Hybridization and Specialization Theme Analysis

The whole family tree of Darwin’s finches is marked by this kind of eccentric specialization, and each species has a beak to go with it. Robert Bowman, an evolutionist who studied the finches before the Grants, once drew a chart comparing the birds' beaks to different kinds of pliers. Cactus finches carry a heavy-duty lineman’s pliers. Other species carry analogues of the high-leverage diagonal pliers, the long chain-nose pliers, the parrot-head gripping pliers, the curved needle-nose pliers, and the straight needle-nose pliers.

Only varieties. If so, they would fit comfortably within the orthodox view of life. But what if they were something more than varieties? […] What if there were no limits to their divergence? What if they had diverged first into varieties, and then gone right on diverging into species. new species, each marooned on its own island?

“—If there is the slightest foundation for these remarks,” Darwin wrote, “the zoology of Archipelagoes—will be well worth examining; for such facts undermine the stability of Species.” Then, in a scribble that foreshadowed two decades of agonized caution, Darwin inserted a word: “would undermine the stability of Species.”

According to [Darwin’s] theory, even the slightest idiosyncrasies in the shape of an individual beak can sometimes make a difference in what that particular bird can eat. In this way the variation will matter to the bird its whole life—most of which, when it is not asleep, it spends eating. The shape of its particular beak will either help it live a little longer or cut its life a little shorter, so that, in Darwin's words, "the smallest grain in the balance, in the long run, must tell on which death shall fall, and which shall survive."

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.

Half a millimeter can decide who lives and who dies. Since these slight variations are passed down from one generation to the next, the brood of a small beak and a medium beak would be likely to have intermediate beaks, equipment that would sometimes differ from their parents' not by one or two tenths of a millimeter but by whole millimeters, maybe by many millimeters. […] Daphne Major is not a forgiving place. A line of misfits should not last.

[…]

That is why the Grants are so puzzled now.

Selection will act in this way on all neighboring varieties, […] and the effect will be continually to move varieties apart and repel them. Even if they never actually jostle and joust, […] natural selection will gradually magnify their differences.

At last the two varieties will move so far apart that competition will slack off. It will slack off when the two varieties have evolved in new directions: when they have diverged. Natural selection will have led in effect to another adaptation—the mutual adaptation of two neighbors to the pressures of each other existence. And the result of this sort of adaptation would be forks in the road, partings of the ways, new branches on the tree of life: the pattern now known as an adaptive radiation.

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.

So there is a simple trade-off here for a stickleback. If the fish specializes in the muck, it cannot compete in the open water; if it specializes in the open water, it is outclassed down in the muck. The fish is in much the same position as a finch in the Galápagos, where specializing in big seeds unfits you for the small ones, and specializing in the small seeds unfits you for the big ones.

To Dolph all this evidence powerfully suggests that the colonists in these lakes have altered the course of each other's evolution, just as the finches have altered each other's courses in the Galápagos.

These two oscillations are driven by the same events. They are both governed by the same changes in the adaptive landscape. In an adaptive landscape that is wrinkling and rolling as fast as Daphne, a landscape in which the peaks are in geological upheaval, it can pay to be born different, to carry a beak 3, 4, or 5 millimeters away from the tried and true. Since the super-Niño, some of the old peaks have turned into valleys, and some of the old valleys are peaks. Now a hybrid has a chance of coming down on the summit of a new peak. It can luck onto a piece of the new shifting ground.

In times of stress, when the temperature shoots up or down, for instance, or the environment goes suddenly more wet or dry colonies of bacterial cells in a Petri dish will begin to mutate wildly. This is known as the SOS response, for the international distress signal Save Our Souls, Save Our Ship. It increases the chance that at least a few of the cells in the Petri dish will survive the disaster of the new conditions.

The SOS response has been observed in the DNA of maize when it is shocked by hot or cold temperatures. Recently it has been discovered in yeast. Apparently many different kinds of living cells can switch up their mutation rate under stress and relax it again when the stress dies down.

You don't find situations that chaotic under natural conditions, but you do find them in the havoc that human beings bring in their train. […] Thus, our disturbances hybridize both the environment and the species.

We are hybridizing the planet.

A pesticide applies selection pressure as surely as a drought or flood. The poison selects against traits that make a species vulnerable to it, because the individuals that are most vulnerable are the ones that die first. The poison selects for any trait that makes the species less vulnerable, because the least vulnerable are the ones that survive longest and leave the most offspring. In this way the invention of pesticides in the twentieth century has driven waves of evolution in insects all over the planet.

In the world's oceans, Norwegian cod, chinook salmon, Atlantic salmon, red snapper, and red porgy are getting smaller, very likely through the selection pressures of the net. Fishermen are not happy with the trend toward small fish, any more than elephant poachers are pleased with the trend toward tusklessness. But both resistance movements are direct results of Darwinian law.

The black mutants swept up through the mort populations wherever the air was black with the soot of the industrial revolution. Their numbers did not rise in rural parts of Cornwall, Scotland, and Wales. In rural Kent, Darwin's adopted county the black form of the moth was not recorded during his lifetime; but by the middle of this century, nine out of ten Biston betularia were black in Bromley, and seven out of ten in Maidstone.

Manchester, of course, was one of the grimy hubs of the industrial revolution.