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The evolution of biological complexity is an 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.[1][2] This observation that complex organisms can be produced from simpler ones has led to the common misperception of evolution being progressive and having a direction that leads towards what are viewed as "higher organisms".[3]
Nowadays, this idea of "progression" in evolution is 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.[4] 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.
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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 malaria and mycoplasma; these organisms often dispense with traits that are made unnecessary through parasitism on a host.[5]
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.[6] 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,[7] 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.[8]
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[7] towards more complexity.[9] 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.
If evolution possessed an active trend toward complexity, then we would expect to see an increase over time in the most common value (the mode) of complexity among organisms, as shown to the right.[10] Indeed, some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution.[11][12] This is sometimes referred to as evolutionary self-organization. Self-organization is the spontaneous internal organization of a system. This process is accompanied by an increase in systemic complexity, resulting in an emergent property that is distinctly different from any of the constituent parts.
However, the idea of increasing production of complexity in evolution can also be explained through a passive process.[10] As shown on the left, this involves an increase in variance but the mode does not change. The trend towards higher complexity over time exists, but also involves increasingly smaller portions of biological life.
In this hypothesis, any appearance of evolution acting with an intrinsic direction towards increasingly complex organisms is a result of people concentrating on the small number of large, complex organisms that inhabit the right-hand tail of the complexity distribution and ignoring simpler and much more common organisms. This passive model predicts that the majority of species are microscopic prokaryotes, which is supported by estimates of 106 to 109 extant prokaryotes[13] compared to diversity estimates of 106 to 3·106 for eukaryotes.[14][15] Consequently, in this view, microscopic life dominates Earth, and large organisms only appear more diverse due to sampling bias.
In the 19th century, some scientists such as Jean-Baptiste Lamarck and Ray Lankester believed that all Nature had an innate striving to become more complex with evolution. This belief may reflect then-current ideas of Hegel and Herbert Spencer that all creation was gradually evolving to a higher, more perfect state.
According to this view, the evolution of parasites from an independent organism to parasite was seen as "devolution" or "degeneration", and contrary to Nature. This view has sometimes been used metaphorically by social theorists and propagandists to decry a class of people as "degenerate parasites". Today, "devolution" is regarded as nonsense; rather, lineages will become simpler or more complicated according to whatever forms have a selective advantage.[16]
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