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A large variety of mechanisms have been demonstrated to reinforce reproductive isolation between closely related plant species that either historically lived or currently live in sympatry. This phenomenon is driven by strong selection against hybrids, typically resulting from instances in which hybrids suffer reduced fitness.
Costus plants: Costus allenii, C. laevis, and C. guanaiensis; [62] [63] C. pulverulentus and C. scaber [64] A unique case of post-zygotic instead of prezygotic isolation has been observed in both Gossypium and Gilia, suggesting that in plants, post-zygotic isolation's role in reinforcement may play a larger role. [3]: 361
Increased prezygotic isolation, which is associated with reproductive character displacement, has been observed in cicadas of genus Magicicada, stickleback fish, and the flowering plants of the genus Phlox.
It is possible that the pattern of enhanced isolation could simply be a temporary outcome of secondary contact where two allopatric species already have a varying range of prezygotic isolation: with some exhibiting more than others. [42] Those that have weaker prezygotic isolation will eventually fuse, losing their distinctiveness. [7]
All three species are separated by intrinsic reproductive barriers [1] 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.
Reproductive isolation is the process of species evolving mechanisms to prevent reproduction with other species. In plants, this is accomplished through the manipulation of the pollinator’s behavior (ethological isolation) or through morphological characteristics of flowers that favor intraspecific pollen transfer (morphological isolation).
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 thought to be particularly difficult to achieve, and thus hybrid species were thought to be very rare.
In plants, this can effectively be a new species, reproductively isolated from its parents, and able to reproduce. Main article: Polyploid Polyploidy is a mechanism that has caused many rapid speciation events in sympatry because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny. [ 76 ]