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 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. [cite journal |author=Adami C |title=What is complexity? |journal=Bioessays |volume=24 |issue=12 |pages=1085–94 |year=2002 |pmid=12447974 |doi=10.1002/bies.10192] 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". [cite journal |author=McShea D |title=Complexity and evolution: What everybody knows |journal=Biology and Philosophy |volume=6 |issue=3 |pages=303–324 |year=1991 |doi=10.1007/BF00132234]

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.cite journal |author=Ayala FJ |title=Darwin's greatest discovery: design without designer |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 Suppl 1 |issue= |pages=8567–73 |year=2007 |pmid=17494753 |url=http://www.pnas.org/cgi/content/full/104/suppl_1/8567 |doi=10.1073/pnas.0701072104] 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) has remained constant.

election 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 made unnecessary through parasitism on a host. [cite journal |author=Sirand-Pugnet P, Lartigue C, Marenda M, "et al" |title=Being Pathogenic, Plastic, and Sexual while Living with a Nearly Minimal Bacterial Genome |journal=PLoS Genet. |volume=3 |issue=5 |pages=e75 |year=2007 |pmid=17511520 |doi=10.1371/journal.pgen.0030075] .

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 by genetic drift 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 may be fixed in the population through genetic drift if no disadvantage is incurred by loss of that pathway.

However, 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. [cite journal |author=Pays E |title=Regulation of antigen gene expression in Trypanosoma brucei |journal=Trends Parasitol. |volume=21 |issue=11 |pages=517–20 |year=2005 |pmid=16126458 |doi=10.1016/j.pt.2005.08.016]

Types of trends in complexity

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.cite journal |author=Carroll SB |title=Chance and necessity: the evolution of morphological complexity and diversity |journal=Nature |volume=409 |issue=6823 |pages=1102–9 |year=2001 |pmid=11234024 |doi=10.1038/35059227] Indeed, some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution. [cite journal |author=Furusawa C, Kaneko K |title=Origin of complexity in multicellular organisms |journal=Phys. Rev. Lett. |volume=84 |issue=26 Pt 1 |pages=6130–3 |year=2000 |pmid=10991141 |doi=10.1103/PhysRevLett.84.6130] [cite journal |author=Adami C, Ofria C, Collier TC |title=Evolution of biological complexity |url=http://www.pnas.org/cgi/content/full/97/9/4463 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=9 |pages=4463–8 |year=2000 |pmid=10781045 |doi=10.1073/pnas.97.9.4463] 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. As shown on the left, this involves an increase in variance but the mode 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 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 emphasises that the overwhelming majority of species are microscopic prokaryotes, [cite journal |author=Oren A |title=Prokaryote diversity and taxonomy: current status and future challenges |url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1693353&blobtype=pdf |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=359 |issue=1444 |pages=623–38 |year=2004 |pmid=15253349 |doi=10.1098/rstb.2003.1458] which comprise about half the world's biomass [cite journal |author=Whitman W, Coleman D, Wiebe W |title=Prokaryotes: the unseen majority |url=http://www.pnas.org/cgi/content/full/95/12/6578 |journal=Proc Natl Acad Sci U S A |volume=95 |issue=12 |pages=6578–83 |year=1998 |pmid=9618454 |doi=10.1073/pnas.95.12.6578] and constitute the vast majority of Earth's biodiversity. [cite journal |author=Schloss P, Handelsman J |title=Status of the microbial census |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15590780#r6 |journal=Microbiol Mol Biol Rev |volume=68 |issue=4 |pages=686–91 |year=2004 |pmid=15590780 |doi=10.1128/MMBR.68.4.686-691.2004] 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. [ [http://www.sciam.com/askexpert_question.cfm?articleID=00071863-683B-1C72-9EB7809EC588F2D7 Scientific American; Biology: Is the human race evolving or devolving?] retrieved 2007-06-11]

ee also

* Biocomplexity
* Biodiversity
* Biosphere
* Complex adaptive system
* Complexity
* Ecosystem
* Orthogenesis

References