Editing Complex adaptive system (section)

== Evolution of complexity ==
[[File:Evolution of complexity.svg|thumb|300px|Passive versus active trends in the evolution of complexity. CAS at the beginning of the processes are colored red. Changes in the number of systems are shown by the height of the bars, with each set of graphs moving up in a time series.]]

{{Main|Evolution of biological complexity}}

Living organisms are complex adaptive systems. Although complexity is hard to quantify in biology, [[evolution]] has produced some remarkably complex organisms.<ref>{{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|doi-access=free }}</ref> This observation has led to the common misconception of evolution being progressive and leading towards what are viewed as "higher organisms".<ref>{{cite journal |author=McShea D |title=Complexity and evolution: What everybody knows |journal=Biology and Philosophy |volume=6 |issue=3 |pages=303–24 |year=1991 |doi=10.1007/BF00132234|s2cid=53459994 }}</ref>

If this were generally true, evolution would possess an active trend towards complexity. As shown below, in this type of process the value of the most common amount of complexity would increase over time.<ref name=Carroll>{{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|bibcode = 2001Natur.409.1102C |s2cid=4319886 }}</ref> Indeed, some [[artificial life]] simulations have suggested that the generation of CAS is an inescapable feature of evolution.<ref>{{cite journal |vauthors=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 |bibcode=2000PhRvL..84.6130F|arxiv = nlin/0009008 |s2cid=13985096 }}</ref><ref>{{cite journal |vauthors=Adami C, Ofria C, Collier TC |title=Evolution of biological complexity |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 |pmc=18257|arxiv = physics/0005074 |bibcode = 2000PNAS...97.4463A |doi-access=free }}</ref>

However, the idea of a general trend towards complexity in evolution can also be explained through a passive process.<ref name=Carroll/> This involves an increase in [[variance]] but the most common value, the [[mode (statistics)|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. This type of random process is also called a bounded [[random walk]].

In this hypothesis, the apparent trend towards more complex organisms is an illusion resulting from concentrating on the small number of large, very complex organisms that inhabit the [[Skewness|right-hand tail]] of the complexity distribution and ignoring simpler and much more common organisms. This passive model emphasizes that the overwhelming majority of species are [[microorganism|microscopic]] [[prokaryote]]s,<ref>{{cite journal |author=Oren A |title=Prokaryote diversity and taxonomy: current status and future challenges |pmc=1693353 |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}}</ref> which comprise about half the world's [[Biomass (ecology)|biomass]]<ref>{{cite journal |vauthors=Whitman W, Coleman D, Wiebe W | title = Prokaryotes: the unseen majority | journal = Proc Natl Acad Sci USA | volume = 95 | issue = 12 | pages = 6578–83 | year = 1998 |pmid = 9618454 | doi = 10.1073/pnas.95.12.6578 | pmc = 33863|bibcode = 1998PNAS...95.6578W | doi-access = free }}</ref> and constitute the vast majority of Earth's biodiversity.<ref>{{cite journal |vauthors=Schloss P, Handelsman J |title=Status of the microbial census  |pmc=539005 |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}}</ref> Therefore, simple life remains dominant on Earth, and complex life appears more diverse only because of [[sampling bias]].

If there is a lack of an overall trend towards complexity in biology, this would not preclude the existence of forces driving systems towards complexity in a subset of cases. These minor trends would be balanced by other evolutionary pressures that drive systems towards less complex states.