Natural Selection Darwin named his mechanism for evolution natural selection because of its similarities to artificial selection. Natural selection is the process by which organisms with variations most suited to their local environment survive and leave more offspring. In both artificial and natural selection, only certain individuals in a population produce new individuals. But in natural selection, the environment—not a farmer or animal breeder—influences fitness.

When does natural selection occur? Natural selection occurs in any situation in which more individuals are born than can survive (the struggle for existence), there is natural heritable variation (variation and adaptation), and there is variable fitness among individuals (survival of the fittest). Well-adapted individuals survive and reproduce. From generation to generation, populations continue to change as they become better adapted, or as their environment changes. Figure 16–10 uses a hypothetical example to show the process of natural selection. Notice that natural selection acts only on inherited traits because those are the only characteristics that parents can pass on to their offspring.

Natural selection does not make organisms “better.” Adaptations don't have to be perfect—just good enough to enable an organism to pass its genes to the next generation. Natural selection also doesn't move in a fixed direction. There is no one, perfect way of doing something, as demonstrated by Figure 16–11. Natural selection is simply a process that enables species to survive and reproduce in a local environment. If local environmental conditions change, some traits that were once adaptive may no longer be useful, and different traits may become adaptive. And if environmental conditions change faster than a species can adapt to those changes, the species may become extinct. Of course, natural selection is not the only mechanism driving evolution. You will learn about other evolutionary mechanisms in the next chapter.

In Your Notebook Give at least two reasons why the following statement is NOT true: “The goal of natural selection is to produce perfect organisms.”

FIGURE 16–11 No Such Thing as Perfect Many different styles of pollination have evolved among flowering plants. Oak tree flowers (right) are pollinated by wind. Apple tree flowers (left) are pollinated by insects. Neither method is “better” than the other. Both kinds of pollination work well enough for these plants to survive and reproduce in their environments.

Table of Contents