Other Types of Selection

Strict natural selection is not the only type of selection that takes place in nature or that has an effect on evolution. Other types of selection, all corollaries or variations of natural selection, are also present in the rich tapestry of evolution. The major variant types of selection are artificial selection, sexual selection, and kin selection.

Artificial Selection

Artificial selection is the process by which a non-natural selection pressure on a population of organisms is set up by human activities, especially domestication of animals and plants. Humans breed from domestic organisms possessing the most desirable qualities in a process known as selective breeding, which then sets up a survival and reproductive advantage for those organisms that possess the desirable qualities. The result is an increase in the number of organisms possessing such qualities until these alleles come to dominate the gene pool of the population.

There are many examples of artificial selection in the modern world. Dachshunds and greyhounds both descend from the same ancestral wild dogs, but both breeds have been created by human selective breeding. In the case of dachshunds, the process has yielded unusually low and elongated bodies; in the case of greyhounds, the process has yielded unusual swiftness in running.

In all artificial selection processes, human criteria determine the likelihood of survival and reproduction because, in general, humans control which organisms survive and reproduce. This generally leads to attributes being favored by artificial selection that would not have been favored - or would have been fatal - under pure natural selection. An example from the plant world illustrates this fact. Corn originally had seeds that came off the cob and dispersed into the surrounding environment, thus propagating the species. Some plants, however, could not disperse their seeds due to a mutation; this mutation always ended that lineage of plants because the plant was effectively sterile. When humans discovered that corn could be eaten, they sought out the cobs that did not lose their seeds. Over generations, humans continually selected for the plants that did not lose their seeds, and therefore a strong selective advantage came to exist for those that were incapable of self-propagating. A mutation that was disastrous to a wild plant suddenly became a potent selective advantage under artificial selection. Before long the process had continued to such an extent that the corn variety raised for food could no longer survive in the wild, but rather relied on humans for its propagation into the next generation.

Artificial Selection and Genetic Diversity

Artificial selection also tends to reduce genetic diversity in the gene pools of populations that have undergone (or are undergoing) domestication. The continual selection for one particular type of attribute, and the continual "weeding out" of mutations or variant types, leads to a constriction in the available variety in the gene pool. Because, in general, humans always select for the same qualities and try to breed from the best animals (i.e., those that best exhibit such qualities), there is a constant pressure in the same direction over many generations. Moreover, new mutations are always removed from the breeding line, so no new alleles are introduced into the population. The end result is a decrease in genetic variation and a quite uniform population.

Population uniformity is certainly beneficial from some perspectives. Uniformity helps farmers plant and raise crops more efficiently and ensures quality in animals on the market. Generally beneficial traits, such as speed, strength, or resistance to disease, are those that gain a "monopoly" in the population. Thus, genetic uniformity is beneficial to humans - and, consequently, to domesticated plants and animals - when seen from an immediate perspective.

However, in the long run genetic uniformity can be extremely damaging to populations of organisms and sometimes to entire species. Species rely on their "stores" of genetic variation to resist a variety of diseases and adapt to a variety of environments. Without such resources, the populations are vulnerable to damage or destruction by even a minor change in environmental conditions because the genes the organisms all share are ill-suited to those circumstances. The same genes that confer resistance to one disease may confer vulnerability to another. While the second disease may not have been a threat when humans established the population, it may become threatening and thus wipe out the population entirely. Additionally, excessive inbreeding with organisms of the same genetic constitution can weaken individual organisms and the entire population. Thoroughbred horses and dogs sometimes encounter these problems because they have been inbred for too long.

In sum, artificial selection can reduce or almost eliminate genetic diversity in a population. This situation can eliminate the raw material needed for natural selection and adaptation, thus resulting in a precarious predicament for many organisms. Artificial selection is a great boon to the furtherance of human interests, but must be undertaken responsibly and with a good understanding of the principles involved if it is not to put the domesticated organisms in dire straits.

Sexual Selection

Sexual selection is another recognized type of selection that is a potent force in nature. It involves the competition of one gender, usually the male, for the preference or favor of another, usually the female. Ultimately the goal of such competition, often in the form of attractive appearance, beneficial habits, or courtship behavior, is the "right" to reproduce with the selected mate. In one sense, sexual selection is "natural selective breeding," since it ensures that the most attractive (usually the most fit) members of the species are reproductively active. However, this is not the basis or justification of sexual selection, and it does not always act in this way.

Sexual selection's most common form is the competition among males for the favors of the females of a species. Such competition may involve the male's display of his virility, strength, desirability, etc., or may take the form of attraction by way of physical or chemical characteristics. For example, in some bird species such as bower birds, the male builds a nest or bower and the female comes to inspect it. If she "likes" the bower, she will become the male's mate; if she does not, she simply flies away, leaving the male to try again. This is a simple example of attraction by way of display - in this case, the males' display of their ability to design a nest to care for future young. An example of the chemical method is the release of pheromones, common in insect species.

Sexual selection obviously affects the frequency of alleles in a given gene pool. If females consistently choose males with certain characteristics, then the genes for these characteristics will become more common, because these males will reproduce more often. Also, the genes that caused the females to choose they way they did in the first place will become more frequent, since they will outnumber their counterparts to begin with and will pass along their genes. If this process achieves total or near-total saturation of the gene pool with one allele, selection usually focuses on another characteristics.

Sexual selection often leads to an increase in the overall fitness of the members of a species, since the females' criterion is often strength, fitness, or some suitable display thereof. However, the preference for a display of strength can also cause sexual selection to offset or counterbalance the force of pure natural selection. For example, suppose that a species of bird exists in which females prefer feathers on the crest of the head to heads without feathers. The crested males will come to out-reproduce their uncrested counterparts, and crests will typically become the norm over many generations. However, the females' preference may continue to tend toward more and more elaborate feathers - perhaps the ability to maintain attractive feathers is a good proxy measure for a bird's overall health. Suppose further that overly elaborate feathers conferred a disadvantage in survival because the feathers impeded the bird's sight, or tended to attract fleas, or any other disadvantage. The opposing forces of natural and sexual selection generally eventually find a "happy medium" between the large feathers preferred by females and the smaller feathers that confer a survival advantage.

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