Evolution of complexity

The evolution of complexity is an important outcome of the process of evolution. Evolution has produced some remarkably complex organisms, although this feature is hard to measure 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] This observation that complex organisms can be produced from simpler ones has led to the common idea of evolution being progressive and leading towards what are viewed as "higher organisms".^[2]

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.^[3] Any apparent trend in complexity over the history of life is due to our sampling bias in concentrating on a few large organisms and ignoring the large majority of smaller organisms.

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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 more simple and thus multiply faster and produce more offspring, as they require less resources to reproduce. A good example are parasites such as malaria and mycoplasma; these organisms often dispense with traits that are provided by their host organism to allow faster multiplication.^[4]

However, evolution can also produce more complex organisms. Complexity often arises in the co-evolution in 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.^[5]

Types of trends in complexity

If it were generally true that evolution tended to move towards increasingly complex organisms, evolution would possess an active trend towards complexity. As shown to the right, in this type of process the value of the most common amount of complexity would increase over time.^[6] Indeed, some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution.^[7][8] 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 a general trend towards complexity in evolution can also be explained through a passive process.^[6] As shown on the right, this involves an increase in variance but the most common value does not change. Thus, the maximum level of complexity increases over time, but only as an indirect product of there being more organisms in total.

In this hypothesis, any apparent trend towards complex organisms over the history of life is an illusion resulting from 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 emphasises that the overwhelming majority of species are microscopic prokaryotes,^[9] which comprise about half the world's biomass^[10] and constitute the vast majority of Earth's biodiversity.^[11] Consequently, microscopic life dominates Earth, and large organisms only appear more diverse due to sampling bias.

History

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 Georg 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.^[12]

See also

• Complexity
• Complex adaptive system
• Orthogenesis

References