Cracking the AP Biology Exam
CAUSES OF EVOLUTION
Natural selection, the evolutionary mechanism that “selects” which members of a population are best suited to survive and which are not, works both “internally” and “externally”: internally through random mutations and externally through environmental pressures.
To see how this process unfolds in nature, let’s return to the moth case. Why did the dark moths in the north survive? Because they were dark-colored. But how did they become dark-colored? The answer is, through random mutation. One day, a moth was born with a dark-colored shell. As long as a mutation does not kill an organism before it reproduces (most mutations, in fact, do), it may be passed on to the next generation. Over time, this one moth had offspring. These, too, were dark. The dark- and the light-colored moths lived happily side by side until something from the outside—in our example, the environment—changed all that.
The initial variation came about by chance. This variation gave the dark moths an edge. However, the edge did not become apparent until something made it apparent. In our case, that something was the intensive pollution due to the burning of coal. The abundance of soot made it easier for predators to spot the light-colored moths, thus effectively removing them from the population.
Eventually, over long stretches of time, these two different populations might change so much that they could no longer reproduce together. At that point, we would have two different species, and we could say, definitively, that the moths had evolved. As a consequence of random mutation and the pressure put on the population by an environmental change, evolution occurred.
TYPES OF SELECTION
The situation with our moths is an example of directional selection. One of the phenotypes was favored at one of the extremes of the normal distribution.
In other words, directional selection “weeds out” one of the phenotypes. In our case, dark moths were favored and light moths were practically eliminated. Here’s one more thing to remember: Directional selection can happen only if the appropriate allele—the one that is favored under the new circumstances—is already present in the population. Two other types of selection are stabilizing selection and disruptive selection.
Stabilizing selection means that organisms in a population with extreme traits are eliminated. This type of selection favors organisms with common traits. It “weeds out” the phenotypes that are less adaptive to the environment. A good example is birth weight in human babies. If babies are abnormally small or abnormally large, they have a low rate of survival. The highest rate of survival is found among babies with an average weight.
Disruptive selection, on the other hand, does the reverse. It favors both the extremes and selects against common traits. For example, females are “selected” to be small and males are “selected” to be large in elephant seals. You’ll rarely find a female or male of intermediate size.