Arrival of the Fittest: Solving Evolution's Greatest Puzzle Read Online Free

inheritance everywhere around them, in the yield of crops, the weight of eggs, the sizes of leaves—in brief, in most features of organisms. 25 This kind of variation is clearly important in nature.
    The controversy raged around the question of which kind of variation, continuous or discrete, was more important for evolution. The naturalist or gradualist school of thought—Darwin was an early adherent—emphasized the small, continuous variation that we see all around us. The other school—“Mendelists,” “mutationists,” or “saltationists”—believed in the large, discrete variants that Mendel had studied. In a cartoon version of this dispute, a gradualist would imagine that the many petals of a garden rose emerged from its five-petaled ancestors through gradual additions of petals over many generations. A mutationist, on the other hand, would argue that the multifoliate rose could have appeared in a single saltational “macromutation” from this ancestor. 26
    Looking back, this debate seems just as important as the question that kept medieval scholastics busy: How many angels can dance on the head of a pin? But it just about pierced the heart of Darwinism. For the Mendelists believed less in natural selection than in the power of mutations to bring forth new traits. In their view, the real drivers behind life’s evolution were large mutations that created individuals far outside the norm of their species. “Hopeful monsters” is what the German-born zoologist Richard Goldschmidt would call them, citing as one of his examples the benthic flatfish that live on the ocean floor, which have both eyes on the same side of the head. 27
    Although the Mendelists would turn out to be wrong—most evolutionary change does indeed occur gradually and involves natural selection—they did have a point. The real mystery of evolution is not selection, but the creation of new phenotypes. But they were born too early. They could speculate wildly, but had no way to solve the mystery, and the controversy between the two camps continued well into the twentieth century until powerful new insights would dissolve it. That process began when a long-known fact became newly appreciated: Genetic change happens not just in individuals, but in populations.

     
    The white-bodied peppered moth is a perfectly inconspicuous insect whose white wings are sprinkled with flecks of black. Against a background of tree bark and lichen, this mottled pattern camouflages the moth against ravenous birds. In some moths, a gene affecting wing color can mutate to produce a dark-colored wing. This mutation is usually bad news for a moth, because mutant moths are no longer camouflaged, and birds can rapidly pick them off. But in nineteenth-century England the Industrial Revolution gave the dark mutant moths a much-needed break. During this time, air pollution became so severe that it wiped out most lichen and turned tree bark black. Now the dark moths were well hidden, and the white moths had turned into bird food.
    If natural selection mattered, we would expect that the black moths would become more frequent over time. They would sweep through a moth population, whereas white moths would become rare. This is indeed what happened in nineteenth-century England, as the proportion of black moths in the population rose from 2 percent in 1848 to 95 percent by 1895. 28 But this information isn’t nearly as important as the questions it triggers: Can we predict how rapidly they sweep through the population? Or conversely, if we have observed how fast they sweep, can we infer how strongly the dark color affects fitness, a moth’s chances of remaining hidden from birds? These were quantitative, mathematical questions, new to evolutionary thinking. And they created a new quantitative discipline within biology:
population genetics
.
    One of the central insights of population genetics is to view a population not just as a collection of distinct
organisms
but as a