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Response to stimuli: a response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional, stabilizing, and disruptive selection. [103] Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population.
Recent human evolution refers to evolutionary adaptation, sexual and natural selection, and genetic drift within Homo sapiens populations, since their separation and dispersal in the Middle Paleolithic about 50,000 years ago. Contrary to popular belief, not only are humans still evolving, their evolution since the dawn of agriculture is faster ...
Culturally-driven evolution can defy the expectations of natural selection: while human populations experience some pressure that drives a selection for producing children at younger ages, the advent of effective contraception, higher education, and changing social norms have driven the observed selection in the opposite direction. [217]
A bottleneck can reduce or eliminate genetic variation from a population. Further drift events after the bottleneck event can also reduce the population's genetic diversity. The lack of diversity created can make the population at risk to other selective pressures. [36] A common example of a population bottleneck is the Northern elephant seal ...
However, even if the original value of α is accurate for a particular time period, this extrapolation is still invalid. This is because there has been a large acceleration in the amount of positive selection in the human lineage over the last 40,000 years, in terms of the number of genes that have undergone adaptive evolution (Hawks et al. 2007).
In immunology, clonal selection theory explains the functions of cells of the immune system (lymphocytes) in response to specific antigens invading the body. The concept was introduced by Australian doctor Frank Macfarlane Burnet in 1957, in an attempt to explain the great diversity of antibodies formed during initiation of the immune response .
Mathematical modeling suggests that under certain circumstances, natural selection favours the evolution of canalization that is designed to fail under extreme conditions. [21] [22] If the result of such a failure is favoured by natural selection, genetic assimilation occurs. In the 1960s, Waddington and his colleague the animal geneticist J. M ...