Info

- - ►

23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways Each curve plots the distribution of body size in a population before selection (top) and after selection (bottom). Natural selection, by favoring the phenotype shown in yellow in the top graphs, changes the shape and position of the original curve (bottom graphs).

(b) Directional selection changes the mean value of the character (in this case, toward larger size).

(c) Disruptive selection favors both extremes and produces two peaks in the distribution of a character.

Directional Selection

Body size

Body size Body size

23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways Each curve plots the distribution of body size in a population before selection (top) and after selection (bottom). Natural selection, by favoring the phenotype shown in yellow in the top graphs, changes the shape and position of the original curve (bottom graphs).

Body size ample, the distribution of the sizes of individuals in a population, a character that is influenced by genes at many loci as well as by the environment, is likely to approximate the bell-shaped curves shown in the top row of Figure 23.12.

Natural selection can act on characters with quantitative variation in any one of several different ways, producing quite different results:

► Stabilizing selection preserves the average characteristics of a population by favoring average individuals.

► Directional selection changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population.

► Disruptive selection changes the characteristics of a population by favoring individuals that vary in both directions from the mean of the population.

stabilizing selection. If both the smallest and the largest individuals in a population contribute relatively fewer offspring to the next generation than those closer to the average size do, then stabilizing selection is operating (Figure 23.12a). Stabilizing selection reduces variation, but does not change the mean. Natural selection frequently acts in this way, countering increases in variation brought about by genetic recombination, mutation, or migration. Rates of evolution are typically very slow because natural selection is usually stabilizing. Stabilizing selection operates, for example, on human birth weight. Babies born lighter or heavier than the population mean die at higher rates than babies whose weights are close to the mean (Figure 23.13). This was especially true before modern medical advances.

Mean birth weight

Was this article helpful?

0 0
The Ketosis Plan Diet

The Ketosis Plan Diet

Top Low Carb Internet Guru Speaks Out. An Open Letter To Anyone Who Wants To Lose Up To 20 Pounds In 30 Days The 'Low Carb' Way. 30-Day Low Carb Diet 'Ketosis Plan' has already helped scores of people lose their excess pounds and inches faster and easier than they ever thought possible. Why not find out what 30-Day Low Carb Diet 'Ketosis Plan' can do for you by trying it out for yourself.

Get My Free Ebook


Post a comment