Cospeciation is a form of

coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...

in which the

speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...

of one species dictates speciation of another species and is most commonly studied in host-

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...

relationships. In the case of a host-parasite relationship, if two hosts of the same species get within close proximity of each other, parasites of the same species from each host are able to move between individuals and mate with the parasites on the other host. However, if a speciation event occurs in the host species, the parasites will no longer be able to "cross over" because the two new host species no longer mate and, if the speciation event is due to a geographic separation, it is very unlikely the two hosts will interact at all with each other. The lack of proximity between the hosts ultimately prevents the populations of parasites from interacting and mating. This can ultimately lead to speciation within the parasite. According to Fahrenholz's rule, first proposed by Heinrich Fahrenholz in 1913, when host-parasite cospeciation has occurred, the phylogenies of the host and parasite come to mirror each other. In host-parasite phylogenies, and all species phylogenies for that matter, perfect mirroring is rare. Host-parasite phylogenies can be altered by host switching, extinction, independent speciation, and other ecological events, making cospeciation harder to detect. However, cospeciation is not limited to parasitism, but has been documented in symbiotic relationships like those of gut microbes in primates.

Fahrenholz's rule

In 1913, Heinrich Fahrenholz proposed that the phylogenies of both the host and parasite will eventually become congruent, or mirror each other when cospeciation occurs. More specifically, more closely related parasite species will be found on closely related species of host. Thus, to determine if cospeciation has occurred within a host-parasite relationship, scientists have used comparative analyses on the host and parasite phylogenies. In 1968,

Daniel Janzen Daniel Hunt Janzen (born January 18, 1939 in Milwaukee, Wisconsin) is an American evolutionary ecologist, and conservationist. He divides his time between his professorship in biology at the University of Pennsylvania, where he is the DiMaura ...

proposed an opposing theory to Fahrenholz's rule. Studying cospeciation within plant-insect relationships, he proposed that species have a physiological range of conditions and environments. Over time, conserved traits within a parasitic species allows for survival in a range of conditions or environments. "Ecological fitting", as it is known, means more closely related parasites will share similar traits that pertain to surviving on a particular host. This provides explanation for the congruence of the host-parasite phylogenies.

Parasitic cospeciation

Fahrenholz's rule appears to be observed in the parasitic cospeciation of pocket gophers and

chewing lice The Mallophaga are a possibly paraphyletic section of lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been reco ...

. It is seen, too, between ''

Poaceae Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns an ...

'' grasses and '' Anguininae''

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...

s, and between some plants and ''

Phyllonorycter ''Phyllonorycter'' is a genus of moths in the family Gracillariidae. Diversity The genus comprises about 400 species, with a worldwide distribution. The vast majority of species are found in the temperate regions, with about 257 species describ ...

'' leaf-mining moths.

Symbiotic cospeciation

Among animals, symbiotic cospeciation is seen between '' Uroleucon'' (aphids) and '' Buchnera'' (plants in the

Orobanchaceae Orobanchaceae, the broomrapes, is a family of mostly parasitic plants of the order Lamiales, with about 90 genera and more than 2000 species. Many of these genera (e.g., ''Pedicularis'', ''Rhinanthus'', '' Striga'') were formerly included in ...

), between deep sea

clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shel ...

s and chemoautotrophic

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

, and between ''

Dendroctonus ''Dendroctonus'' is a genus of bark beetles. It includes several species notorious for destroying trees in the forests of North America. The genus has a symbiotic relationship with many different yeasts, particularly those in the genera '' Cand ...

'' bark beetles and certain fungi. Symbiotic cospeciation is found between ''

Crematogaster ''Crematogaster'' is an ecologically diverse genus of ants found worldwide, which are characterised by a distinctive heart-shaped gaster (abdomen), which gives them one of their common names, the Saint Valentine ant. Members of this genus are ...

'' ants and ''

Macaranga ''Macaranga'' is a large genus of Old World tropical trees of the family Euphorbiaceae and the only genus in the subtribe Macaranginae (tribe Acalypheae). Native to Africa, Australasia, Asia and various islands of the Indian and Pacific Oceans, t ...

'' plants, between ''

Ficus ''Ficus'' ( or ) is a genus of about 850 species of woody trees, shrubs, vines, epiphytes and hemiepiphytes in the family Moraceae. Collectively known as fig trees or figs, they are native throughout the tropics with a few species extendi ...

'' fig trees and

fig wasp Fig wasps are wasps of the superfamily Chalcidoidea which spend their larval stage inside figs. Most are pollinators but others simply feed off the plant. The non-pollinators belong to several groups within the superfamily Chalcidoidea, while the ...

s, and between the ''

'' grasses and '' Epichloe'' fungi.

False incongruence

The two main hurdles to determining cospeciation using Fahrenholz's rule are instances of false congruence and false incongruence. False congruence occurs when parasite and host phylogenies mirror each other but not due to cospeciation, for instance, if the parasites were to colonize the hosts after the host species had diverged and congruent phylogenies resulted by chance, but this is unlikely. False incongruence, when cospeciation has occurred, but the phylogenies do not mirror each other, is more common, and can be caused by a number of factors; it might also appear to be present if parasites that are present on a host are not detected by the experimenter.

Host switching

Though parasites have been thought to be specialized to a certain host species, it is common for a parasite to colonize a different host that was not previous colonized by the parasite species. If a " host switch" occurs after a cospeciation event, the presence of the parasite on other host species will disrupt any potential congruence in the two phylogenies. Coupled with extinction or independent speciation, phylogenetic comparisons can become complicated and entirely mask the cospeciation event.

Independent speciation

Typically independent speciation does not significantly alter the phylogenetic analysis used to measure cospeciation. However, in combination with extinction, independent speciation can become very problematic when trying to sort out host and parasite phylogenies. Independent speciation occurs when a single population on a single host undergoes speciation resulting in two sister lineages of parasite on a particular host. In other words, the parasite lineage speciates while the host lineage does not. This becomes complicated when the two lineages of parasites then undergo cospeciation with the host. If one of the two parasite lineages goes extinct from the new host lineage, the phylogenies of the host and parasite will begin to break apart. Even though the parasite and host cospeciated together, the phylogenies will not be congruent.

Extinction

After cospeciation, it is possible for a parasite (or symbiont) to become extinct while its host survives. This can happen if, for example, the host species adapts to a new habitat.

"Missing the boat"

Prior to speciation of the hosts, if the distribution of the parasite population among the host population is sporadic, it is possible that when host speciation occurs, it will occur with hosts that do not have the parasite population. This phenomenon is known as "missing the boat". The parasites could potentially cospeciate with their host down the line, however, the parasites could potentially be absent from some host lineages. Like extinction and independent speciation, "missing the boat" alone will probably have a minimal effect on mapping phylogenies, however, in conjunction with independent speciation, parasite and host phylogenies can begin to break apart.

References

{{Biological rules, show=no Speciation Biological rules