17.3 Population genetics

Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms. Populations with two or more variations of particular characteristics are called polymorphic. The distribution of phenotypes among individuals, known as the population variation     , is influenced by a number of factors, including the population’s genetic structure and the environment ( [link] ). Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures.

Figure 19.2

17.3a patterns of natural selection

As mentioned previously, natural selection is the preferential survival and reproduction of individuals who are most well adapted to their environment. Over time, natural selection generally weeds out unfavorable alleles, and increases the frequency of favorable ones. However, sometimes it's not so simple as an "unfavorable allele" vs. a "favorable allele."

Balanced polymorphism and heterozygote advantage

Sometimes an allele provides a benefit only in certain environments, like the sickle-cell allele. Two copies of the sickle-cell allele give a person sickle-cell anemia, which causes their blood cells to be shaped like a sickle. This reduces the oxygen they can carry, and can cause severe pain and possibly an early death. You would expect this allele to not be favored by natural selection, and in most environments that would be a correct prediction. However, in places where malaria is common, having one or both sickle-cell alleles is actually a benefit. The parasite that causes sickle-cell anemia can live inside of healthy blood cells, but not so much the sickled ones. Therefore, people with both copies of the sickle-cell allele are reistant to malaria, which can be fatal. Because the sickle-cell allele protects against malaria, and because the normal blood cell allele means an individual does not have sickle cell anemia, both of these alleles are favored in areas where malaria is prevalent. Selection for two different alleles keeps both of them in the population -- neither allele is expected to be lost forever. This is called balanced polymorphism     .

Individuals who carry one sickle-cell allele and one normal blood cell allele do not exhibit sickle cell anemia. Some of their blood cells take on the sickled shape, but not enough to cause problems. These people are said to have "sickle cell trait." They are also resistant to malaria. Therefore, being heterozygous for these alleles in an area where malaria is prevalent is the best possible genotype. These individuals are unlikely to die from malaria, and they won't die from sickle-cell anemia. In these areas, the heterozygote is the most fit, a pattern called heterozygote advantage     . However, in places where malaria is not prevalent, their advantage goes away. In these areas, they are just as fit as individuals homozygous for the normal blood cell allele.