∞ generated and posted on 2016.09.04 ∞
Biased declines in the frequency of alleles within populations.
Natural selection operates on units of selection, which in most but certainly not all instances consist of individual organisms. Natural selection thus typically consists of a biased loss of organisms from populations. Furthermore, this bias is in terms of which organism genotypes better prosper.
Natural selection fundamentally is a response to present ecological circumstances rather than being in any way anticipatory ("the resolute urgency of now" [listen]). Natural selection, in other words, "cares" solely about what is going on, ecologically, in the present, rather than what might occur some time in the future. This means that even adaptations that were once useful, and/or which could be useful sometime in the future, nevertheless at any given moment may or may not be actively favored by natural selection.
Different forms of natural selection include:
See in addition the natural-selection related terms of artificial selection, clonal selection, direct selection, indirect selection, thymic selection, and, simply, selection.
While mutation puts genetic variation into populations, natural selection takes that variation out. The result is that populations possess fewer alleles that are poorly "adapted" to the environments in which they are found. If reproduction is biased against poorly adapted individuals, so long as these poor adaptations are genetically based – that is, are the products of poorly adapted alleles – then the series of individuals making up a population through time will, on average, become increasingly better adapted to that environment.
On a much simpler level, take a mixture of blue and red marbles. If you slowly remove the red marbles, then the population of marbles gradually will become bluer. This constitutes selection against red marbles, and the result is an adaptation by the marble population to this selection against redness. That is, if a marble "wants" to remain in the population while red marbles are being actively removed, then it had better not be red.
The following video presents a brief overview of the story of the peppered moth:
The following video discusses evolution of the eye by means of surprisingly rapid natural selection: