Pre zygotic mechanism:
Geographic isolation: In this, species are isolated by physical boundaries to stop interbreeding between them, such as water sources, rivers, deserts, and so on. Thus, mechanisms such as natural selection arise on both sides independently and contribute to the creation of new organisms or sub-species, for example, Darwin’s Galapagos Island Finches.
Spatial isolation: Owing to their failure to reach large distances, people separated by long distances are incapable of reproducing in spatial isolation.
Seasonal or Temporal isolation: The breeding season of animals belonging to one genus varies from that of other species due to factors such as photoperiod, temperature, and humidity. This is because they are isolated reproductively, for example, in the early period of the rainy season, American toad (Bufo Americanus) breed, while Fowler’s toad (B .fowleri) breeds in the latter period of the rainy season; therefore, these two are isolated reproductively.
Ecological or Habitat isolation: Inability to mate with the individuals belonging to closely related species or sub-species inhabiting the same area but separate inhabiting environment, For example, two Rana species, that is, Rana grigio (Pig Frog) and Rana areolata (Gopher Frog) survive in dissimilar habitats. Hence, these two exhibit ecological isolation.
Ethological isolation: Ethological isolation is referred to the distinction between members of closely associated species during courtship or sexual behavioral patterns, for example, various species males of Sticklebacks have different nest-building modes, and the way of conducting their courtship dance differs. Likewise, Pintail duck and Mallard duck also reflect ethological separation.
Physiological isolation: Inability to mate due to mechanical incompatibility in mating. For example: swelling of the Drosophila vaginal mucous membrane during inter-specific mating.
Mechanical isolation: Due to variations in morphological features such as outer genitalia, this form of separation occurs. For example, in glossina and drosophila, this type of isolation is observed.
Genetic isolation: Genetic isolation is due to interspecific sterility arising from chromosomal incompatibility, mating inability is observed among members of similar species.
If the premating mechanisms fail at any point, then the hybrid creation is prevented by post-mating mechanisms.
Gametic mortality: During external fertilization, as a result of unfavorable climatic conditions, the sperm may not be able to fertilize the eggs. In the case of internal fertilization, sperm of various species may be immobilized or destroyed in female genital structures.
Zygotic mortality: Zygotic mortality causes the death of zygotes as in the case of sheep and goats.
Hybrid zygotes do not mature in this as the embryos produced by inter-specific breeding abort themselves in the course of development, for example, hybrids formed by crossbreeding between the Rana pipiens and Rana sylvatica do not get through past the early gastrula stage.
Hybrid sterility: Hybrids are infertile in most species because of chromosome incompatibility, for example, a mule (2N = 63) is formed by a cross between male ass (2N = 66) and a mare (2N = 60) and a hinny is formed by the cross between she-ass (Janet) and a male horse (stallion). There are several examples where interspecific crosses may create viable interspecific hybrids in captivity because, even after their long separation from each other, the reproductive obstacle has not been formed between these species. The example is as follows:
Fertile ‘tigons’-a a cross of the African lioness (Panthera leo) and the Asian tigers (P. Tigris). Due to varying geographical arrangements, they do not breed in nature
Owing to separate breeding and nesting patterns, fertile hybrids of mallard duck (Anas basis) and pintail duck (A. acuta) do not breed naturally.
Fertile crosses between the polar bear (Ursus maritimus) and the brown bear of Alaska (U. arctos) do not procreate in nature (ecological isolation) due to different environments.
Hybrid breakdown: Hybrids are productive in some cases, but the offspring are frail, abnormal, or infertile. This condition is termed hybrid breakup, for example, Gossypium which is a cotton interspecific hybrids. At the genetic stage, because of incompatibility among the chromosomes of two distinct species and their inability to pair during meiosis, the offspring formed after interspecific hybrids are typically sterile.
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