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The evolution of biological complexity is one important outcome of the process of evolution. Evolution has produced some remarkably complex organisms - although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of cell types or morphology all being used to assess an organism's complexity. Some used to believe that evolution was progressive and had a direction that led towards so-called "higher organisms," despite a lack of evidence for this viewpoint. This idea of "progression" and "higher organisms" in evolution is now regarded as misleading, with natural selection having no intrinsic direction and organisms selected for either increased or decreased complexity in response to local environmental conditions. Although there has been an increase in the maximum level of complexity over the history of life, there has always been a large majority of small and simple organisms and the most common level of complexity (the mode) appears to have remained relatively constant. ==Selection for simplicity and complexity== Organisms that reproduce more quickly and plentifully than their competitors have an evolutionary advantage. Consequently, organisms can evolve to become simpler and thus multiply faster and produce more offspring, as they require fewer resources to reproduce. A good example are parasites such as Plasmodium - the parasite responsible for malaria - and mycoplasma; these organisms often dispense with traits that are made unnecessary through parasitism on a host. A lineage can also dispense with complexity when a particular complex trait merely provides no selective advantage in a particular environment. Loss of this trait need not necessarily confer a selective advantage, but may be lost due to the accumulation of mutations if its loss does not confer an immediate selective disadvantage. For example, a parasitic organism may dispense with the synthetic pathway of a metabolite where it can readily scavenge that metabolite from its host. Discarding this synthesis may not necessarily allow the parasite to conserve significant energy or resources and grow faster, but the loss may be fixed in the population through mutation accumulation if no disadvantage is incurred by loss of that pathway. Mutations causing loss of a complex trait occur more often than mutations causing gain of a complex trait. With selection, evolution can also produce more complex organisms. Complexity often arises in the co-evolution of hosts and pathogens, with each side developing ever more sophisticated adaptations, such as the immune system and the many techniques pathogens have developed to evade it. For example, the parasite ''Trypanosoma brucei'', which causes sleeping sickness, has evolved so many copies of its major surface antigen that about 10% of its genome is devoted to different versions of this one gene. This tremendous complexity allows the parasite to constantly change its surface and thus evade the immune system through antigenic variation. More generally, the growth of complexity may be driven by the co-evolution between an organism and the ecosystem of predators, prey and parasites to which it tries to stay adapted: as any of these become more complex in order to cope better with the diversity of threats offered by the ecosystem formed by the others, the others too will have to adapt by becoming more complex, thus triggering an on-going evolutionary arms race〔 towards more complexity.〔Heylighen, F. (1999a) ("The Growth of Structural and Functional Complexity during Evolution )", in F. Heylighen, J. Bollen & A. Riegler (eds.) The Evolution of Complexity Kluwer Academic, Dordrecht, 17-44.〕 This trend may be reinforced by the fact that ecosystems themselves tend to become more complex over time, as species diversity increases, together with the linkages or dependencies between species. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolution of biological complexity」の詳細全文を読む スポンサード リンク
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