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Zo 6.7

Types of Selection


Stabilizing Selection

¨      ______________________________________________________________________________

¨      Genetic diversity __________________________________

¨      Ex: Human birth weight

¤      Babies of low weight become sick more easily

¤      Babies of high weight have a harder time being born


Stabilizing selection takes place when individuals near the center of a curve have higher fitness than individuals at either end.  This example shows that human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average.


Directional Selection

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¨      Genetic diversity ____________________________________

¨      Ex: beak sizes

¤      If food supply runs low, than birds may have to find different food, and individuals with a large or small beak may have higher fitness

Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.  In this example, a population of see-eating birds experiences directional selection when a food shortage causes the supply of small seeds to run low.  The dotted line shows the original distribution of beak sizes.  The solid line shows how the distribution of beak sizes would change as a result of selection.


Disruptive Selection

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¨      Genetic diversity _________________________________________

¨      Ex:  seed-eating birds

¤      If medium sized seeds become scarce, birds with large and small beaks will have a higher fitness


When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.  In this example, average-sized seeds become less common, and larger and smaller seeds become more common.  As a result, the bird population splits into two subgroups specializing in eating different-sized seeds.

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