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The types of barriers that can cause this isolation include: different habitats, physical barriers, and a difference in the time of sexual maturity or flowering. [ 6 ] [ 7 ] An example of the ecological or habitat differences that impede the meeting of potential pairs occurs in two fish species of the family Gasterosteidae (sticklebacks).
[3]: 354 [4] [5] Differences in behavior or biology that inhibit formation of hybrid zygotes are termed prezygotic isolation. Reinforcement can be shown to be occurring (or to have occurred in the past) by measuring the strength of prezygotic isolation in a sympatric population in comparison to an allopatric population of the same species.
Isolation by distance is distantly related to speciation. Multiple types of isolating barriers, namely prezygotic isolating barriers, including isolation by distance, are considered the key factor in keeping populations apart, limiting gene flow. [4]
Reinforcement, under his definition, included prezygotic divergence and complete post-zygotic isolation. [18] Servedio and Noor include any detected increase in prezygotic isolation as reinforcement, as long as it is a response to selection against mating between two different species. [4]
Three-spined stickleback fish have been a frequently studied species in ecological speciation.. Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species.
The lack of geographic isolation as a definitive barrier between sympatric species has yielded controversy among ecologists, biologists, botanists, and zoologists regarding the validity of the term. As such, researchers have long debated the conditions under which sympatry truly applies, especially with respect to parasitism .
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.
A major aspect of speciation research is to determine the nature of the barriers that inhibit reproduction. Botanists often consider the zoological classifications of prezygotic and postzygotic barriers as inadequate. [259] The examples provided below give insight into the process of speciation.