Evolution (进化) can perform extraordinary makeovers; today's airborne songbirds evolved from the wingless, earthbound dinosaurs that wandered millions of years ago. But some organisms seem to be unchanged—in other words, escape natural selection. The coelacanth, a modern-day fish, is nearly identical to its 410-million-year-old fossils.
Scientists have long wondered how these species do so. It has been assumed that natural selection keeps some species unchanged by selecting for moderate or average qualities (stabilizing selection) rather than selecting for more extreme qualities that would cause a species to change (directional selection).
But a study published in the National Academy of Sciences USA contradicts this idea, showing that evolution constantly favors different qualities in seemingly unchanging animals to improve short-term survival. In the long term, though, "all that evolution cancels out and leads to no change," says the study's lead author, James Stroud.
Stroud and his colleagues studied four lizard (蜥蜴) species, all relatively unchanged for 20 million years. The researchers caught members of these populations every six months for three years. They measured each lizard's head size, leg length, mass and height, as well as the size of its sticky toes (脚趾头), noting which individuals survived. Stroud expected to observe stabilizing selection at work preserving moderate qualities. Instead he saw clearer evidence of directional selection: some lizards with unique characteristics, such as stickier toes, survived better.
The study offers "a good explanation for why we see what we think is stabiliring selection," says Tadashi Fukami, an ecologist studying evolution at Stanford University. Many new qualities are evolving in the short term, but they don't provide a crucial advantage over the long term. In other words, species staying unchanged may simply have found the best possible combination of qualities for lasting success in their environment. So what happens when the lizards' environment changes more dramatically? To help answer this bigger question, Stroud is still making trips to visit the lizards.
