Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to raise their own reproductively successful offspring.
As implied, natural selection involves the natural (but often complex) pressures present in an organism's environment. Artificial selection is the conscious manipulation of mating, manipulation, and fusion of genetic material to produce a desired result.
Evolution requires genetic variation, and these variations or changes (mutations) are usually deleterious because environmental factors already support the extent genetic distribution within a population.
Natural selection is based upon expressed differences in the ability of organisms to thrive and produce biologically successful offspring. Importantly, selection can only act to exert influence (drive) on those differences in genotype that appear as phenotypic differences. In a very real sense, evolutionary pressures act blindly.
There are three basic types of natural selection: directional selection favoring an extreme phenotype; stabilizing selection favoring a phenotype with characteristics intermediate to an extreme phenotype (i.e., normalizing selection); and disruptive selection that favors extreme phenotypes over intermediate genotypes.
The evolution of pesticide resistance provides a vivid example of directional selection, wherein the selective agent (in this case DDT) creates an apparent force in one direction, producing a corresponding change (improved resistance) in the affected organisms. Directional selection is also evident in the efforts of human beings to produce desired traits in many kinds of domestic animals and plants. The many breeds of dogs, from dachshunds to shepherds, are all descendants of a single, wolf-like ancestor, and are the products of careful selection and breeding for the unique characteristics favored by human breeders.
Not all selective effects are directional, however. Selection can also produce results that are stabilizing or disruptive. Stabilizing selection occurs when significant changes in the traits of organisms are selected against. An example of this is birth weight in humans. Babies that are much heavier or lighter than average do not survive as well as those that are nearer the mean (average) weight.
On the other hand, selection is said to be disruptive if the extremes of some trait become favored over the intermediate values. Perhaps one of the more obvious examples of disruptive selection is sexual dimorphism, wherein males and females of the same species look noticeably different from each other. One sex may be larger, have bright, showy plumage, bear horns, or display some kind of ornament that the other lacks. The male peacock, for instance, has deep green and sapphire blue plumage and an enormous, fanning tail, while the female is a drab brown, with no elaborate tail.
Sexual dimorphism is considered the result of sexual selection, the process in which members of a species compete for access to mates. Sexual selection and natural selection may often operate in opposing directions, producing the two distinct sex phenotypes. Males, who are typically the primary contestants in the competition for mating partners, usually bear the ornaments such as showy plumage in spite of the potential costs of these ornaments, such as increased visibility to predators, and attacks from rival males. Females are less often involved in direct competition for mates, and they are not generally subject to the forces of sexual selection (although there are role reversals in a few species). Females are believed to play a critical role in the evolution of many elaborate male traits, however, because if the female preference has a genetic basis, female choice of particular males as mating partners will cause those male traits to spread in subsequent generations.
Sometimes the fitness of a phenotype in some environment depends on how common (or rare) it is; this is known as frequency-dependent selection. Perhaps an animal enjoys an increased advantage if it conforms to the majority phenotype in the population; this occurs when, for example, predators learn to avoid distasteful butterfly prey, because the butterflies have evolved to advertise their noxious taste by conforming to a particular wing color and pattern. Butterflies that deviate too much from the "warning" pattern are not as easily recognized by their predators, and are eaten in greater numbers. Interestingly, frequency-dependent selection has enabled butterflies who are not distasteful to mimic the appearance of their noxious brethren and thus avoid the same predators. Conversely, a phenotype could be favored if it is rare, and its alternatives are in the majority. Many predators tend to form a "search image" of their prey, favoring the most common phenotypes, and ignoring the rarer phenotypes. Frequency-dependent selection provides an interesting case in which the gene frequency itself alters the selective environment in which the genotype exists.
Many people attribute the phrase "survival of the fittest" to Darwin, but in fact, it originated from another naturalist/philosopher, Herbert Spencer (1820–1903). Recently, many recent evolutionary biologists have asked: Survival of the fittest what? At what organismal level is selection most powerful? What is the biological unit of natural selection-the species, the individual, or even the gene?
Although it seems rational that organisms might exhibit parental behavior or other traits "for the good of the species." In his 1962 book Animal Dispersion in Relation to Social Behaviour, behavioral biologist V. C. Wynne-Edwards proposed that animals would restrain their reproduction in times of resource shortages, so as to avoid extinguishing the local supply, and thus maintain the "balance of nature." However, Wynne-Edwards was criticized because all such instances of apparent group-level selection can be explained by selection acting at the level of individual organisms. A mother cat who suckles her kittens is not doing so for the benefit of the species; her behavior has evolved because it enhances her kittens' fitness, and ultimately her own as well, since they carry her genes.
Under most conditions, group selection will not be very powerful, because the rate of change in gene frequencies when one individual replaces another in the population is greater than that occurring when one group replaces another group. The number of individuals present is generally greater than the number of groups present in the environment, and individual turnover is greater. In addition, it is difficult to imagine that individuals could evolve to sacrifice their reproduction for the good of the group; a more selfish alternative could easily invade and spread in such a group.
However, there are some possible exceptions; one of these is reduced virulence in parasites, who depend on the survival of their hosts for their own survival. The myxoma virus, introduced in Australia to control imported European rabbits (Oryctolagus cuniculus), at first caused the deaths of many individuals. However, within a few years, the mortality rate was much lower, partly because the rabbits became resistant to the pathogen, but also partly because the virus had evolved a lower virulence. The reduction in the virulence is thought to have been aided because the virus is transmitted by a mosquito, from one living rabbit to another. The less deadly viral strain is maintained in the rabbit host population because rabbits afflicted with the more virulent strain would die before passing on the virus. Thus, the viral genes for reduced virulence could spread by group selection. Of course, reduced virulence is also in the interest of every individual virus, if it is to persist in its host. Scientists argue that one would not expect to observe evolution by group selection when individual selection is acting strongly in an opposing direction.
Some biologists, most notably Richard Dawkins (1941–), have argued that the gene itself is the true unit of selection. If one genetic alternative, or allele, provides its bearer with an adaptive advantage over some other individual who carries a different allele then the more beneficial allele will be replicated more times, as its bearer enjoys greater fitness. In his book The Selfish Gene, Dawkins argues that genes help to build the bodies that aid in their transmission; individual organisms are merely the "survival machines" that genes require to make more copies of themselves.
This argument has been criticized because natural selection cannot "see" the individual genes that reside in an organism's genome, but rather selects among phenotypes, the outward manifestation of all the genes that organisms possess. Some genetic combinations may confer very high fitness, but they may reside with genes having negative effects in the same individual. When an individual reproduces, its "bad" genes are replicated along with its "good" genes; if it fails to do so, even its most advantageous genes will not be transmitted into the next generation. Although the focus among most evolutionary biologists has been on selection at the level of the individual, this example raises the possibility that individual genes in genomes are under a kind of group selection. The success of single genes in being transmitted to subsequent generations will depend on their functioning well together, collectively building the best possible organism in a given environment.
When selective change is brought about by human effort, it is known as artificial selection. By allowing only a selected minority of individuals to reproduce, breeders can produce new generations of organisms featuring particular traits, including greater milk production in dairy cows, greater oil content in corn, or a rainbow of colors in commercial flowers. The repeated artificial selection and breeding of individuals with the most extreme values of the desired traits may continue until all the available genetic variation has been exhausted, and no further selection is possible. It is likely that dairy breeders have encountered the limit for milk production in cattle—eventually, a cow's milk production will increase more slowly for a given increase in feed-but the limit has not yet been reached for corn oil content, which continues to increase under artificial selection.
Seemingly regardless of the trait or characteristic involved (e.g. zygotic selection), there is almost always a way to construct a selectionist explanation of the manifest phenotype.
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K. Lee Lerner Susan Andrew