Consequences of secondary contact Schematic

Secondary contact is the process in which two allopatricaly 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, the formation of a hybrid zone, and the formation of a new species through hybrid speciation.

Extinction

One of the two populations may go extinct 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'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

'' and Neanderthals, 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 and Middle Paleolithic. Denisovans are known from few physical remains and consequently, most of what is kn ...

, 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, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offsprin ...

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 Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...

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. 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 behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher fr ...

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 Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. It is used in horticulture, animal breeding, and produ ...

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 one pair of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...

, 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