8.15: Natural selection can change the traits in a population in several ways.

Certain traits are easily categorized—blue eyes and brown eyes, or white tiger fur and orange tiger fur, for example. Other traits, such as height, are influenced by many genes and environmental factors, so a continuous range of phenotypes occurs (FIGURE 8-25) Whether a given trait is influenced by one gene or by a complex interaction of many genes and the environment, it can be subject to natural selection and be changed in any of several ways.

Directional Selection In directional selection, individuals with one extreme of the range of variation in the population have higher fitness. Milk production in cows is an example. There is a lot of variation in milk production from cow to cow. As you might expect, farmers select for breeding those cows with the highest milk production and have done so for many decades. The result of such selection is not surprising: between 1920 and 1945, average milk production increased by about 50% in the United States (FIGURE 8-26).

Those at the other end of the range, the cows that produce the smallest amounts of milk, have reduced fitness, since the farmers do not allow them to reproduce, limiting the number of “less milk” alleles in the next generation. In subsequent generations, the average value for the milk-production trait increases. Note, however, that the variation for the trait in the cow population decreases a bit, too, because the alleles contributing to one of the phenotypic extremes are eventually removed from the population.

Hundreds of experiments in nature, in laboratories, and on farms demonstrate the power of directional selection. In fact, it is one of the marvels of laboratory selection and animal breeding that nearly any trait chosen to be exaggerated through directional selection responds dramatically—sometimes with absurd results (FIGURE 8-27). Turkeys, for instance, have been selected for increased size of their breast muscles, which makes them more profitable for farmers. Directional selection has been so successful that the birds’ breast muscles are now so large that it has become impossible for them to mate. All turkeys on large-scale, industrial poultry farms must now reproduce through artificial insemination!

Stabilizing Selection How much did you weigh at birth? Was it more than 10 pounds? More than 11? Was it close to the 22 ½ pounds that Carmelina Fedele’s child weighed when born in Italy in 1955? Or was it 5 pounds or less? Unlike directional selection, for which there is increased fitness at one extreme and reduced fitness at the other, stabilizing selection is said to take place when individuals with intermediate phenotypes are the most fit. The death rate among babies, for example, is lowest between 7 and 8 pounds, and is higher for both lighter and heavier babies (FIGURE 8-28). The variation in birth weight has decreased as stabilizing selection has reduced the frequencies of genes associated with high and low birth weights.

Modern technologies, including Caesarean deliveries and premature-birth wards, allow many babies to live who would not have survived without such technology. This has the unintended consequence of reducing the selection against any alleles causing those “extreme” traits, reducing the rate of their removal from the population.

Disruptive Selection There is a third kind of natural selection, in which individuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the lowest. Although examples of this type of selection, called disruptive selection, are rare in nature, the results are not surprising. Among some species of fish—the coho salmon, for instance—only the largest males acquire good territories. They generally enjoy relatively high reproductive success. While the intermediate-size fish regularly get run out of the good territories, some tiny males are able to sneak in and fertilize eggs before the territory owner detects their presence. Consequently, we see an increase in the frequency of small and large fish, with a reduction in the frequency of medium-size fish (FIGURE 8-29).