In 1858, two men, Charles Darwin and Alfred Russel Wallace, independently proposed a mechanism for evolution. Darwin named this mechanism natural selection. Natural selection requires variation among individuals in a population. In a particular environment, some traits of individuals are more advantageous than others. The individuals with the advantageous traits are able to survive better, and therefore they are also able to produce more offspring. In subsequent generations, there are relatively more individuals with the inherited advantageous traits. In this way, the population changes, or evolves, from one generation to the next.
In the accompanying animation, we examine natural selection in Texas Longhorn cattle.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.
The long horns of Texas Longhorn cattle are an example of a trait that has evolved through directional selection.
Texas Longhorns are descendants of cattle brought by Christopher Columbus from the Canary Islands to the island of Hispaniola in 1493. Ships continued to supply the New World with additional cattle up through 1511. Since then, all of the Iberian-derived cattle of the New World developed from these founding cattle. Similar to modern Portuguese and Spanish breeds, the founding cattle likely had horns spanning 24–30 inches tip-to-tip as adults. Compare that to Texas Longhorns, with spans of 30–80 inches.
The cattle multiplied, and their descendants were taken to the mainland of Mexico. Spaniards exploring what would become Texas and the southwestern United States brought these cattle with them, but some of them escaped and formed feral herds. Populations of feral cattle increased greatly over the next few hundred years, and in the wild they were subjected to natural selection. Here we show just cows and their calves.
For natural selection to occur, several requirements must be met for any particular trait. First, the population must show variation in the trait of interest—in this case, horn length. Some of the cattle had short horns, while others had longer horns. Second, the trait must be heritable, able to be passed on from parent to offspring, a property that is true of horn length. Third, the trait must affect the reproductive success of the individuals in the population.The trait did, indeed, affect the reproductive success of the individual, as there was heavy predation from bears, mountain lions, and wolves, especially on the young calves. Cows with shorter horns were less successful in protecting their calves against attacks. As a result, the next generation contained more individuals with longer horns. Over a few hundred years the average horn length in the feral herds increased considerably.
In addition to increases in horn length, the cattle evolved resistance to endemic diseases of the Southwest, and they had higher fecundity and longevity. Texas Longhorns often live and produce calves well into their twenties—about twice as long as many breeds of cattle that have been artificially selected by humans for traits such as high fat content or high milk production (which are examples of artificial directional selection).
In the accompanying animation, we examined several examples of natural selection, each of which produces strikingly different results in the level of variation in the population.
· In stabilizing selection, natural selection preserves the characteristics of a population by favoring average individuals. Stabilizing selection reduces variation, but does not change the position of the population's mean.
· In directional selection, natural selection changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population. If directional selection operates over many generations, then an evolutionary trend results in the population.
· In disruptive selection, natural selection changes the characteristics of a population by favoring individuals that vary in both directions from the mean of the population.
The mechanism underlying each is the same: individuals with advantageous traits survive better and therefore are able reproduce more, increasing the frequency of individuals with the advantageous traits in the next generation.