Reproductive+Barriers

Reproductive Barriers There are two main reproductive barriers, prezygotic and postzygotic. Prezygotic reproductive barriers hinder the mating or fertilization of organisms and no offspring are produced. Postzygotic reproductive barriers prevent the hybrid zygote, or offspring produced by two species, from reproducing or even surviving. These barriers hinder the development of new species and populations in many ways.

There are five different process of prezygotic reproductive barriers; habitat, temporal, behavioral, mechanical, gametic. Habitat barriers occur when species are found in the same environment but cannot survive in the same habitat. Because of this separation of habitats the species are also separated and do not have the opportunity to interbreed. This is the case with insects that only feed on certain plants. If the one of the plants only survives in dry, sunny places and the other only survives in a damp, dark place then the bugs that feed on them will never come into contact with each other and therefore never breed. Temporal reproductive barriers happen when species have varying mating seasons. Because different subspecies have different times of the year that they are fertile breeding is completely impossible. This is the case with frog populations, wood frogs are fertile in late winter, while leopard frogs do not mate until early spring, because of this small difference mating will never occur between these frogs. Behavioral reproductive barriers are when a species behavior can trigger mating, such is the case with mating calls or color of different genders. Although these mating rituals allow species to breed they may have no effect on the subspecies of another environment. When the other species do not react to these mating rituals it creates even more of an obstacle for interbreeding. This is the case with the prairie chicken mating dance, although the same species in the same environment will react to this dance, other species around do not. Mechanical reproductive barriers are when anatomical differences stop process of mating. If the structure of the organism does not allow the subspecies to transmit gametes reproduction is not possible. This is the case with some flower species, the structure does not allow the sperm to reach the eggs due to a lengthened or shortened stigma. Gametic reproductive barriers are when gametes are incompatible. There must be a chemical signal between sperm and eggs in order for them to fuse, without this interbreeding does not occur. This is the case with fish, some do not breed because the gametes are not recognizable.

There are three different types of postzygotic reproductive barriers; reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. Each of these occur after offspring is produced. Reduced hybrid viability is when a hybrid zygote is produced but the genes of the offspring are weak and the new species cannot survive or reproduce. The generation is lost because of this. The process of reduced hybrid viability happens in the case of two different species of irises, a hybrid zygote can be formed but the embryo dies before seeds can be produced. Reduced hybrid fertility, a hybrid zygote is able to reach adult status but is sterile and not able to produce offspring. Therefore the gene cannot be passed on and the generation is lost. This is the case with the species zebroid, a hybrid zygote produced from a zebra and a horse. The zebroid is sterile and will never be produced past that first organism. Hybrid breakdown is when a first generation offspring is produced but the second generation will always be too unhealthy or infertile. The generation is then lost, which is the case with different sunflower hybrids.