Evolution Happens More Quickly Than You Think
Biologist Jonathan Losos tells the story of two biologists who witnessed evolution unfold before their eyes.
The following is an excerpt from Improbable Destinies: Fate, Chance, and the Future of Evolution, by Jonathan Losos.
Ironically, it was research on the birds bearing Darwin’s name—the Galápagos finches—that drove the dagger through the heart of the idea that evolution is always slow. Like the peppered moth, Darwin’s finches have become one of the poster-child examples of evolution, and not just because of their name and history. Rather, much of their fame stems from the extraordinary forty-year research program of Princeton biologists Rosemary and Peter Grant.
Starting in 1973, the Grants spent several months each year on the small, crater-shaped Galápageian island of Daphne Major. Their goal was to study the population of the medium ground finch (so named because there are both larger and smaller ground finch species) to see whether and how the population changed from one generation to the next and to attempt to measure natural selection driving such change.
To do so, the Grants had to capture and measure all of the finches on the island every year. Only in that way could they see if the characteristics of the population—body mass, beak size, wing length, and so on—were changing from one generation to the next.
Catching a bird is a more passive process than capturing a moth or a lizard. Instead of actively seeking out the quarry and snaring it with some sort of contraption, be it net or noose, ornithologists let birds catch themselves. The trick is to put up what looks like an oversized badminton net, except that the latticework is very thin. So thin, in fact, that a bird often fails to see it until it’s too late, flying straight in and becoming hopelessly entangled. Then one of the Grants would come along and carefully extricate the protesting finch, popping it into a cloth sack and bringing it back to camp for processing.
The camp itself wasn’t much—a rock shelter, some tarps for shade, and folding chairs. Using calipers, the Grants would carefully measure the dimensions of the beak: how long, how high, how wide. Then they would deftly extend the wing to record its length and measure the leg bones as well. Finally, they would place several colored bands around each bird’s leg, giving the bird its own, individual identification card.
Returning year after year, the Grants were able to watch the evolutionary process unfold. Natural selection occurs when the survival and reproductive success of an individual is related to its phenotype. The Grants’ data allowed them to ask whether natural selection was operating. Each year, they tabulated who had survived from the previous year and who hadn’t. They already had all of their phenotypic measurements for the birds, so they could simply correlate the two data sets: was there a relationship between how long a bird’s legs were or how wide its beak was, and whether it survived or perished?
It didn’t take the Grants long to find out. The fourth year of their study was extraordinarily dry. Whereas in a normal year, the wet season brings five inches of rain, in 1977 the amount was less than an inch. Daphne Major turned into a barren wasteland. Plants dried up. Water was even more scarce than usual. Seeds—the staple diet of these finches—became few and far between.
The birds died in droves. Starvation and lack of water were a powerful one-two punch, especially because hungry birds couldn’t produce new feathers, and as the old ones wore out, water loss through the exposed skin increased. In January 1977, Daphne Major had 1,200 medium ground finches; twelve months of drought later, the total was 180.
But the mortality was not random. Rather, the largest birds and those with the biggest beaks survived better. The reason is that the supply of small seeds was eaten up first, and as they disappeared, smaller-beaked birds were out of luck—they didn’t have the jaw power to crack open the remaining larger seeds. This was among the strongest episodes of natural selection ever detected in the wild.
Natural selection doesn’t necessarily lead to evolutionary change. If birds with big beaks survive and reproduce better, then average beak size should increase through time. But this expectation only holds true if big-beaked birds give rise to big-beaked offspring. That is, variation in a trait must have a genetic basis so that trait values are inherited from parent to offspring. Often this is the case, but not always. In humans, for example, the children of bodybuilders don’t necessarily have big muscles.
Or think about the sun-loving houseplant in your kitchen window. Put it in a shady corner, and it will grow much more slowly. The amount of water and fertilizer you give it will also matter. Better yet, take a dozen genetically identical plants, created by grafting or taking cuttings, and give them different combinations of light, water, and fertilizer. After a few months, you’ll almost surely end up with very different-looking potted plants.
The phenomenon in which genetically identical organisms produce different phenotypes depending on their environmental circumstances is called “phenotypic plasticity.” This is the nurture part of the nature-versus-nurture debate.
In the case of Darwin’s finches, however, phenotypic variation was genetically based, inherited from parent to offspring. The Grants’ team demonstrated that by comparing parents and offspring—they knew whose parents were whose because they had been banding birds shortly after they hatched, while still in the nest. And what they found was a strong correlation between parent and progeny—overall body size was highly heritable, as were beak, wing, and leg dimensions.
Consequently, the larger body size and beaks of the drought survivors were passed on to the next generation, and in subsequent years, the finches were larger and with bigger beaks. Strong selection had led to rapid evolutionary change.
The Grants continued studying the finches of Daphne Major for another thirty-five years. And what they found was that such strong selection was not uncommon. Just a few years later, for example, one of the strongest El Niño events ever brought fifty-four inches of rain— that’s right, ten times the normal. The deluge produced a glut of small seeds that led to strong selection for small-beaked birds with the delicate touch necessary to efficiently harvest small seeds. And once again, the population rapidly evolved in response.
The Grants’ work was so influential not only for what they documented, but for what they showed was possible. Contrary to conventional wisdom, they demonstrated that evolution can be studied in nature as it occurs, in real time. Their work has been an inspiration to several generations of field biologists, and the result is that the number of people conducting similar research has exploded, providing a previously unavailable wealth of information on rates of evolution in nature.
From IMPROBABLE DESTINIES: Fate, Chance, and the Future of Evolution. Published by arrangement with Riverhead Books, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC. Copyright © 2017 by Jonathan B. Losos.