3.2 Stabilising selection

Stabilising selection occurs where there is an optimal value for the trait of interest. This means that selection acts against both extreme phenotypes and favours intermediate variants which have the highest fitness (survive to reproduce). For example, goldenrod gall flies (Eurosta solidaginis, Figure 14a) lay their eggs individually in developing shoots of the tall goldenrod plant (Solidago sp.). The fly larva that hatches from the egg makes a chemical that causes the plant tissue to swell around it, and the result is a mass of plant tissue called a gall (Figure 14b), which serves as a home for the larva. Downy woodpeckers attack large galls and eat the gall fly larva. Parasitic wasps lay eggs inside small galls and the wasp larvae that hatch from the eggs eat the gall fly larva when it hatches. Only galls of intermediate size remain intact because the wall of the gall is too thick for the ovipositor of a parasitic wasp to penetrate, and the size of the gall and hence the larva within makes it less attractive to woodpeckers than a larger gall containing a larger larva and hence a larger meal. So, selection pressures act on gall size and only larvae in galls of intermediate size survive (Figure 14c).

Maximise

Figure 14 (a) Goldenrod gall fly. (b) Gall on the goldenrod plant in which gall fly larvae hatch. (c) Stabilising selection favours intermediate-sized galls, since these are not attacked by wasps or birds. The distribution of gall sizes therefore 'stabilises' around an optimal trait - in this case, intermediate gall size.

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Part (a) is a photo of a goldenrod gall fly, a small brown fly about five millimetres long. Part (b) is a photo of a gall on the goldenrod plant; this is visible as a hard ball-shaped swelling of the stem. In part (c), a dashed line shows the frequency versus gall size plot (distribution curve) that would be expected in the absence of stabilising selection. Selection by parasitic wasps for small galls leads to a lower frequency of small gall sizes, steepening the left arm of the distribution curve (as indicated by the rightward arrow). Balancing this effect is selection by woodpeckers for large gall sizes, which leads to a lower frequency of large gall sizes, steepening the right arm of the distribution curve (as indicated by the leftward arrow). The distribution curve with this stabilising selection (shown as a continuous line) is thus narrower and taller (greater frequency of the mean size) but the mean value is unchanged.

Figure 14 (a) Goldenrod gall fly. (b) Gall on the goldenrod plant in which gall fly larvae hatch. (c) Stabilising selection favours ...