Bird Beaks and Guppy Lifetimes

Darwin never imagined that anyone could witness natural selection. He thought that the variations in his pigeons were the closest that he could get. In the wild, Darwin assumed evolution was too slow and gentle for our short‑term minds to perceive, just as we can’t see rain wash away a mountain. But these days biologists building on the modern synthesis can actually witness flashes of evolutionary change taking place before their eyes.

David Reznick, a biologist at the University of California at Riverside, gets a glimpse in the forests of Trinidad, where guppies swim in the streams and pools. At the lower elevations, the guppies face the assault of predatory fishes, but the ones in higher waters live in peace, because few of the predators can move upstream past the waterfalls and craggy rocks. In the late 1980s, Reznick began to use the guppies as a natural experiment.

Like all animals, guppies have a timetable for their lives–how long they take to reach sexual maturity, how fast they grow during that time, how long they live as adults. Theoretical biologists have predicted that the life history of animals can evolve if mutations that alter it bring the animals more reproductive success. Reznick put their predictions to the test.

In ponds with a lot of predators, guppies that live fast should be more successful than slow‑growing ones. With the threat of death hanging over a guppy, it will grow as quickly as possible so that it can start mating as soon as possible and have as many offspring as possible. Of course, this strategy comes with a heavy price. By growing so quickly, a guppy may shorten its own natural lifespan, and by quickly giving birth to babies, a female guppy can’t take time to supply her offspring with much energy, which puts them at risk of dying young. But Reznick reasoned that the threat of an early death offset these risks.

To see whether this trade‑off was real, Reznick rescued guppies that were being terrorized in the downstream pool and put them in pools with relatively few predators. Eleven years in these conditions produced guppies that were, on average, in less of a rush. They took 10 percent longer to mature than their ancestors and were over 10 percent heavier by the time they were fully grown. They were also laying smaller broods of eggs, but each of the new guppies that hatched from those eggs was bigger.

To spend 11 years watching guppies become 10 percent bigger may seem at first like a dull way to pass the time. But in the history of life, 11 years is a fraction of a flash. The rate of evolution that Reznick has witnessed is thousands of times faster than the rate that George Gaylord Simpson documented in the fossil record. When Simpson estimated the rate of evolutionary change among fossils, he could compare it only to the rate at which fruit flies evolved in laboratories. No one could say whether the flies were evolving unnaturally or not. But now scientists like Reznick have shown that even in the wild, animals can change rapidly.

Sometimes nature runs evolutionary experiments of its own, without any help from humans whatsoever. In these cases, biologists simply have to observe. After Darwin left the Galápagos, scientists came back every few decades to study his puzzling finches. In 1973 Peter and Rosemary Grant, husband and wife biologists now at Princeton University, arrived on the islands to study the effects of natural selection on the birds.

Most years on the Galápagos, the weather follows a standard pattern. For the first five months of the year it is hot and rainy, followed by a cool, dry period. But in 1977 the wet season never came. A periodic disturbance of the Pacific Ocean called La Niña altered weather patterns over the Galápagos, causing a disastrous drought.

On Daphne Island, where the Grants worked, the drought was lethal. Out of the 1,200 medium ground finches (Geospiza fortis) that lived on the island, more than 1,000 died. But the Grants discovered that the decimation wasn’t random. G. fortis lives mainly on seeds, which it cracks with its strong beak. Small G. fortis can break only small seeds, but larger birds have beaks that are strong enough to break big ones. After the drought had lingered for a few months, the small finches ran out of small seeds and began dying off. But the big finches managed to survive, because they could eat seeds that the smaller ones couldn’t get to. (In particular, they depended on a plant called caltrop, which grows spiked shells to protect its seeds.)

The survivors of the 1977 drought mated in 1978, and the Grants could see evolution’s mark on their offspring. A new generation of G. fortis was born, and the Grants’ student Peter Boag discovered that on average their beaks were 4 percent larger than those of the previous generation. The big‑beaked finches, which had fared better during the drought, had passed their trait to their offspring and altered the profile of the entire population.

In the years since the drought, the finches have continued to change. In 1983, for example, a season of heavy rains and abundant seeds favored finches with smaller beaks, and the Grants found that by 1985 their average size had dropped 2.5 percent. The finches can change quickly, but it seems that they are swinging back and forth like a pendulum. After tracking 4,300 medium ground finches on Daphne Island between 1976 and 1993, the Grants have found no overall trend in their beak size. If a finch has a beak that helps it survive its first crucial year of life, it will probably go on to have lots of offspring. But big beaks are good in some years, and small ones are good in others.

Short‑term climate fluctuations can make natural selection drive a population of animals in circles. But under other conditions, it can push them in one direction for a long time. Instead of a cycle of droughts and rains, for example, the climate of an island may become progressively damper for centuries. It’s also possible for a group of finches to settle on an island where other finches are already specialized for eating certain kinds of seeds; in that case, evolution might favor genes in the newcomers that allowed them to eat other kinds of food. They would be able to avoid having to compete with the resident finches and risk being driven to extinction. And in either case, with enough time a new kind of finch might emerge.