Evolution

To an evolutionary biologist, evolution is a change in the proportion of genes present in an existing population. Each individual in a breeding population possesses a unique genotype, the set of paired genetic alternatives that determines its physical attributes, or phenotype. According to a principle known as the Hardy-Weinberg equilibrium, if certain conditions prevail, the frequency of each genotype in the population will be constant over generations, unless evolution has occurred to cause a shift in the gene frequencies. What factors can cause the proportion of genotypes in a population to change?

Natural selection is one example. An organisms environment—including its habitat, diseases, predators, and others of its own kind—present numerous challenges to its survival and to its ability to reproduce. Individual organisms of the same species have slightly different phenotypes (observable attributes), and some of these individuals may be slightly better equipped to survive the hazards of existence than others. The survivors produce proportionally more offspring than do the others of their species, and so their traits will be better represented in subsequent generations. These traits lead to an outnumbered or skewed representation of the offspring compared to less successful individuals (in terms of reproductive capacities) with alternative genetic complements. The genotype that is better adapted to the prevailing conditions will spread throughout the population—at least until conditions change and new genotypes are favored.

Another means by which gene frequencies may change is genetic mutation. When organisms reproduce, a copy of the DNA from each parent is transmitted to the offspring. Normally, the parental DNA is copied exactly; but occasionally, errors occur during replication in their sex cells, the egg and the sperm. Many of these errors occur in non-coding regions of the DNA, and so may have no known effect on the offspring's phenotype; others may be lethal. In some cases, however, an offspring with a slightly modified genetic makeup survives. Mutation is thought to be an important source of new variation, particularly in rare cases where the mutation confers a selective survival advantage or a gain of function. After all, not all mutations are disadvantageous. In fact, natures method for ensuring the survival of a species is to rely on gene mutations that might enhance the function of the protein it encodes in such a way that there is a survival advantage. The determination of this advantage does not only rely on the type of mutation but the environment circumstances that prevail.

Evolution may also occur by genetic drift, a random process in which gene frequencies fluctuate due to chance alone. For example, if a certain genetic background is over-represented in one generation, perhaps because the region happened to be colonized by a small, homogeneous population, these genetic complements are more likely to remain high in the future generations. Genetic drift has more potent effects in small, isolated populations where some characteristic are not necessarily the result of natural selection and therefore do not represent a selective survival advantage.

Finally, the proportions of genotypes in a population may change as a result of migration, resulting in gene flow, or the movement of individuals (and their sex cells) into and out of the population.