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Selection generally refers to the pressures on cells and organisms to evolve. These pressures include natural selection, and, in eukaryotic cells that reproduce sexually, sexual selection. Certain phenotypic traits (characteristics of an organism)—or, on a genetic level, alleles of genes—segregate within a population, where individuals with adaptive advantages or traits tend to succeeded more than their peers when they reproduce, and so contribute more offspring to the succeeding generation. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring inherit them from their parents. When selection is intense and persistent, adaptive traits become universal to the population or species, which may then be said to have evolved.
Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, juveniles, embryos, and gamete eggs and sperm all undergo selection. Factors fostering natural selection include sexual selection, primarily caused by mate choice in the mating phase of sexual reproduction, limits on resources (nourishment, habitat space, mates) and the existence of threats (predators, disease, adverse weather). Biologists often refer to such factors as selective or evolutionary pressures.
Natural selection has, since the 1930s, included sexual selection because biologists at the time did not think it was of great importance though it has become to be seen as more important in the 21st Century. Other subcategories of natural selection include ecological selection, stabilizing selection, disruptive selection and directional selection. Selective breeding can be seen in the breeding of dogs, and the domestication of farm animals and crops, now commonly known as selective breeding.
Selection occurs only when the individuals of a population are diverse in their characteristics—or more specifically when the traits of individuals differ with respect to how well they equip them to survive or exploit a particular pressure. In the absence of individual variation, or when variations are selectively neutral, selection does not occur.
Meanwhile, selection does not guarantee that advantageous traits or alleles become prevalent within a population. Another process of gene frequency alteration in a population is called genetic drift, which acts over genes that aren't under selection. But, this drift can't overcome natural selection itself, as it is a 'random sampling' process and Natural Selection is actually an evolutive force. In the face of selection even a so-called deleterious allele may become universal to the members of a species. This is a risk primarily in the case of "weak" selection (e.g., an infectious disease with only a low mortality rate) or small populations.
Though deleterious alleles may sometimes become established, selection may act "negatively" as well as "positively." Negative selection or purifying selection decreases the prevalence of traits that diminish individuals' capacity to succeed reproductively (i.e., their fitness), while positive selection increases the prevalence of adaptive traits.
In biological discussions, traits subject to negative selection are sometimes said to be "selected against," while those under positive selection are said to be "selected for," as in the scenario: During the severe drought affecting the Galapagos Islands, short beaks were selected against due to their relative inability to access tribulus seed nutrients. The following season, female finches displayed a change in their preferences for mates. During that mating season, greater sizes were selected for, as evident by the increase in the number of mating events per season for larger birds.
Aspects of selection may be divided into effects on a phenotype and their causes. The effects are called patterns of selection, and do not necessarily result from particular causes (mechanisms). In fact, each pattern can arise from a number of different mechanisms. Stabilizing selection favors individuals with intermediate characteristics while its opposite, disruptive selection, favors those with extreme characteristics. Directional selection occurs when characteristics lie along a phenotypic spectrum, and individuals at one end are more successful.Balancing selection is a pattern in which multiple characteristics are favored. Diversifying selection occurs when rare variants are favored, and is similar to balancing selection.
Distinct from patterns of selection are mechanisms of selection; for example, disruptive selection often is the result of disassortative sexual selection, and balancing selection may result from frequency-dependent selection and overdominance.
Units of selection
|Unit of selection||Role||Quantity maximised|
|Individual organism||'Vehicle'||Inclusive fitness|
- Miller, Geoffrey (2000). The mating mind: how sexual choice shaped the evolution of human nature, London, Heineman, ISBN 0-434-00741-2 (also Doubleday, ISBN 0-385-49516-1) p.8
- SEXUAL SELECTION AND THE MIND
- Richard Dawkins, Brief Candle in the Dark: My Life in Science, Bantam Press, 2015, page 318 (ISBN 978-0-59307-256-1).