Microevolution

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Microevolution is the occurrence of small-scale changes in gene frequencies in a population over a few generations, also known as change at or below the species level. These changes may be due to several processes: mutation, gene flow, genetic drift, as well as natural selection. Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution. Biologists distinguish between microevolution and macroevolution, which is the occurrence of large-scale changes in gene frequencies in a population over a long period of time (and may culminate in the evolution of new species).

Typically, observable instances of evolution are examples of microevolution; for example, bacterial strains that have become resistant to antibiotics. Because microevolution can be observed directly, both pro-evolution and some anti-evolution groups agree that it is a fact of life.

Notes: Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the case of artificial selection, whereby various breeds of animals and varieties of plants have been produced that are different in some respect from their ancestors, or in the often-cited, but somewhat problematic case of systematic color change in the peppered moth, Biston betularia, which was observed over a 50-year period in England. The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level. That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years. If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors. One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.

Punctuational models of speciation

Historically, the process of speciation has been viewed as involving the accumulation of small, microevolutionary changes in a population over time until a descendant species arises, either from the transformation of the ancestral population, or splitting from the ancestral population. Generally, the favored method for this was considered geographic isolation, such that a population becomes separate from the parental population, and develops into a new species by natural selection until reproductive isolation occurs, and there are two species. Reproductive isolation is therefore a secondary by-product of geographic isolation, with the process involving gradual allelic substitution.

Contrasted with this view are recent punctuational models for speciation, whereby reproductive isolation arises rapidly, and not through gradual selection, but without selective significance (Gould 1980a; Gould and Eldredge 1977). That is, natural selection does not play a creative role in initiating speciation, nor in the definitive aspect of reproductive isolation, although it is usually postulated as the important factor in building subsequent adapation. One example of this is polyploidy, where there is a multiplication of the number of chromosomes beyond the normal diploid number. Another models is chromosomal speciation, involving large changes in chromosomes due to various genetic accidents.

One view, put forth by Stephen Jay Gould, is based on the fact that there are critical genes (such as the homeobox) in all living organisms, and a small change in them could cause drastic changes in the organism, resulting in a new species quite rapidly.

Single small mutations are sometimes the main difference between one species and another. Scientists have discovered very important genes, such as the homeobox, which regulate the growth of animals in their embryonic state. Scientists have managed to create new species of fly by irradiating the homeobox gene, causing a radical mutation in the development of the segments of the body. The fly may grow an extra thorax, or grow legs out of its eyestalks, all due to a single base pair alteration. The additional information needed for these structures did not arise from the mutation, of course, but existed elsewhere in the animal's DNA and was replicated at the novel location. It has been proposed that centipedes and millipedes originated from insect precursors, but their homeobox gene mutated and they ended up growing dozens of body segments instead of just one. A very small change, and an entire species is formed.

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