Genetic Variation within Populations

For a population to evolve, its members must possess heritable genetic variation which is the raw material on which agents of evolution act. In everyday life, we do not directly observe the genetic compositions of organisms. What we do see in nature are phenotypes, the physical expressions of organisms’ genes. The features of a phenotype are its characters — eye color, for example. The specific form of a character, such as brown eyes, is a trait.Aheritable trait is a characteristic of an organism that is at least partly determined by its genes. The agents of evolution generally act on phenotypes but for the moment we will concentrate on genetic variation within populations. We will do so because genetic variation is what is passed on to offspring via gametes — eggs and sperm. The genetic constitution that governs a character is called its genotype. A population evolves when individuals with different genotypes survive or reproduce at different rates. Those different forms of a gene, called alleles, may exist at a particular locus. A single individual has only some of the alleles found in the population to which it belongs. The sum of all copies of alleles found in the population constitutes its gene pool. The gene pool contains the variation that produces the phenotypic characters on which agents of evolution act. To understand evolution, we need to know how much genetic variation populations have, the sources of that genetic variation, and how genetic variation is maintained and expressed in populations over space and time.

Most populations are genetically variable

Nearly all populations contain some level of genetic variation for many characters. Artificial selection on different characters in a European species of wild mustard produced many important crop plants. Plant and animal breeders could achieve such results because the original population had genetic variation for the characters of interest.

Laboratory experiments also demonstrate the existence of considerable genetic variation in populations. In one such experiment, investigators chose fruit flies (Drosophila melanogaster) with either high or low numbers of bristles on their abdomens as parents for subsequent generations of flies. After 35 generations, all flies in both the high-bristle and lowbristle lineages had bristle numbers that fell well outside the range found in the original population. Thus, there must have been considerable variation in the original fruit fly population for selection to act on. The study of the genetic basis of evolution is difficult because genotypes do not uniquely determine phenotypes. With dominance, for example, a particular phenotype can be produced by more than one genotype (e.g., AA and Aa individuals may be phenotypically identical). Similarly, different phenotypes can be produced by a given genotype, depending on the environment encountered during development. For example, the cells of all the leaves on a tree or shrub are normally genetically identical, yet leaves on the same tree often differ in shape and size. Leaves closer to the top of an oak tree, where they receive more wind and sunlight, may be more deeply lobed than the shaded leaves growing lower down on the same tree. The same differences can be seen between the leaves of individuals growing in sunny and in shady sites. Thus, the phenotype of an organism is the outcome of a complex series of developmental processes that are influenced by both the environment and its genes.