Editing Parasitism (section)

=== Coevolution ===

{{further|Host–parasite coevolution}}

As hosts and parasites evolve together, their relationships often change. When a parasite is in a sole relationship with a host, selection drives the relationship to become more benign, even mutualistic, as the parasite can reproduce for longer if its host lives longer.<ref name=Rook2007/> But where parasites are competing, selection favours the parasite that reproduces fastest, leading to increased virulence. There are thus varied possibilities in [[host–parasite coevolution]].<ref name=MasseyBuckling2004/>

[[Evolutionary epidemiology]] analyses how parasites spread and evolve, whereas [[Darwinian medicine]] applies similar evolutionary thinking to non-parasitic diseases like [[cancer]] and [[Autoimmune disease|autoimmune conditions]].<ref name="Ewald1994">{{cite book |last=Ewald |first=Paul W. |title=Evolution of Infectious Disease |url=https://archive.org/details/evolutionofinfec0000ewal|url-access=registration |year=1994 |publisher=Oxford University Press |isbn=978-0-19-534519-3 |page=8}}</ref>

==== 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>

==== Competition favouring virulence ====

Competition between parasites can be expected to favour faster reproducing and therefore more [[Virulence|virulent]] parasites, by [[natural selection]].<ref name=MasseyBuckling2004>{{cite journal |last1=Massey |first1=R. C. |last2=Buckling |first2=A. |last3=ffrench-Constant |first3=R.|author1-link=Ruth Massey |title=Interference competition and parasite virulence |journal=Proceedings of the Royal Society B: Biological Sciences |volume=271 |issue=1541 |year=2004 |pages=785–788 |doi=10.1098/rspb.2004.2676 |pmc=1691666 |pmid=15255095}}</ref><ref name=Rigaud2010>{{cite journal |last1=Rigaud |first1=T. |last2=Perrot-Minnot |first2=M.-J. |last3=Brown |first3=M. J. F. |title=Parasite and host assemblages: embracing the reality will improve our knowledge of parasite transmission and virulence |journal=Proceedings of the Royal Society B: Biological Sciences |volume=277 |issue=1701 |year=2010 |pages=3693–3702 |doi=10.1098/rspb.2010.1163|pmid=20667874 |pmc=2992712 }}</ref>

[[File:Pink Flamingos with Duck - Camargue, France - April 2007 (cropped).jpg|thumb|Biologists long suspected [[cospeciation]] of [[flamingo]]s and [[duck]]s with their parasitic [[louse|lice]], which were similar in the two families. Cospeciation did occur, but it led to flamingos and [[grebe]]s, with a later [[host switch]] of flamingo lice to ducks.]]

Among competing parasitic insect-killing bacteria of the genera ''[[Photorhabdus]]'' and ''[[Xenorhabdus]]'', virulence depended on the relative potency of the antimicrobial [[toxin]]s ([[bacteriocins]]) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when [[caterpillar]]s were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.<ref name=MasseyBuckling2004/>

==== Cospeciation ====

A parasite sometimes undergoes [[cospeciation]] with its host, resulting in the pattern described in [[Fahrenholz's rule]], that the phylogenies of the host and parasite come to mirror each other.<ref name=Page>{{cite book |last=Page |first=Roderic D. M. |publisher=John Wiley |date=27 January 2006 |isbn=978-0-470-01617-6 |doi=10.1038/npg.els.0004124|title=Encyclopedia of Life Sciences |chapter=Cospeciation }}</ref>

An example is between the [[simian foamy virus]] (SFV) and its primate hosts. The phylogenies of SFV polymerase and the mitochondrial [[cytochrome c oxidase subunit II]] from African and Asian primates were found to be closely congruent in branching order and divergence times, implying that the simian foamy viruses cospeciated with Old World primates for at least 30&nbsp;million years.<ref name="SwitzerSalemi2005">{{cite journal |last1=Switzer |first1=William M. |last2=Salemi |first2=Marco |last3=Shanmugam |first3=Vedapuri |last4=Gao |first4=Feng |last5=Cong |first5=Mian-er |last6=Kuiken |first6=Carla |last7=Bhullar |first7=Vinod |last8=Beer |first8=Brigitte E. |last9=Vallet |first9=Dominique |last10=Gautier-Hion |first10=Annie |last11=Tooze |first11=Zena |last12=Villinger |first12=Francois |last13=Holmes |first13=Edward C. |last14=Heneine |first14=Walid |display-authors=3 |title=Ancient co-speciation of simian foamy viruses and primates |journal=Nature |volume=434 |issue=7031 |year=2005 |pages=376–380 |doi=10.1038/nature03341|pmid=15772660 |bibcode=2005Natur.434..376S |s2cid=4326578 |url=https://zenodo.org/record/1233279 }}</ref>

The presumption of a shared evolutionary history between parasites and hosts can help elucidate how host taxa are related. For instance, there has been a dispute about whether [[Phoenicopteriformes|flamingos]] are more closely related to [[Ciconiiformes|storks]] or [[Anseriformes|ducks]]. The fact that flamingos share parasites with ducks and geese was initially taken as evidence that these groups were more closely related to each other than either is to storks. However, evolutionary events such as the duplication, or the extinction of parasite species (without similar events on the host phylogeny) often erode similarities between host and parasite phylogenies. In the case of flamingos, they have similar lice to those of [[grebe]]s. Flamingos and grebes do have a common ancestor, implying cospeciation of birds and lice in these groups. Flamingo lice then [[host switch|switched hosts]] to ducks, creating the situation which had confused biologists.<ref name=JohnsonKennedy2006>{{cite journal|last1=Johnson|first1=K. P.|last2=Kennedy|first2=M.|last3=McCracken|first3=K. G|title=Reinterpreting the origins of flamingo lice: cospeciation or host-switching?|journal=Biology Letters|volume=2|issue=2|year=2006|pages=275–278|doi=10.1098/rsbl.2005.0427|pmc=1618896|pmid=17148381}}</ref>

[[File:Toxoplasma gondii (2).jpg|thumb|The protozoan ''[[Toxoplasma gondii]]'' facilitates its transmission by [[Behavior-altering parasites|inducing behavioral changes]] in rats through infection of neurons in their [[central nervous system]].]]

Parasites infect [[sympatry|sympatric]] hosts (those within their same geographical area) more effectively, as has been shown with [[Digenea|digenetic trematodes]] infecting lake snails.<ref name="Lively00"/> This is in line with the [[Red Queen hypothesis]], which states that interactions between species lead to constant natural selection for coadaptation. Parasites track the locally common hosts' phenotypes, so the parasites are less infective to [[allopatric speciation|allopatric]] hosts, those from different geographical regions.<ref name="Lively00">{{cite journal |last1=Lively |first1=C. M. |last2=Dybdahl |first2=M. F. |url=https://public.wsu.edu/~dybdahl/nature00.pdf |archive-url=https://web.archive.org/web/20160607131917/http://public.wsu.edu/~dybdahl/nature00.pdf |archive-date=7 June 2016 |url-status=live |title=Parasite adaptation to locally common host genotypes |journal=Nature |volume=405 |issue=6787 |pages=679–81 |year=2000 |pmid=10864323 |doi=10.1038/35015069 |bibcode=2000Natur.405..679L |s2cid=4387547 }}</ref>