Sexual Selection Term Analysis

Now it became of great significance that variations of body and beak are passed on from one generation to the next with fidelity. As a result, the males' unequal luck in love helped to perpetuate the effects of the drought. The male and female fortis that survived in 1978 were already significantly bigger birds than the average fortis had been before the drought. Of this group the males that became fathers were bigger than the rest. And the young birds that hatched and grew up that year turned out to be big too, and their beaks were deep. The average fortis beak of the new generation was 4 or 5 percent deeper than the beak of their ancestors before the drought.

So the birds were not simply magnified by the drought: they were reformed and revised. They were changed by their dead. Their beaks were carved by their losses.

In most places on this planet, the sight of a dead bird is so rare that it shocks us, even scares us. […]

But on the desert island of Daphne Major, dead birds are commonplace. They are everywhere. […] Each generation lies where it falls, and the next generation builds on the ruins of the one before.

In the dry season, natural selection metaphorically scrutinizes these birds, “daily and hourly,” as they strive to keep body and beak together. Some birds make it, and some don't. In the wet season, which is also the breeding season, the survivors are scrutinized daily and hourly by one another, not metaphorically but literally, as males begin jousting for territory building nests, and singing from the highest cactus in their territories, while females troop by and inspect the males' nests and plots of lava and listen to their songs.

In other words, as soon as nature stops selecting among these birds, the birds start selecting among one another. Again, some make it and some don't.

The answer is that a male guppy has more to do in life than merely survive. It also has to mate. To survive it has to hide among the colored gravel at the bottom of its stream and among the other guppies of its school. But to mate it has to stand out from the gravel and stand out from the school. It has to elude the eyes of the cichlid or the prawn while catching the eyes of the female guppy.

The conclusion is inescapable: the feature that makes the finches most interesting to us is also the feature that makes them most interesting to each other. When they are courting, head to head, making decisions that are fateful for the evolution of their lines, Darwin’s finches are studying the same thing as the finch watchers. They are looking at each other's beaks.

Thus the Grants suspect that the finches here are perpetually being forced slightly apart and drifting back together again. A drought favors groups of one beak length or another. It splits the population and forces it onto two slightly separate adaptive peaks. But because the two peaks are so close together, and there is no room for them to widen farther apart, random mating brings the birds back together again.

These two forces of fission and fusion fight forever among the birds. The force of fission works toward the creation of a whole new line, a lineage that could shoot off into a new species. The force of fusion brings them back together.