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Genetic drift is the change in allele frequency from one generation to the next that occurs because of the role that chance plays in determining whether a given individual will survive and reproduce. In mathematical terms,[link widoczny dla zalogowanych] alleles are subject to sampling error. As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.[133]
The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[134] The precise measure of population that is important is called the effective population size. The effective population is always smaller than the total population since it takes into account factors such as the level of inbreeding, the number of animals that are too old or young to breed, and the lower probability of animals that live far apart managing to mate with each other.[135]
An example of when genetic drift is probably of central importance in determining a trait is the loss of pigments from animals that live in caves,[link widoczny dla zalogowanych] a change that produces no obvious advantage or disadvantage in complete darkness.[136] However, it is usually difficult to measure the relative importance of selection and drift,[137] so the comparative importance of these two forces in driving evolutionary change is an area of current research.[138] These investigations were prompted by the neutral theory of molecular evolution, which proposed that most evolutionary changes are the result of the fixation of neutral mutations that do not have any immediate effects on the fitness of an organism.[139] Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.[140] This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.
Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by co-operating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.
These outcomes of evolution are sometimes divided into macroevolution, which is evolution that occurs at or above the level of species, such as extinction andspeciation, and microevolution, which is smaller evolutionary changes, such as adaptations, within a species or population.[143] In general,[link widoczny dla zalogowanych], macroevolution is regarded as the outcome of long periods of microevolution.[144] Thus, the distinction between micro- and macroevolution is not a fundamental one – the difference is simply the time involved.[145] However,[link widoczny dla zalogowanych] in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated.