African black-bellied seedcracker finches have beaks that are either large or small.Only large-beaked birds can crack open hard seeds,and small-beaked birds are more adept at handling small seeds.Both have an advantage over intermediate-sized beaks.This is an example of a response to:

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This is an example of disruptive selection because it favors extremes (large or small beaks) over intermediates. The intermediate-sized beaks are not as effective at cracking hard seeds as the larger beaks, nor are they as adept at handling small seeds as the smaller beaks. Therefore, the extremes are selected for while the intermediates are selected against.

African Black-Bellied Seedcracker Finches Have Beaks That Are Either Large