Reinforcement (speciation)

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Speciation by Reinforcement Schematic

Alfred-Russel-Wallace-c1895

Consequences of secondary contact Schematic

Reinforcement Parameter Space (annot)

Reproductive Character Displacement

Enhanced pre-zygotic isolation in Drosophila (allopatric & sympatric plots-vert)

Reinforcement (speciation) is an evolutionary process by which natural selection increases the reproductive isolation between two populations of species. This process occurs when hybrid offspring between the populations are less fit than their parents, leading to selection against hybridization. This strengthens prezygotic barriers to reproduction, effectively reducing gene flow between the populations and potentially leading to the formation of new species. Reinforcement is considered a critical mechanism in the final stages of speciation, contributing to the biodiversity observed in nature.

Overview

The concept of reinforcement in speciation is grounded in the biological principle that species evolve through the accumulation of genetic differences which, over time, can lead to reproductive isolation. When two populations of the same species become geographically separated (allopatric speciation), they may evolve independently through mutations, genetic drift, and natural selection. If these populations come into contact again, they might have diverged sufficiently that they can no longer produce viable or fertile offspring, a state known as reproductive isolation.

Reinforcement comes into play when there is still some level of hybridization between these populations. If the hybrids are less fit than their parents, natural selection will favor individuals in each population that avoid mating with the other population, thus reinforcing reproductive isolation.

Mechanisms

The mechanisms of reinforcement can be broadly categorized into prezygotic and postzygotic barriers. Prezygotic barriers include any factors that prevent fertilization from occurring, such as differences in mating behavior, physical incompatibilities, or temporal differences in mating periods. Postzygotic barriers occur after fertilization and include factors that reduce the viability or fertility of hybrid offspring.

Reinforcement typically emphasizes the evolution of prezygotic barriers because they directly prevent the formation of less fit hybrids. This can involve changes in behavioral traits (e.g., mating calls in frogs) or physical traits (e.g., flower morphology in plants) that make it easier for individuals to distinguish between members of their own population and those of a diverging population.

Evidence

Evidence for reinforcement has been observed in various species, including insects, birds, and plants. One of the classic examples is the case of two species of fruit flies, Drosophila pseudoobscura and Drosophila persimilis, which show stronger prezygotic isolation in regions where they co-occur than in regions where each species exists alone.

Controversy

The concept of reinforcement has been controversial among evolutionary biologists. Some argue that it is difficult to distinguish between reinforcement and other processes that could lead to increased reproductive isolation, such as ecological speciation or the independent evolution of prezygotic barriers without hybridization. However, the accumulation of empirical evidence supporting reinforcement, particularly from studies that demonstrate increased reproductive isolation due to reduced hybrid fitness, has helped to solidify its role in speciation.

Conclusion

Reinforcement is a significant evolutionary process that contributes to the diversity of life by promoting the formation of new species. Through the strengthening of reproductive barriers, reinforcement reduces gene flow between diverging populations, facilitating the accumulation of genetic differences necessary for speciation. As research continues, our understanding of the complexities and nuances of reinforcement and its contribution to the speciation process is likely to deepen.