3.1 Directional selection

Directional selection occurs when conditions favour individuals expressing one extreme of a phenotypic range, thereby shifting a population's frequency distribution for the phenotypic character in one direction or the other. For example, in long tailed widowbirds (Euplectes progne, Figure 13a) males with long tails have a reproductive advantage over males with shorter tails because females prefer to mate with males with long tails. Directional selection therefore favours long tails and may shift the distribution of the population so that tail length increases over time (Figure 13b).

Figure 13 (a) The long tailed widowbird (Euplectes progne). (b) Because directional selection favours long tails, over time the distribution may shift to the right (shown by the arrow) so that mean tail size increases.

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Part (a) is a photo of a long-tailed widowbird in flight. It is almost entirely black with orange and white markings on the upper part of the wings. It has a bluish-white bill and a long tail with numerous tail feathers. Part (b) shows two partially overlapping bell-shaped frequency versus tail length plots (distribution curves). For each plot, the mean length, corresponding to the frequency peak, is marked as a vertical line. A left-to-right arrow spanning these two lines indicates a shift of the mean tail length to a larger value.

Figure 13 (a) The long tailed widowbird (Euplectes progne). (b) Because directional selection favours long tails, over time the ...

With respect to migration, studies have shown that under strong directional selection, populations can switch from sedentary to migratory or vice versa relatively quickly and that factors such as climate change could lead to the evolution of sedentariness in many bird populations that are currently migratory.