Better adapted genotypes replace less adapted genotypes in an altered environmental situation. For example, the local climatic change associated with forest thinning favors colonizing species over climax species.

process by which one state or value of a trait becomes more common in a population.

A process of natural selection that tends to favor phenotypes at one extreme of the phenotypic range. PICTURE

Preferential change in a population, favoring the increase in frequency of one allele over another.

selection resulting in a shift in the population mean in the direction desired by the breeder or in the direction of greater adaptation by nature

Selection that changes the frequency of an allele in a constant direction, either toward or away from fixation for that allele.

Selection for a higher or lower value of a character than its current mean.

A selective process that changes the frequency of an allele in a specific direction, either toward fixation or toward elimination.