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| Macroevolution, the process of long-term evolution, can occur in two ways: anagenesis, the evolution of a species over time, or cladogenesis, the splitting off of one or more new species from the original parent species. In cladogenesis, new species form through the process of reproductive isolation followed by genetic divergence. Both steps are understood in terms of evolutionary forces. Reduction or elimination of gene flow provides for the beginning of reproductive isolation. Mutation, genetic drift, and selection can then act on this isolation to produce a new species. The relative importance of the evolutionary forces in speciation is still debated. Two models of macroevolutionary change can be applied to the fossil record. Gradualism states that most evolutionary change is the result of slow but constant change over many generations. New species are believed to form as a by-product of natural selection operating over time. Punctuated equilibrium states that there are long periods with little evolutionary change (stasis), punctuated by rapid evolutionary events. New species are seen as forming in small, isolated populations. The most common evolutionary pattern is extinction. Some scientists have argued that the evolutionary record is best understood as the process of new species forming from old, with many species becoming extinct. The evolutionary trends we observe in the fossil record may reflect the differential survival of species with certain adaptations. There are many misconceptions regarding natural selection and evolution. Some of the more common of these are that bigger is better, that newer is better, that natural selection always works, and that there is an inevitable direction to natural selection. There are also misconceptions regarding the relationship of biological structures, their functions, and their evolutionary origin. Species are classified according to similarity in traits that arose because of descent from a common ancestor (homology) and not according to similar traits that evolved independently (homoplasy). Homologous traits can be categorized as primitive (unchanged since the time of a common ancestor) or derived (having changed since the time of a common ancestor). Different approaches to classification place different weight on primitive or derived traits depending on whether the purpose of classification is to provide a measure of overall similarity or to reveal evolutionary relationships. |