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The Collective Set of Alleles in a Population Is Its Gene Pool

A schematic shows the dorsal side of 14 fruit flies in silhouette with their wings outstretched. Eight flies have a brown body color, red eyes, and normal wings. One fly has a brown body color, brown eyes, and normal wings. One fly has a brown body color, brown eyes, and short wings. One fly has a brown body color, red eyes, and short wings. One fly has a black body color, brown eyes, and normal wings. One fly has a black body color, red eyes, and short wings. One fly has a black body color, red eyes, and normal wings.

The fact that genes exist in alternate forms, called alleles, forms the basis for the study of population genetics. Populations are made up of members of the same species that interbreed. Population geneticists study the variation that naturally occurs among the genes within a population. The collection of all the genes and the various alternate or allelic forms of those genes within a population is called its gene pool.

What is variation in a gene pool?

Genetic variation within a population is measured according to the number of different alleles of all genes and the frequency with which they appear. Variation is high when there are many different allelic forms of all genes and when there are many different combinations of those alleles. However, genetic variation is constantly changing. Different allelic forms of a single gene can appear and disappear from time to time within a single group of organisms. This means that the gene pool of a population is dynamic and can change at any moment for a variety of reasons. In addition, the rate of change within a gene pool can vary at different points in time.

Nineteen black letters of random sizes are shown in a random arrangement on a white background. They include: five uppercase B's; four lowercase b's; three uppercase W's; two lowercase w's; three uppercase V's; one lowercase v, and one lowercase a.

Figure 1: The concept of a gene pool includes multiple alleles in a group of organisms.

Consider for a moment a population of the fruit fly (Drosophila melanogaster). The collective set of genes in the fruit fly population includes a variety of allelic forms: B and b, V and v, W and w, and more (Figure 1). Accounting for all of these alleles and their relative proportions within the population are measures of genetic variation. Scientists, however, cannot always determine which alleles are present in a population based on outward appearance (i.e. phenotypes) of the organisms that comprise it. Some phenotypes associated with certain alleles are masked or deemphasized when other alleles are present, or are less visible under certain environmental conditions. For this reason, it is important to understand a gene pool as the collective set of alleles whose phenotype may or may not be observable.

A photograph montage shows two rows of images separated into three columns. Each column represents a species of butterfly: Araschnia levana is shown in the far left column, Precis octavia is shown in the middle column, and Bicyclus anynana is shown in the far right column. The top and bottom rows show the same butterflies born in different seasons.

Figure 2: Genetic variation within three butterfly species. Three different butterfly species (top row) show distinct wing colors and patterns. When individuals from the same three species are born in a different season, they each show different wing color and pattern phenotypes (bottom row). This is a reflection of the variation that exists in the gene pool.

Figure Detail

Tropical butterflies provide an excellent example of genetic variation within species. In these butterflies, temperature and light can influence gene expression. Consequently, wing color and pattern can vary depending on the season during which a butterfly is born. The top row of Figure 2 shows examples of three different butterfly species. The bottom row shows butterflies from the same three species, but these individuals were born under different temperature and light conditions than those in the top row. How can this happen? The differences in wing colors and patterns of butterflies of the same species reflect the underlying genetic variability within a population. Even though each butterfly within a species has the potential to develop the wide variety of colors and patterns shown above, its environment influences the phenotypic expression of its genetic characteristics. The gene pool of each species, therefore, contains a collection of many different alleles whose phenotype may or may not be observable.

Can gene pools vary within populations?

From the perspective of a geneticist, a population is a group of organisms of the same species that interbreed. This may mean that the group of organisms all live in the same area, or that they can travel over long distances to mate. Over time, a population's gene pool may change. A variety of different factors can account for these changes, including migration of new individuals into the population, death of a large number of individuals within the population, or environmental factors that favor certain traits over others.

The factors that affect the composition of the gene pool are shaped both by the physical environment and by time. Consequently, the definition of populations can be refined to include groups of organisms whose genetic makeup can change over time, and groups of organisms that tend to interbreed.

Further Exploration

Key Questions

How can mutation cause genetic variation?

Key Concepts

genetic variation | species

eBooks

This page appears in the following eBook

Essentials of Genetics, Unit 5.2

A Brief History of Genetics: Defining Experiments in Genetics, Unit 9.2