Some definitions to begin with:
Sympatry: Phylogenetically closely related taxa living in the same area or in overlapping areas are called sympatric.
Allopatry: Phylogenetically closely related taxa living in different areas are called allopatric.
Vicariation: Phylogenetically closely related taxa replacing each other ecologically in different areas are called vicarious.
It is distinguished between prezygotic and postzygotic barriers, that foster reproductive isolation. The first type prevents fertilization and the development of a zygote, the latter is the cause of feeble or sterile hybrids. Some prezygotic barriers depend on the existence of extern factors, others are controlled by the genome of the respective individual (endogenous factors). Postzygotic barriers are without exception controlled genetically. Among the prezygotic barriers of reproduction are:
- geographical (spatial) isolation,
- isolation of the biotope, i.e. the organisms live in the same area, but have different needs regarding the biotop,
- isolation by season or daytime, i.e. the opening of flowers is staggered depending on season or daytime,
- ethological isolation, i.e. the species to be pollinated are visited by different pollinators,
- mechanical isolation, i.e. a genetically reproductive isolation within a species is diminished or prevented by differences in the flowers’ shape,
- isolation by sterility..
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