range of seeds once more became available, as plants grew, flowered, and set seed. There was no longer any selective advantage in possessing a larger bill – and the average bill size did indeed decline. Just as evolutionary theory would predict. In other words, Wells’ argument doesn’t stack up. Again.

J. Weiner (1994) The beak of the finch: a story of evolution in our time. Knopf.

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