Types of Isolation

Speciation needs reproductive isolation: something must stop two groups exchanging alleles. Isolating mechanisms are divided into those that act before fertilisation and those that act after it.Prezygotic mechanisms stop a zygote being formed:1. Geographical isolation: a physical barrier, such as a mountain range, river, sea or new road, separates populations so they never meet.2. Ecological (habitat) isolation: groups live in the same area but occupy different habitats, for example one on the soil and one in trees.3. Temporal (seasonal) isolation: groups breed or flower at different times of day or year.4. Behavioural isolation: groups have different courtship rituals, songs or displays, so they do not recognise each other as mates.5. Mechanical (morphological) isolation: differences in reproductive organs, or in flower shape for pollinators, prevent mating or pollination.6. Gametic isolation: sperm and eggs, or pollen and stigma, are incompatible, so fertilisation fails.Postzygotic mechanisms act after a zygote is formed:1. Hybrid inviability: the hybrid embryo does not develop or dies young.2. Hybrid sterility: the hybrid survives but cannot produce gametes. A mule, the offspring of a horse and a donkey, is healthy but sterile, because its chromosomes cannot pair properly in meiosis.3. Hybrid breakdown: first-generation hybrids are fertile, but their offspring are weak or sterile.Geographical isolation leads to allopatric speciation. All the other mechanisms can act between groups living in the same area, which allows sympatric speciation.