Explain the concept of "directional selection." How do changing environmental conditions act as selection pressure on the phenotypic characters of a group? Explain how selection pressure can play a very creative role in the evolution of a group.
Most phenotypic characters on which selection acts are determined by a variety of different genes many of which have multiple alleles within a population. Because phenotypic characters are greatly influenced by environmental conditions, the characters often show a wide variation of expressions. Expressions favored by the environment are more prevalent within the population while expressions less favored by the environment are less prevalent. When graphed, the frequency of phenotypic expressions approximates a bell-curve in shape. Changes in environmental conditions cause shifts in selection pressures which, as selection pressures affect allelic frequencies, cause the curve of phenotypic variations to shift as well. In other words, new environmental conditions tend to favor a different allele or gene combination and, thus, a new phenotypic expression becomes more prevalent while the phenotypic expressions coded for by alleles or gene combinations that used to be favored by the old environmental conditions and that are now less favored become less prevalent. The shift in prevalent expressions of a phenotypic character caused by changing environmental conditions is called directional selection. In response to changing
