Darwin's Legacy and Mendel's Mechanism

"Nothing in biology makes sense except in the light of evolution."

THEODOSIUS DOBZHANSKY, 1973

HURRICANE KATRINA was a new kind of experience for most Americans—a huge natural disaster that demonstrated how poorly prepared the United States was, in 2005, for natural disasters. It also underlined that something strange is going on with the weather, even if Katrina's destructiveness itself might just have been a rare event in normal variability in the size and paths of hurricanes. If you regularly watch TV, read newspapers, or go to the movies, you can hardly have missed news about unusual weather. Global temperatures are going up, glaciers are melting, storms seem more frequently intense and so do droughts, and sea level is gradually rising.

Other animals are noticing too, at some level. Polar bears are finding it more difficult to make a living as the sea ice from which they hunt seals disappears. Coral animals in some places are dying as seawater warms, threatening the existence of coral reefs. If you are a bird-watcher, you might have noticed that some migratory birds from Latin America are arriving earlier each spring on their North American breeding grounds. In the Yukon of Canada, red squirrels are having their young earlier because of the great quantities of spruce seeds made available by a warming climate. In Europe, flowers are blooming about a week earlier in spring than they did in 1975—and in 2006 they were still blooming in Moscow in November.

POPULATIONS EVOLVING

That populations of organisms do not remain static in the face of environmental change is a recurring theme in the study of the natural world. Today, numerous animals and plants are changing their lives in response to the human-caused alteration of climatic regimes—the annual sequence of changes in temperature and rainfall—in their environments. While the average temperature increase of Earth has been less than a degree Celsius (?°C is 1.8°F) over the past fifty years, in the same period at the high-altitude laboratory in Colorado where we've worked since 1960, magpies have arrived from the lowlands for the first time and many flowers are blooming earlier, all apparently in response to earlier spring melting of the alpine snow.

Pitcher-plant mosquitoes (Wyeomyia smithii) are a good example of such climate-related changes. They inhabit eastern North America, and their early stages (eggs, larvae, and pupae) all live in water trapped in the "pitchers" of a carnivorous plant, Sarracenia purpurea. The plants grow primarily in nitrogen-poor soil and supplement their "diet" by digesting insects, mostly ants and flies, that die when trapped by downward-pointing hairs in their tubular leaf, which the small, hovering mosquitoes easily avoid. Southern populations of the mosquitoes produce five or more generations per year; northern populations, just one.

The larvae (wigglers), which hibernate in the leaves of the host plant, use the length of the day to determine when to enter that dormant state and when to resume activity. The critical day lengths at which the mosquitoes carry out these functions are quite rigidly controlled by their genes. But over the past thirty years, the genetically controlled clocks of more northerly populations have shifted their response so that hibernation does not begin until the day length becomes even shorter. As a result, the northern populations now behave much as the southern populations do, waiting until later in the fall before hibernating. Without that shift, as a warming climate in North America lengthens the growing season, the larvae would hibernate too soon. They would need to survive on the fat they had stored—not just survive through the winter as previously, but also without feeding through a warm period at the end of summer. This would lessen their chances of being alive when warm weather returned in the spring. With the shift, their necessary hibernation is shorter, helping them to survive without eating.

Other animals have not been able to change their behavior in response to a warming climate. Some populations of pied flycatchers (Ficedula hypoleuca), an attractive brown, black, and white insect-eating bird in Europe, have declined in size by 90 percent. The reason? The peak of insect abundance is occurring ever earlier as the environment warms, and the birds' customary breeding time is now too late for their offspring to have an optimal food supply. They have not successfully adjusted to the challenge of climate change, but neither have their populations remained static; they, along with other slow adjusters, are blinking out.

DARWIN AND WALLACE'S GREAT IDEA

Why do some organisms adjust successfully to environmental change while others do not? For a biologist, there is no more basic or fascinating question, and it now has a pretty good answer—thanks in large part to scientific foundations laid by two Victorian Englishmen. In 1859, Charles Darwin (1809–82) and the brilliant polymath Alfred Russel Wallace (1823–1913) simultaneously proposed the first basically correct model for the mechanism that causes shifts such as that in mosquito hibernation. Historically, Darwin has received most of the credit for this world-changing idea—justifiably, since he supported his conjecture with an abundantly documented book, On the Origin of Species. He had formed many of his ideas on the basis of a five-year trip around the world as a naturalist on the British naval survey ship HMS Beagle (1831–36) and had subsequently corresponded about them with numerous colleagues.

It came as a shock to him when, in 1858, Wallace sent him a paper outlining Darwin's basic idea. Darwin, at the urging of friends, had his and Wallace's idea presented jointly at a meeting of a scientific society. But he followed this in 1859 with Origin, which sold out in a day and cemented his reputation as the greatest of all biologists. Like many great ideas, the Darwin-Wallace theory, which has come down to us identified by the term "natural selection," was disarmingly simple. Basically, it recognized that variation ordinarily exists among individuals within a...