▸ Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection occurs. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than are others.

Consider how limited resources, such as food, can affect individuals' fitness. Among seed-eating birds such as Darwin's finches, birds with bigger, thicker beaks can feed more easily on larger, harder, thicker-shelled seeds. Suppose the supply of small and medium-size seeds runs low, leaving only larger seeds. Birds with larger beaks would have an easier time feeding than would small-beaked birds. Big-beaked birds would therefore be more successful in surviving and passing genes to the next generation. Over time, the average beak size of the population would probably increase.

▸ Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end, stabilizing selection takes place. This situation keeps the center of the curve at its current position, but it narrows the curve overall.

For example, the mass of human infants at birth is under the influence of stabilizing selection. Very small babies are likely to be less healthy and, thus, less likely to survive. Babies who are much larger than average are likely to have difficulty being born. The fitness of these smaller or larger babies is, therefore, lower than that of more average-size individuals.

FIGURE 17–7 Selection on Polygenic Traits Natural selection on polygenic traits has one of three patterns—directional selection, stabilizing selection, or disruptive selection.

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▸ Disruptive Selection When individuals at the outer ends of the curve have higher fitness than individuals near the middle of the curve, disruptive selection occurs. Disruptive selection acts against individuals of an intermediate type. If the pressure of natural selection is strong and lasts long enough, this situation can cause the single curve to split into two. In other words, disruptive selection creates two distinct phenotypes.

Suppose a bird population lives in an area where medium-size seeds become less common and large and small seeds become more common. Birds with unusually small or large beaks would have higher fitness. As shown in the graph, the population might split into two groups: one with smaller beaks and one with larger beaks.

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