托福口语 Task 3

In biology, organisms often develop traits that are not advantageous for survival but are nonetheless passed on to offspring due to a phenomenon called genetic drift. Genetic drift refers to random fluctuations in the frequency of a gene variant in a small population, driven by chance rather than natural selection. Unlike natural selection, which favors beneficial traits, genetic drift can cause neutral or even slightly harmful traits to become common or disappear purely by luck. This effect is most pronounced in small, isolated populations where random events can drastically alter gene frequencies over generations.

Professor: Let me give you a concrete example from a study on island foxes. On the Channel Islands off California, there are six species of foxes, each on a different island. One island, San Nicolas, has a very small fox population—only about a hundred individuals. Researchers noticed that many of these foxes have a distinctive white patch on their tails, a trait that is completely neutral—it doesn’t help them hide from predators or find food. In fact, on the mainland, where fox populations are large and connected, this white patch is rare, appearing in less than 1% of foxes. But on San Nicolas, over 60% of the foxes have it. Why? Because a few hundred years ago, a storm or a disease might have killed off many foxes, leaving a tiny group. By pure chance, more of the survivors happened to carry the gene for the white patch. As the population recovered, the descendants of those few foxes passed on that gene, and now it’s common. No natural selection favored it—it was just random luck who survived.