Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Disruptive (diversifying, centrifugal)selection is the opposite of stabilizing selection (Fig. IV.7).In this case, individuals with an average value of the trait are affected most and individuals with values far from the average are affected least (however, this need not necessarily be the most extreme groups, e.g. the largest and smallest organisms).This situation occurs, e.g., when the members of a single species exhibit two different life strategies.For example, small individuals are capable of hiding from predators, while large individuals cannot fit in the available hiding places but can try to fight with predators, with greater or lesser success.Medium-sized individuals are at a disadvantage – they cannot fit in hiding places and they are not strong enough to fight predators.

            A similar situation can occur in species using mimicry.If the forest contains dark-coloured spruce trees and light-coloured birch trees, it is advantageous for a butterfly to be either dark or light in colour, to optically merge with the bark of spruce or birch trees.Butterflies with medium-coloured wings are easily visible on both spruce and birch trees.

            Disruptive selectionis disadvantageous from the standpoint of the population and of a typical individual because it has the greatest effect on the most numerous frequency class.Thus, it is probable that this kind of selection pressure will sooner or later lead to the development of genetic, ethological or other mechanisms that reduce the frequency of individuals with an average value of the given trait.For example, it can increase the importance of one of the genes determining the value of the trait, so that value of the trait will finally be determined predominantly (or exclusively) by a pair of alleles, one of which will be dominant and the other recessive.Preferential mating between individuals with the same phenotype (positive assortative pairing) is an example of an ethological mechanism.This mechanism could possibly lead to speciation, in which two new species can be formed from one original polymorphous species through disruptive selection.