Reproductive isolation
5146476
207928131
2008-04-24T19:05:29Z
128.189.251.194
/* Fertilization barriers (pre-zygotic) */
An important concept in evolutionary biology, '''reproductive isolation''' is a category of mechanisms that prevent two or more [[populations]] from exchanging genes. The separation of the [[gene pool]]s of populations, under some conditions, can lead to the genesis of distinct [[species]]. Reproductive isolation can occur either by preventing [[fertilization]], or by the creation of a degenerate or sterile [[Hybrid (biology)|hybrid]], such as the case with the common [[mule]] and the [[hinny]].
==Fertilization barriers (pre-zygotic)==
Pre-zygotic barriers to fertilization include everything from a physical barrier (such as an ocean) formed between two populations, to ethological (behavioral) differences, to errors in cell division that cause incompatibility between populations.
[[Speciation]] by reproductive isolation is frequently seen in plants, with errors in division during [[mitosis]] doubling the number of [[chromosome]]s and thereby preventing even pairing of chromosomes with normal [[gamete]]s during fertilization. For various species that [[blossom|bloom]] seasonally, the time of gamete release can prevent hybridization, a temporal isolation. For animal species, mating might be stymied. Incompatible [[genitalia]] forms a mechanical reproductive isolation, and members of opposite sexes often fail to recognize one another, due to some morphological difference used to identify a potential mate.
The gametes of different species are frequently incompatible, and do not form a viable [[zygote]]. Sperm may not possess the proper enzymes for penetrating the coat of the [[ovum]], or have the proper chemical markers to signal the egg cell to accept it.
==Hybrid barriers (post-zygotic)==
If fertilization does occur, several other barriers for the hybrid exist. The first is gametic: the [[gamete]]s successfully combine, but then immediately die before any cell division can occur. The second is zygotic: the [[zygote]] forms but quickly dies. The third is embryonic or larval: which is spontaneous abortion of the hybrid [[fetus]]. The fourth is hybrid inviability: the offspring is born but is [[Fitness (biology)|unfit]], quickly succumbs to environmental pressures, and dies. The fifth is hybrid sterility: the offspring can produce no offspring of its own, isolating its genes from both parental groups. Finally, hybrids that do produce offspring can, potentially, produce sterile progeny. This is known as F<sub>2</sub> breakdown. These mechanisms prevent the recycling of genetic material in distinct species.
==Interspersed repeats==
Insertion of [[interspersed repeat]]s creates non-homologies between otherwise homologous DNA sequences, creating barriers to [[gene conversion]]. This barrier acts as an isolating mechanism protecting nascent novel genes from being overwritten by the progenitors of this gene. This uncoupling allows the evolution of new genes, both within [[Gene family|gene families]] and also [[allele|allelic]] forms of a gene. The importance is that this allows the splitting of a [[gene pool]] without requiring physical isolation of the organisms harboring those gene sequences.
==See also==
*[[Assortative mating]]
[[Category:Evolutionary biology]]
[[Category:Ecology]]
[[Category:Population genetics]]
[[Category:Speciation]]
[[de:Isolation (Biologie)]]
[[ja:生殖的隔離]]
[[pl:Izolacja rozrodcza]]
[[zh:生殖隔离]]