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 environmental conditions, selection can even create entirely new phenotypic expressions of a character by establishing new gene combinations over successive
generations that had not existed prior to the change in the environment. In this way selection plays a very creative role in directing the evolution of a group.
But, selection also plays a conservative role in directing the evolution of a group. In a sexually reproducing population, recombination has the tendency to disperse favorable gene combinations and to form less adaptively favorable combinations.
