IV.6.2.3 Directional selection removes individuals with values of a trait at either end of the distribution curve from the population.

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 selectionleads 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.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.