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 pools 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, 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 chromosomes and thereby preventing even pairing of chromosomes with normal gametes during fertilization. For various species that 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 gametes 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 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 F2 breakdown. These mechanisms prevent the recycling of genetic material in distinct species.
Insertion of interspersed repeats 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 families and also 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.