Beaks in African black-bellied seedcracker finches are small or large, but not intermediate in size. This is an example of

disruptive selection

Explanation:

Disruptive selection may be defined as a type of a natural selection which selects against some average individual in a given population. These makeup of such a type of the population shows the phenotypes of both the extremes of characteristics but they have very few individuals in the middle.

Disruptive selection is also known as diversifying selection.

In the given context, the beaks of an African seedcracker finches may be small or may be large but they are not of the intermediate size. Such a selection is known as disruptive selection in species.

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