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Part (a) is a photo of a male lazuli bunting. This bird has a bright blue head and back, white bars across its wings, a pale rust-coloured breast and a white underside. In part (b) a dashed line shows the frequency versus plumage brightness plot (a bell-shaped distribution curve) that would be expected in the absence of disruptive selection. Selection for dull-coloured plumage pulls the curve to the left, while selection for bright plumage pulls the curve to the right. The combined result of these two selection pressures is a curve with two frequency peaks separated by a frequency trough centred on the mean plumage brightness value.

 3.3 Disruptive selection