Is natural selection the prime mover behind evolution? Darwin’s great insight into the mechanics of evolution was that a population of creatures always has a lot of variation--more feathers here, less fat there, more urge to kill there--and some of these variations allow the individuals bearing them to thrive and have more offspring than others. After many generations these traits become more common among the population as a whole. If one imagines fitness as a beckoning peak on a given ecological landscape, then natural selection should be a process that moves species steadily uphill.
Countless experiments have proved that natural selection is real and quite powerful. Yet given the complexity of the interplay of genes that determine fitness, biologists have always suspected that natural selection seldom guides a species straight up the evolutionary heights. It has been hard, though, to find data to support such suspicions--until now.
Biologist Dolph Schluter of the University of British Columbia has been studying three-spined sticklebacks, a genus of fish that lives in western Canada. When glaciers pulled back from the region 13,000 years ago, sticklebacks invaded the newly carved lakes and over time formed new species. Schluter captured some individuals from one species that has evolved into a big, bulky, and mean-looking form that makes its living by sucking sediment into its wide mouth. Schluter measured a number of traits important to its way of life and then bred the fish. When their offspring matured, he repeated the measurements. Naturally, the mixing of genes meant that the second generation was not a carbon copy of its parents--the range of different forms was an expression of the built-in variability of stickleback genes.
But the variations between the generations were not random; instead, all the traits varied together. A stickleback that was unusually long was also unusually fat and had a wide mouth. Similarly, a short stickleback was invariably slender and had a narrow mouth. Variation works like this because of the way genes build our bodies. A wide mouth is the product of many genes, not just one, and many of these genes play a part in creating other traits as well.
The odd thing about this pattern of variability among individual sticklebacks is that the species as a whole has evolved in the same manner. The fat, wide-mouthed species that Schluter studied descended from a shorter, slimmer, smaller-mouthed species that first invaded the region’s lakes. The traits of shortness, slimness, and narrow mouths have remained linked in the sticklebacks for at least 13,000 years. Even though natural selection might favor, say, a long, slender, wide-mouthed stickleback, the linked genes for these traits prevent--for millennia--such a form from arising.
The connection between evolution and variability was no coincidence, Schluter found. He looked at similar measurements taken from other species, such as sparrows, finches, and mice. In every case, evolution was biased toward the kinds of body shapes produced most easily by the variability of the genes. For some of the species he studied, the genetic constraints persisted for 4 million years. This genetic rein on natural selection, Schluter’s work shows, is far more persistent than anyone had thought.
Schluter likes to call the bias the genetic path of least resistance. As natural selection tries to change a feature on an animal-- say, the mouth’s width--other traits change with it because the same genes control many traits. It’s easier to change along some directions than others, says Schluter.
Over very long periods of time, says Schluter, natural selection does eventually break down the constraints imposed by such linked genetic traits. It tries to steer the species steadily upward, but linked variability inevitably delays this upwardly mobile tendency for millions of years. We do expect a species to get to the point where selection is directing it, says Schluter. It’s just going to go in a roundabout way.