A thin-shelled crab can more readily move to escape a predator than can a thick-shelled crab, but it is more vulnerable to predators that drill through the shell. Because of these opposing forces, shell thickness for many types of crabs has remained within a narrow range over a long time. This would be an example of ___.

Stabilizing selection

Explanation:

Stabilizing selection is the most common form of natural selection that is not easy to notice in a population as the change is less drastic.  It occurs when average or intermediate phenotypes of a trait in a population are favored, while the extreme phenotypes of that trait are not favored by the forces of natural selection. Over time, intermediate or non-extreme traits become more common in the population, while extreme traits become less common.

the  frequency of a wave represents  the number of wavelengths that pass a specific point per second. the unit for  frequency  is hertz (hz).

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