Editing Parasitism (section)

==== Long-term partnerships favouring mutualism ====

[[File:Wolbachia.png|thumb|left|''[[Wolbachia]]'' bacteria within an insect cell]]

Long-term partnerships can lead to a relatively stable relationship tending to [[commensalism]] or [[Mutualism (biology)|mutualism]], as, all else being equal, it is in the evolutionary interest of the parasite that its host thrives. A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals parasitised by [[helminth|worms]] are often clearly harmed, such infections may also reduce the prevalence and effects of [[Autoimmunity|autoimmune]] disorders in animal hosts, including humans.<ref name=Rook2007>{{cite journal |last=Rook |first=G. A. |url=https://www.academia.edu/580118 |title=The hygiene hypothesis and the increasing prevalence of chronic inflammatory disorders |journal=Transactions of the Royal Society of Tropical Medicine and Hygiene |year=2007 |volume=101 |issue=11 |pages=1072–1074 |pmid=17619029 |doi=10.1016/j.trstmh.2007.05.014}}</ref> In a more extreme example, some [[nematode]] worms cannot reproduce, or even survive, without infection by ''[[Wolbachia]]'' bacteria.<ref>{{Cite journal |last=Werren |first=John H. |date=February 2003 |title=Invasion of the Gender Benders: by manipulating sex and reproduction in their hosts, many parasites improve their own odds of survival and may shape the evolution of sex itself |journal=[[Natural History (magazine)|Natural History]] |volume=112 |issue=1 |page=58 |oclc=1759475 |url=http://findarticles.com/p/articles/mi_m1134/is_1_112/ai_97174198 |access-date=15 November 2008 |url-status=dead |archive-url=https://archive.today/20120708190307/http://findarticles.com/p/articles/mi_m1134/is_1_112/ai_97174198/ |archive-date=8 July 2012 }}</ref>

[[Lynn Margulis]] and others have argued, following [[Peter Kropotkin]]'s 1902 ''[[Mutual Aid: A Factor of Evolution]]'', that natural selection drives relationships from parasitism to mutualism when resources are limited. This process may have been involved in the [[symbiogenesis]] which formed the [[eukaryote]]s from an intracellular relationship between [[archaea]] and bacteria, though the sequence of events remains largely undefined.<ref>{{cite book |last1=Margulis |first1=Lynn |author1-link=Lynn Margulis |last2=Sagan |first2=Dorion |author2-link=Dorion Sagan |author3=Eldredge, Niles |author3-link=Niles Eldredge |year=1995 |url=https://books.google.com/books?id=4IIpAQAAMAAJ |title=What Is Life? |publisher=Simon and Schuster |isbn=978-0-684-81087-4}}</ref><ref>{{cite book |last1=Sarkar |first1=Sahotra |last2=Plutynski |first2=Anya |title=A Companion to the Philosophy of Biology |url=https://books.google.com/books?id=iN7UYNjbxsYC&pg=PA358 |year=2008 |publisher=John Wiley & Sons |isbn=978-0-470-69584-5 |page=358}}</ref>