The Results of Natural Selection

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 selectionis operating. 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. This was especially true before modern medical advances.

Directional selection

If individuals at one extreme of a character distribution—the larger ones, for example—contribute more offspring to the next generation than other individuals do, then the average value of that character in the population will shift toward that extreme. In this case, directional selectionis operating. If directional selection operates over many generations, an evolutionary trend within the population results. Such directional evolutionary trends often continue for many generations, but they may be reversed when the environment changes and different phenotypes are favored, or they may be halted when an optimum is reached, or when trade-offs oppose further change. The character then falls under stabilizing selection. Directional selection produced the resistance to tetrodotoxin (TTX) by some garter snakes that we discussed at the beginning of this chapter. The common garter snake, Thamnophis sirtalis, is the only predator of the rough-skinned newt, Taricha granulosa, known to be resistant to TTX. Resistance to TTX has evolved independently at least twice within T. sirtalis populations in western North America, once in California and once in Oregon. This resistance is due to genetically based differences in the ability of sodium channels in the snake’s nerves and muscles to continue functioning when exposed to variable concentrations of TTX.

Disruptive selection

When disruptive selectionoperates, individuals at both extremes of a character distribution contribute more offspring to the next generation than do those close to the mean, producing two peaks in the distribution. This type of selection is apparently rare. The strikingly bimodal (two-peaked) distribution of bill sizes in the black-bellied seedcracker (Pyrenestes ostrinus), a West African finch, illustrates how disruptive selection can influence populations in nature. The seeds of two types of sedges (marsh plants) are the most abundant food source for these finches during part of the year. Birds with large bills can readily crack the hard seeds of the sedge Scleria verrucosa. Birds with small bills can crack S. verrucosa seeds only with difficulty, but they feed more efficiently on the soft seeds of S. goossensii than do birds with larger bills. Young finches whose bills deviate markedly from the two predominant bill sizes do not survive as well as finches whose bills are close to one of the two sizes represented by the distribution peaks. Because there are few abundant food sources in the environment, and because the seeds of the two sedges do not overlap in hardness, birds with intermediate- sized bills are inefficient in using either one of the principal food sources. Disruptive selection therefore maintains a bimodal bill size distribution.

Sexual selection results in conspicuous traits

In The Origin of Species, Darwin devoted a few pages to sexual selection, a topic that he developed at length in another book, The Descent of Man, and Selection in Relation to Sex, in 1871. Sexual selectionwas Darwin’s explanation for the evolution of apparently useless but conspicuous traits in males of many species, such as bright colors, long tails, horns, antlers, and elaborate courtship displays. He hypothesized that these traits either improved the ability of their bearers to compete for access to members of the other sex (intrasexual selection) or made their bearers more attractive to members of the other sex (intersexual selection). Darwin argued that female preferences for such features are also the result of sexual selection because “unornamented, or unattractive males would succeed equally in the battle for life and in leaving a numerous progeny, but for the presence of better endowed males.” Sexual selection may result in species that are sexually dimorphic — that is, species in which males and females differ in size, shape, or color. The concept of sexual selection was not well received by Darwin’s contemporaries. However, many examples of sexual selection have been investigated in the century and a half since he first proposed the idea, and Darwin turned out to be right. For example, sexual selection is responsible for different morphological attributes of male birds that compete with each other for available females. One case in point is the remarkable tails of male African long-tailed widowbirds which are longer than their heads and bodies combined. To examine the role of sexual selection in the evolution of widowbird tails, a behavioral ecologist captured some male widowbirds. He shortened the tails of some males by cutting them and lengthened the tails of others by gluing on additional feathers. Male widowbirds normally select and defend from other males, a site where they perform courtship displays to attract females. Both short-tailed and long-tailed males successfully defended their display sites, indicating that a long tail does not confer an advantage in male–male competition. However, males with artificially elongated tails attracted about four times more females than did males with shortened tails. Why do female widowbirds prefer males with long tails? The ability to grow and maintain a costly feature such as a long tail may indicate that the male bearing it is vigorous and healthy. The hypothesis that having well-developed ornamental traits signals vigor and health has been tested experimentally with captive zebra finches. The bright red bills of male zebra finches are the result of red and yellow carotenoid pigments. Zebra finches (and most other animals) cannot synthesize carotenoids and must obtain them from their food. In addition to influencing bill color, carotenoids are antioxidants and components of the immune system. Males in good health may need to allocate fewer carotenoids to immune function than males in poorer health. If so, then females can use the brightness of his bill to assess the health of a male. Investigators manipulated blood levels of carotenoids in male zebra finches by means of carotenoid supplements. Experimental males were given drinking water with carotenoids added; control males were given only distilled water. All the males had access to the same food. After one month, the experimental males had higher levels of carotenoids in their blood, had much brighter bills than the control males and were preferred by female zebra finches. Next, the investigators challenged both groups of males immunologically by injecting phytohemagglutinin (PHA) into the webs of their wings. PHA induces a response by T lymphocytes, resulting in an accumulation of white blood cells and thus a thickening of the skin. Experimental males with enhanced carotenoid levels developed thicker skins because they responded more strongly to PHA than control males did, indicating a heightened immune system. This experiment showed that when a female chooses the male with a bright red bill, she probably gets a mate with a healthy immune system. Such males are less likely to become infected with parasites and diseases, so they are less likely to pass on infections to their mates. Healthier males are also better able to assist with parental care than are males with duller bills.