Speciation and the Rate of Evolution
Speciation is the process by which a new species arises from an ancestral species. The rate and ways in which it takes place vary but are directly related to natural selection as described by Darwin. Biologically the term species refers to a population of similar individuals that in nature interbreed and produce fertile offspring. As a result, all species are reproductively isolated from one another. This definition does not apply to organisms such as bacteria that reproduce asexually, but it is useful for our discussion of plants, animals, fungi, and single-cell organisms called protistans all of which reproduce sexually.
In order for speciation to occur the genetic variation within a parent population must change over time. The amount of variation that occurs in any given population over time is controlled by sexual reproduction, as described by Medel, and the occurence of mutations. The randomness with which alles are transferred through a populationas a result of sexual reproduction is called genetic drift. In large populations genetic drift has bery little effect on the frequency of specific variations within a population. However, as a population becomes smaller and smaller genetic drift can greatly alter the frequency of a specific variation and a can cause some variations to become completely lost and others to become more abundant. The animated image below shows how pure random sampling alters the variation within a small population over relatively few successive generations.
|Animated sequence showing the effect of genetic drift on a population.|
It is important, at this point, to emphasize that in modern evolutionary theory it is the population that evolves not the individuals. Natural selection works on variations within populations. With each successive generation there is a reshuffling of genetic material and, depending on the environmental conditions at the time, some of those individuals will survive while others will not.
According to the concept of allopatric speciation, a new species arises when a small part of a population becomes isolated from its parent population. The isolated population has less available genetic variation and with fewer individuals genetic drift has a much greater influence on variation frequency. As a result natural selection will proceed differently in the isolated population than it does in the parent population given rise to organism that differ greatly from the parent population.
Although there is widespread agreement on the process of allopatric speciation there is some debate regarding how rapidly these changes occur. According to one school of thought known as phylletic gradualism states that gradual accumulations of minor changes in populations eventually brings about a new species. Punctuated equilibrium, on the other hand, holds that there is very little change within a species during most of its existence and then evolution occurs rapidly, in as little as a few thousand years. Evidence exists for both views and as such evolution most likely proceeds through the gradual accumulation of minor changes as well as rapid changes due to catastophic events or rapid environmental changes.