Variation Of Traits Within A Population

Population genetics is the study of evolution from a genetic point of view. Evolution at the genetic level is sometimes called microevolution, defined as a change in the collective genetic material of a population. Recall that the genetic material of organisms consists of many alleles—or variations—of many genes that code for various traits. Recall that a population consists of a group of individuals of the same species that routinely interbreed. Populations are important to the study of evolution because a population is the smallest unit in which evolution occurs.

Within a population, individuals may vary in observable traits. For example, fish of a single species in a pond may vary in size. Biologists often study variation in a trait by measuring that trait in a large sample. Figure 16-1 shows a graph of the frequency of lengths in a population of mature fish. Because the shape of the curve looks like a bell, it is called a bell curve. The bell curve shows that whereas a few fish in this population are very short and a few are very long, most are of average length. In nature, many quantitative traits in a population—such as height and weight— tend to show variation that follows a bell curve pattern.

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