Consequences of secondary contact Schematic

Secondary contact is the process in which two allopatrically distributed populations of a species are geographically reunited. This contact allows for the potential for the exchange of genes, dependent on how reproductively isolated the two populations have become. There are several primary outcomes of secondary contact: extinction of one species, fusion of the two populations back into one,

reinforcement In Behaviorism, behavioral psychology, reinforcement refers to consequences that increase the likelihood of an organism's future behavior, typically in the presence of a particular ''Antecedent (behavioral psychology), antecedent stimulus''. Fo ...

, the formation of a

hybrid zone A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form ''in situ'' due to the evolution of a new lineage but generally they result from secondary cont ...

, and the formation of a new species through

hybrid speciation Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was tho ...

Extinction

One of the two populations may go

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

due to competitive exclusion after secondary contact. This tends to happen when the two populations have strong reproductive isolation and significant overlap in their niche. A possible way to prevent extinction is if there is an advantage to being rare. For example, sexual imprinting and male-male competition may prevent extinction. The population that goes extinct may leave behind some of its genes in the surviving population if they hybridize. For example, the secondary contact between ''

Homo sapiens Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

'' and

Neanderthals Neanderthals ( ; ''Homo neanderthalensis'' or sometimes ''H. sapiens neanderthalensis'') are an extinction, extinct group of archaic humans who inhabited Europe and Western and Central Asia during the Middle Pleistocene, Middle to Late Plei ...

, as well as the

Denisovans The Denisovans or Denisova hominins ( ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower Paleolithic, Lower and Middle Paleolithic, and lived, based on current evidence, from 285 thousand to 25 thou ...

, left traces of their genes in modern human. However, if hybridization is so common that the resulting population received significant amount of genetic contribution from both populations, the result should be considered a fusion.

Fusion

The two populations may fuse back into one population. This tends to occur when there is little to no

reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensu ...

between the two. During the process of fusion, a hybrid zone may occur. This is sometimes called introgressive hybridization or reverse speciation. Concerns have been raised that the homogenizing of the environment may contribute to more and more fusion, leading to the loss of biodiversity.

Hybrid zones

A hybrid zone may appear during secondary contact, meaning there would be an area where the two populations cohabitate and produce hybrids, often arranged in a

cline Cline may refer to: Science * Cline (biology), a measurable gradient in a single trait in a species across its geographical range * Cline (hydrology), a fluid layer with a property that varies * Cline (mathematics) or generalised circle, a ci ...

. The width of the zone may vary from tens of meters to several hundred kilometers. A hybrid zone may be stable, or it may not. Some shift in one direction, which may eventually lead to the extinction of the receding population. Some expand over time until the two populations fuse.

Reinforcement In Behaviorism, behavioral psychology, reinforcement refers to consequences that increase the likelihood of an organism's future behavior, typically in the presence of a particular ''Antecedent (behavioral psychology), antecedent stimulus''. Fo ...

may occur in hybrid zones. Hybrid zones are important study systems for speciation.

Reinforcement

Reinforcement is the evolution towards increased reproductive isolation due to selection against hybridization. This occurs when the populations already have some reproductive isolation, but still hybridize to some extent. Because hybridization is costly (e.g. giving birth and raising a weak offspring), natural selection favors strong isolation mechanisms that can avoid such outcome, such as assortative mating. Evidence for speciation by reinforcement has been accumulating since the 1990s.

Hybrid speciation

Occasionally, the hybrids may be able to survive and reproduce, but not backcross with either of the two parental lineages, thus becoming a new species. This often occur in plants through

polyploidy Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...

, including in many important food crops. Occasionally, the hybrids may lead to the extinction of one or both parental lineages.

References

{{DEFAULTSORT:Speciation Ecology Evolutionary biology Speciation