Evolution of biological complexity
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 reproduce faster. A good example are parasites such as malaria and mycoplasma; these organisms often dispense with traits that are provided by their host organism. 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 that caused sleeping sickness has evolved so many copies of it's 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.
Types of trends in complexity
Evolution has produced some remarkably complex organisms, but 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] The 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] If this were generally true, 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.[3] Indeed, some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution.[4][5]
However, the idea of a general trend towards complexity in evolution can also be explained through a passive process.[3] 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, the apparent trend towards complex organisms 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,[6] which comprise about half the world's biomass.[7] constitute the vast majority of Earth's biodiversity.[8] 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 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 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, as lineages will become simpler or more complicated according to whatever forms have a selective advantage.
- ^ Adami C (2002). "What is complexity?". Bioessays. 24 (12): 1085–94. PMID 12447974.
- ^ McShea D (1991). "Complexity and evolution: What everybody knows". Biology and Philosophy. 6 (3): 303–324. doi:10.1007/BF00132234.
- ^ a b Carroll SB (2001). "Chance and necessity: the evolution of morphological complexity and diversity". Nature. 409 (6823): 1102–9. PMID 11234024.
- ^ Furusawa C, Kaneko K (2000). "Origin of complexity in multicellular organisms". Phys. Rev. Lett. 84 (26 Pt 1): 6130–3. PMID 10991141.
- ^ Adami C, Ofria C, Collier TC (2000). "Evolution of biological complexity". Proc. Natl. Acad. Sci. U.S.A. 97 (9): 4463–8. PMID 10781045.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Oren A (2004). "Prokaryote diversity and taxonomy: current status and future challenges". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 359 (1444): 623–38. PMID 15253349.
- ^ Whitman W, Coleman D, Wiebe W (1998). "Prokaryotes: the unseen majority". Proc Natl Acad Sci U S A. 95 (12): 6578–83. PMID 9618454.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Schloss P, Handelsman J (2004). "Status of the microbial census". Microbiol Mol Biol Rev. 68 (4): 686–91. PMID 15590780.