 In the 1950s, a lively debate broke out among biologists that continues to this day, over what might seem like the most unlikely of organisms: the land snail, Cepaea nemoralis. Yet, there are in fact some interesting aspects to C. nemoralis. This species of snail is polymorphic; the snail’s shell varies in color (pink, brown, and yellow) as well as the number of visible bands (anywhere from 0–5). But the colors and bands are not equally distributed across populations. In some populations, pink predominates, whereas in others, yellow or brown, and similarly, some banding numbers are more prevalent in some populations than in others. Thus, not only are there variations within populations (it is rare to find a population that is all one color or where all the snails have the same number of bands), but there are variations between populations. What is the explanation for this distribution of forms? Those whose knowledge of evolution familiarized them only with the theory of natural selection might assume, for example, that in the populations where yellow snails were the most prevalent, it was because they were fitter than the other colors—that there was some environmental factor that favored yellow over brown and pink. And that in the populations where brown snails were the most prevalent, there was some difference in the environment that led them to be favored over yellow and pink snails. But is there some other explanation? Perhaps the distributions are in some sense due to chance, perhaps even in a way that can be modeled mathematically. What would that mean, and how would you determine which explanation was correct? The attempt to develop “chancy” explanations that are alternatives (perhaps complementary alternatives) to those due to natural selection is what led biologists to develop models of genetic drift.