Extinction speciation is another example of instant speciation (Fig. XXI.2). A great many species form an extended linear series of individual populations in their area of occurrence, where exchange of genetic material (sexual reproduction) occurs solely or almost solely between neighboring populations. Geographically more distant populations can thus be so phenotypically and genetically different that their members are not capable of fertile crossing and can occupy different ecological niches in nature. Nonetheless, they must be considered to be the members of a single species, as exchange of genetic material is mediated by a number of mutually neighboring populations, which geographically connect them. Ring species are an interesting example of species with this type of population structure. Their range of occurrence frequently forms a relatively narrow line of mutually neighboring populations that, for example, encircle a certain geographic obstacle (Californian salamanders of the Ensatina genus) or that even extend in a strip around the entire Earth (the lesser black-backed gull (Larus fuscus) and the herring gull (Larus argentatus)). These species spread in the past in one or both directions from the place of their original occurrence until their populations met secondarily in a certain area and closed the range of occurrence to form its present-day ring shape. Thus the phenotypically and genetically most distant populations met at the site of encounter and their members act as well defined species; in the ideal case, they do not cross and even do not compete much. Extinction speciation occurs when one of the inner populations becomes extinct, interrupting the gene flow from one end of the range to the other.
- more or less1831
- not at all2020