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.