Genetics

Population genetics is the branch of genetics that focuses on the occurrence and interactions of genes in specific populations of organisms. One of its primary concerns is evolution, or how genes change from one generation to the next. By using mathematical calculations that involve an interbreeding population's gene pool (the total genetic information present in the individuals within the species), population geneticists delve into why similar species vary among different populations that may, for example, be separated by physical boundaries such as bodies of water or mountains.

As outlined in the previous section, genetic mutations may cause changes in a population if the mutation occurs in the germ cells. Many scientists consider mutation to be the primary cause of genetic change in successive generations. However, population geneticists also study three other factors involved in genetic change or evolution: migration, genetic drift, and natural selection.

Migration occurs when individuals within a species move from one population to another, carrying their genetic makeup with them. Genetic drift is a natural mechanism for genetic change in which specific genetic traits coded in alleles (alternate states of functioning for the same gene) may change by chance often in a situation where organisms are isolated, as on an island.

Natural selection, a theory first proposed by Charles Darwin in 1858, is a process that occurs over successive generations. The theory states that genetic changes that enhance survival for a species will come to the forefront over successive generations because the gene carriers are better fit to survive and are more likely reproduce, thus establishing a new gene pool, and eventually, perhaps, an entirely new species. One proposed mechanism of natural selection is gradualism, which predicts very slow and steady accumulation of beneficial genes. Punctuated equilibrium, in contrast, depicts natural selection as occurring in brief, but accelerated periods of "survival of the fittest" with lengthy periods of relative stagnation of genetic change in populations. Some scientists hold that both processes occur and have occurred.