The Beak of the Finch Reading #3

“ The Beak of the Finch” reading #3:

Early in his fieldwork he had found a Trinidadian stream that contained the guppy eater Rivulus hartii, but no guppies. About 2 kilometers away was a second stream that contained both guppy eaters and guppies. Endler took a random sample of about two hundred guppies from one of the high-danger zones in the second river. He measured each and every one, as usual, and then he transferred them to the safe place in the first river. He took a sample of their descendants more than a year later, after a passage of fifteen generations.

The males in the safe stream were now much gaudier than their immediate ancestors, who were still living in the stream next door and coping with many enemies. The immigrant males wore bigger spots, and more of them, and each male sported a wider assortment of colors. Natural selection had acted just as predicted. Evolution had run as fast in the wild as in the greenhouse.

Everywhere in those streams, daily and hourly, natural selection in the form of cichlids and prawns is not just metaphorically but literally scrutinizing the male guppies. The result of enemy predations on each generation keeps pushing the males to blend in with the stream bottom. At the same time, daily and hourly, sexual selection in the form of female guppies is scrutinizing those same males. The result of their choices is that generation after generation of males is pushed to stand out.

Now it is clear why there is such virtually infinite range of variation in the way each individual male guppy is spotted. Many different random patterns of splotches will be equally good camouflage, because the streambed patterns are random too. It would not help the guppies to sport the same pattern as all the others, and in fact it would hurt them. If the guppy males all looked alike, their enemies could develop a search image – an inner template. They would search for that pattern, as we search for the face of a friend in a crowd. The rare misfit would have a great advantage. Meanwhile the females would go for the unusual males, too, and that would drive more and more diversity of patterns, So in this respect natural selection and sexual selection cease to oppose each other and push in the same direction: toward almost infinite diversity.

We see here an example of what Darwin saw in the wide world. He understood that his simple process can lead to the most bewildering and chaotic-looking diversification and variety – but underneath, the driver is as simple and plain and commonsense as ever, “small consequences of one general law leading to the advancement of all organic beings, -- namely, multiply, vary, let the strongest live and the weakest die.” The guppy experiments suggested to Endler what Darwin’s finches were suggesting at about the same time to the Galapagos finch watchers: that natural selection can be swift and sure. The process is following along, all around us, much faster than Darwin ever dreamed.

Endler’s study is leading him into deeper and deeper waters. He now suspects that the guppies’ spots, their mating habits, and their color vision are all evolving simultaneously, with change in any one of these factors driving change in all the others. To measure variations in the guppies’ retinas, Endler is collaborating with physiologists. These “hard science” types often remind him how “soft” the science of evolution is perceived to be by the outside world – even by biologists. “I was talking with someone in vision physiology the other day,” Endler says, “and he told me, ‘Wow, I had no idea that the subject was so rigorous. I had no idea that you actually did experiments.’

“We have a serious public-relations problem,” Endler says. “People don’t realize this is real science.”

THE END!