[Post 3] Hybrid species

Hybrid species

Hybrid species are caused by two separate species interbreeding, and this is fairly common within nature (this is known as introgressive hybridisation). Up to 10% of animals, and 25% of plants are involved in some form of hybridisation (classic or introgressive). Hybrids somewhat confuse the current definition of a species, as sometimes the offspring of two species are born fertile.  Often in law, the conservation status of these hybrids are unclear, for example, in the 1973 Endangered Species Act in the United States.  Humans are believed to have exacerbated hybridization through activities such as aquaculture, agriculture, habitat alteration and the accidental introduction of invasive species, for example, via ballast water from boats . Climate change is also likely to have enhanced hybridisation, due to species shifting their home ranges. Usually the conservation issue with hybridisation occurs when a rare or endangered species can form viable offspring with a very common, or non-threatened species. Introgressive hybridisation is the term used to describe when genes flow between the two mating species backwards and forwards, in other words, hybrids are breeding with an individual from one of its parental species. Introgressive hybridisation is different from ‘classic’ hybridisation as it contains a different mix of genes eg. 25% one species, 75% another species, by mating more often with a certain species. Whereas classic hybridisation, that occurs between two different species will contain roughly 50% of each species genes, when there is no preference to mate with a certain species. Hybridisation is a very important part of speciation and maintains genetic diversity by mixing genes. 

The above image is a hybrid individual between a mistle thrush and a blackbird, both species are in the same genus (Turdus). Due to sharing the same genus, the two species are closely related, which is why this individual will have developed well within the egg and survived into adulthood. It is likely that this specimen was infertile. This taxidermy specimen is located in the World Museum, Liverpool.