Pre-zygotic isolation:

Geographical isolation

Population becomes split by a physical barrier into separate demes. The speciation resulting is thus allopatric

• Example: birds of a population reproduce in the cool temperatures of a valley. Peaks are too cold.

• Climate becomes warmer → move to the 2 separate peaks where it’s cooler. Split into 2 separate demes. Each with its own gene pool.

• Over time, the birds experience different selection pressures so allele frequencies of each deme change. Also the populations accumulate different mutations.

• If temperature reverts to original, birds return to valley and come into contact again. Appearances have altered, eg different shaped beaks = different mating calls. No longer attracted.

• The two populations have established different gene pools and can no longer interbreed.

Behavioural isolation

Different mating rituals arise making organisms mate only with those possessing behaviours they recognise. E.g male grasshopper making a song. Different mating groups arising from different mating rituals are likely to accumulate, over time, different mutations that will further prevent successful mating between the groups. The speciation resulting is sympatric.

Morphological isolation

Occurs when differences in size, shape, or structure of reproductive organs prevent mating between species. Also known as mechanical isolation. Sympatric.

Gametic isolation

Gametes in open environments meet other gametes of other species eg corals releasing their gametes in coral reefs.

• In many cases, molecules in gamete membranes prevent inter-species fusion (of gametes)

• In some cases, chemoattractants from female gametes only recognised my male gametes of same species.

• Pollen of many angiosperms may germinate on stigma of another species but pollen tube only goes so far down the style.

• Sperm of many animals does not survive in oviduct of another species.

This isolation leads to sympatric speciation.

Seasonal isolation

Seasonal/temporal isolation. When reproductive organs of different demes mature at different times. Thus unable to hybridise so genetically isolated. Leads to sympatric speciation.

Post-zygotic isolation

Hybrid inviability

Fertilisation occurs but incompatibility between parental genes prevents the development of an embryo.

Hybrid sterility

An embryo formed from gametes of 2 species may develop. However, if the chromosomes are not sufficiently similar, no pairing at prophase I so gametes cannot form = sterile.