Cospeciation

Cospeciation is a form of coevolution in which the speciation of one species dictates speciation of another species and is most commonly studied in host-parasite 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. Cospeciation is a form of coevolution in which the speciation of one species dictates speciation of another species and is most commonly studied in host-parasite 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. 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 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. Fahrenholz's rule appears to be observed in the parasitic cospeciation of pocket gophers and chewing lice. It is seen, too, between Poaceae grasses and Anguininae nematodes,and between some plants and Phyllonorycter leaf-mining moths. Among animals, symbiotic cospeciation is seen between Uroleucon (aphids) and Buchnera (plants in the Orobanchaceae), between deep sea clams and chemoautotrophic bacteria, and between Dendroctonus bark beetles and certain fungi. Symbiotic cospeciation is found between Crematogaster ants and Macaranga plants, between Ficus fig trees and chalcid wasps, and between the Poaceae grasses and Epichloe fungi. 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.