Secondary contact

Consequences of secondary contact Schematic

Secondary contact refers to a stage in the speciation process where two populations that have diverged in isolation from each other, and thus have begun to evolve into separate species, come back into contact. This can lead to a variety of outcomes, including hybridization, reinforcement of species boundaries, the creation of new species, or even the merging back into a single species. The concept of secondary contact is significant in the field of evolutionary biology and biogeography, offering insights into how biodiversity is generated and maintained.

Overview

After an initial period of geographic isolation, which prevents gene flow between two populations of the same species, these populations may evolve independently due to natural selection, genetic drift, or both. If these populations come into contact again, this is known as secondary contact. The outcomes of secondary contact are highly variable and depend on numerous factors, including the degree of genetic divergence between the populations, their ecological niches, and the presence of reproductive barriers.

Outcomes of Secondary Contact

Hybridization

Hybridization occurs when individuals from the two populations interbreed, producing offspring that share genetic material from both parent populations. This can lead to the creation of hybrid zones, areas where hybrids are common. Hybrids may have new combinations of traits that can be advantageous, disadvantageous, or neutral in their environment.

Reinforcement

Reinforcement is the process by which reproductive barriers between the two populations are strengthened as a result of secondary contact. This occurs because hybrids often have reduced fitness compared to purebred individuals, leading natural selection to favor traits that prevent interbreeding. Over time, this can lead to the completion of speciation.

Creation of New Species

In some cases, hybridization in secondary contact zones can lead to the emergence of new species, a process known as speciation by hybridization. This can happen if hybrids possess unique combinations of traits that allow them to exploit different ecological niches or if they become reproductively isolated from both parent populations.

Merging

Conversely, if the genetic and ecological differences between the two populations are not significant, secondary contact might lead to the merging of the populations back into a single species. This can occur if hybrids are as fit or fitter than individuals from the parent populations and there are no strong reproductive barriers.

Factors Influencing Outcomes

The outcome of secondary contact is influenced by several factors, including:

• The extent of genetic divergence between the populations
• The ecological niches of the populations and hybrids
• The strength and types of reproductive barriers
• The fitness of hybrid individuals

Research and Implications

Studying secondary contact provides valuable insights into the mechanisms of speciation and the dynamics of biodiversity. It helps scientists understand how new species form and how genetic and ecological diversity is maintained. Furthermore, understanding secondary contact has implications for conservation biology, especially in the context of habitat fragmentation and climate change, which can force previously isolated populations into contact.

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