Selection disruptive, stabilizing and directional
If we monitor the values of a certain quantitative trait, e.g. body length, in a large population of organisms, we usually find that this trait had normal distribution in the population. There are very few very large and very small individuals in the population, while there are the greatest numbers of individuals of medium size. This is a result of the fact that a quantitative trait is mostly determined by a large number of relatively independent and mutually replaceable genes. It follows from the rules of combinatorics that only a very small number of individuals inherit from all their genes those alleles that have an identical effect, e.g. cause a larger body size. Most individuals inherit part of the alleles causing larger size and part causing smaller size, so that their phenotype will approach the average. If we compare the distribution of a given quantitative trait (number of individuals in individual size classes) prior to commencing the process of natural selection and after its termination, then we frequently find substantial differences. Three types of natural selection can be distinguished on the basis of the character of these differences (Fig. IV.7).
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.
We most often encounter a situation where the distribution of the frequency of individual phenotypes prior to selection and after selection have the same mean (same position of the maximum frequency); however, the distribution following selection is much narrower, as particularly individuals with extreme values of the monitored trait (the smallest and the largest) were removed from the population. This type of selection is called stabilizing or normalizing or centripetal (Fig. IV.7)..If the population is present under unchanging conditions, there is usually an optimal value of each quantitative trait, for example optimal body length. During evolution, the action of natural selection generally establishes a frequency of the alleles of the individual genes affecting the particular quantitative trait, so that most of the progeny formed through genetic recombination exhibit the optimal or almost optimal phenotype and are thus least affected by natural selection. Šmalgauzen and Waddington (Waddington 1953a)used the term stabilizing selection in a somewhat different sense (selection of alleles reducing the ability of aberrant genes to affect the phenotype).
The third type of natural selection of quantitative traits consists in directional selection (Fig. IV.7). In contrast to the two previous types of selection, in this case selection leads to a shift in the frequency maximum towards the left or the right. Directional selection leads to a change, not only in the average value of a particular trait, but also a change (decrease or increase in size) in the variability of the given trait in the population.
A shift in the frequency maximum occurs when natural selection preferentially eliminates individuals with a certain extreme value of a trait (largest or smallest). Through the action of directional selection, a species gradually changes, for example organisms become either larger or smaller. It is clear that this must be a temporary situation from the standpoint of evolution (although it sometimes lasts a very long time). This can most frequently be a reaction to a change in living conditions, a change in a biotic or abiotic factor. In this case, over time, the individuals attain a new optimum value of the particular trait and will remain in the vicinity of this value through stabilizing natural selection.