Thank you for the opportunity to help you with your question!
It is directional selection.
the types are stabilizing, directional, and disruptive.
it's directional because if you think about it, the lighter the bones
are the better adapted the bird will be, so natural selection will just
keep selecting for the lightest bones out of every generation. this will
cause the average bone weight to get progressively lighter over
Please let me know if you need any clarification. I'm always happy to answer your questions.
Aug 3rd, 2015
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