Reinforcement (speciation)


Reinforcement is a process of speciation where natural selection increases the reproductive isolation (further divided to pre-zygotic isolation and post-zygotic isolation) between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation (differences in behavior or biology that inhibit formation of hybrid zygotes). Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly.[1] This aspect has been particularly appealing among evolutionary biologists.[2]

The support for reinforcement has fluctuated since its inception, and terminological confusion and differences in usage over history have led to multiple meanings and complications. Various objections have been raised by evolutionary biologists as to the plausibility of its occurrence. Since the 1990s, data from theory, experiments, and nature have overcome many of the past objections, rendering reinforcement widely accepted,[3]: 354  though its prevalence in nature remains unknown.[4][5]

Numerous models have been developed to understand its operation in nature, most relying on several facets: genetics, population structures, influences of selection, and mating behaviors. Empirical support for reinforcement exists, both in the laboratory and in nature. Documented examples are found in a wide range of organisms: both vertebrates and invertebrates, fungi, and plants. The secondary contact of originally separated incipient species (the initial stage of speciation) is increasing due to human activities such as the introduction of invasive species or the modification of natural habitats.[6] This has implications for measures of biodiversity and may become more relevant in the future.[6]

Reinforcement has had a complex history in that its popularity among scholars has changed over time.[7][8] Jerry Coyne and H. Allen Orr contend that the theory of reinforcement went through three phases of historical development:[3]: 366 

Sometimes called the Wallace effect, reinforcement was originally proposed by Alfred Russel Wallace in 1889.[9]: 353  His hypothesis differed markedly from the modern conception in that it focused on post-zygotic isolation, strengthened by group selection.[10][11][3]: 353  Theodosius Dobzhansky was the first to provide a thorough description of the process in 1937,[3]: 353  though the term itself was not coined until 1955 by W. Frank Blair.[12] In 1930, Ronald Fisher laid out the first genetic description of the process of reinforcement in The Genetical Theory of Natural Selection, and in 1965 and 1970 the first computer simulations were run to test for its plausibility.[3]: 367  Later population genetic[13] and quantitative genetic[14] studies were conducted showing that completely unfit hybrids lead unequivocally to an increase in prezygotic isolation.[3]: 367 


Reinforcement assists speciation by selecting against hybrids upon the secondary contact of two separated populations of a species.
Alfred Russel Wallace proposed in 1889 that isolation could be strengthened by a form of selection.
The four outcomes of secondary contact:
1. An extrinsic barrier separates a species population into two but they come into contact before reproductive isolation is sufficient to result in speciation. The two populations fuse back into one species
2. Speciation by reinforcement
3. Two separated populations stay genetically distinct while hybrid swarms form in the zone of contact
4. Genome recombination results in speciation of the two populations, with an additional hybrid species. All three species are separated by intrinsic reproductive barriers[19]
A parameter space representing the conditions in which speciation by reinforcement can occur. Here, three outcomes can arise: 1) extinction of one of the initial populations; 2) the initial populations can hybridize; 3) the initial populations can speciate. The outcomes are determined by both initial divergence and level of fitness of the hybrids.[23]
Two allopatric populations come into secondary contact. In sympatry, divergence is exhibited by changes in mating traits. These patterns of reproductive character displacement detected in species populations that exist in zones of overlap indicate that the process of speciation by reinforcement has occurred.
Prezygotic isolation in allopatric (red) and sympatric (blue) species pairs of Drosophila. Gradients indicate the predictions of reinforcement for allopatric and sympatric populations.[37]
Phylogenetic signature to distinguish sympatric speciation from reinforcement. Stronger prezygotic isolation (indicated by the red boxes and associated arrows) should be detected between Z and Y and between Z and X if species Z sympatrically speciated (green) from the common ancestor of species Y and X. If Z, Y, and X speciated allopatrically (blue), with Z and Y experiencing secondary contact, strong prezygotic isolation should be found between Z and Y, but not between Z and X.[45]