ALLOPATRIC & SYMPATRIC SPECIATION

=Allopatric & Sympatric Speciation= [image:http://i.imgur.com/LDcxske.png?1] '''Allopatric Speciation''' Allopatric speciation occurs when a species gets split into two '''geographically isolated''' populations. This is often the result of geological events such as mountain building. The isolated populations may then be subjected to slightly different conditions and therefore selection pressures. Genetic drift and mutations may also contribute to differences in each population. The populations eventually become so different that even if they come into contact again they can no longer interbreed. They are reproductively isolated and are no longer capable of exchanging genes (they have become two different species). '''Sympatric Speciation''' Sympatric speciation occurs when a single species develop into two distinct species as a result of '''reproductive isolation''', despite living in the same geographical area. Species do not have to be geographically isolated for speciation to occur. As long as two groups are reproductively isolated genotypic and phenotypic differences can accumulate until the groups are so different they are considered separate species. =Isolating Mechanisms= Members of the same species can become reproductively isolated for a variety of reasons. These mechanisms are discussed below and can be put into two broad categories: '''Pre-Zygotic Isolation''' This occurs when some mechanism prevents fertilisation (the egg and sperm from fusing) ''Geographical Isolation'' -organisms become separated by some geological feature (mountain, river, etc.) ''Temporal Isolation'' -organisms are active at different times so they do not come into contact ''Ecological Isolation'' -organisms occupy slightly different habitats (e.g. feed / mate on different plants in the same area) ''Behavioural Isolation'' -different courtship or mating behaviours prevent fertilisation ''Structural Isolation'' -differences in the structure of sex organs make them incompatible ''Biological Isolation'' -sperm don't survive in the reproductive tract or gametes are incompatible (don't fuse) '''Post Zygotic Isolation''' Post-zygotic isolation occurs when some mechanism prevents the development of the fertilised egg (zygote) into a fertile offspring ''Hybrid Inviability'' -The hybrid zygote that is formed does not develop properly ''Hybrid Infertility'' -The hybrid formed is infertile (often because the chromosomes can't pair correctly) ''Hybrid Breakdown'' -The hybrid formed is fully fertile but the next generation is not