Cracking the AP Biology Exam

12

Evolution

GENETIC VARIABILITY

In Chapter 8, we saw how traits are passed from parents to offspring. You’ll recall from our discussion of heredity that different alleles are passed from parents to their progeny. For example, you might have an allele for brown eyes from your mother and an allele for blue eyes from your father. Since brown is dominant, you’ll wind up with brown eyes. We also saw how these alleles are in fact just different forms of the same gene.

As you know, no two individuals are identical. The differences in each person are known as genetic variability. All this means is that no two individuals in a population have identical sets of alleles (except, of course, identical twins). How did all this wonderful variation come about? Through random mutation.

It might be hard to think of it in this way, but this is the very foundation of evolution, as we’ll soon see. Now that we’ve reintroduced genes, we can refine our definition of evolution. More specifically:

Evolution is the change in the gene pool of a population over time.

THE PEPPERED MOTHS

Let’s take an example. During the 1850s in England there was a large population of peppered moths. In most areas, exactly half of them were dark, or carried “dark” alleles, while the other half carried “light” alleles. All was fine in these cities until air pollution, due primarily to the burning of coal, changed the environment. What happened?

Imagine two different cities: one that was unpolluted, City 1, in the south of the country, and the other that was heavily polluted, City 2, in the north. Prior to the Industrial Revolution, both of these cities had unpolluted environments. In these environments, dark moths and light moths lived comfortably side by side. For simplicity’s sake, let’s say our proportions were a perfect fifty-fifty, half dark and half light. At the height of the Industrial Revolution, City 2, our northern city, was heavily polluted, whereas City 1, our southern city, was nearly the same as before.

In the north, where all the trees and buildings were thick with soot, the light moths didn’t stand a chance. They were impossible for a predator to miss! As a result, the predators gobbled up light-colored moths just as fast as they could reproduce, sometimes even before they reached an age where they could reproduce. However, the dark moths were just fine. With all the soot around the predators couldn’t even see them; they continued doing their thing—above all, reproducing. And when they reproduced, they had more and more offspring carrying the dark allele.

After a few generations, the peppered moth gene pool in City 2 changed. Although our original moth gene pool was 50 percent light and 50 percent dark, excessive predation changed that. By about 1950, the gene pool reached 90 percent dark and only 10 percent light. This occurred because the light moth didn’t stand a chance in an environment where it was so easy to spot. The dark moths, on the other hand, multiplied just as fast as they could.

In the southern city, you’ll remember, there was very little pollution. What happened there? Things remained pretty much the same. The gene pool was unchanged, and the population continued to have roughly equal proportions of light moths and dark moths.