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Speciation Speciation The formation of new species – populations reproductively isolated = no gene flow. May occur: - Gradually by slow accumulation of small changes - Instantly by changes in chromosome numbers through polyploidy A reproductive isolating mechanism (RIM) is any factor stopping members of populations from breeding. May be pre/post zygotic. Prezygotic isolating mechanisms Geographical isolation: - Physical barriers between populations. - If they come back together, they are sufficiently different and unable to breed. - EG – Kea & Kaka. Sea level rising and cutting off North and South Islands – two genetically isolated populations from one ancestral population. Kaka in the north and kea in the south. Prezygotic isolating mechanisms Structural (Morphological) isolation: - Differences in external reproductive organs that don’t allow transfer of sperm. - Common in insects which have genitals that are shaped in a speciesspecific way. - In flowering plants (angiosperms) the flowers often have adaptations for pollination by a specific type or species of animal pollinator = co-evolution. Flowers attracting particular pollinator and pollinators only going to particular flower. Prezygotic isolating mechanisms Ecological isolation: - Differences in habitat within same geo area, so populations rarely come into contact. - EG – Two species of herbivorous insects feed and mate on different species of plants. Prezygotic isolating mechanisms Temporal isolation - Results from breeding behaviour occurring at different, non-overlapping times. - Can be seasonal or daily differences in timing of mating; there can be differences in the season when pollination occurs. - EG – Two species of petrel nest on sub-Antarctic islands. One lays eggs 6 weeks before the other. Prezygotic isolating mechanisms Behavioural isolation - Differences in behaviour, typically mating behaviour – courtship. - Females only recognise and respond to the courtship behaviour of males of their own species. - EG – Moths release species-specific pheromones to attract a mate. - Although 2 species are sympatric, one could be diurnal and the other nocturnal in their feeding habits. - Many flowers secrete nectar at a particular time of day. Prezygotic isolating mechanisms Gametic isolation - Results from incompatibility of gametes. Sperm may not fertilise an egg or another species through: • Egg’s surface not having the right chemical receptors • Sperm can’t penetrate the surface of the egg • Sperm can’t survive the chemical environment of the female repro system - EG. Kina can’t be fertilised by any other species’ sperm present in the water. - EG. Pollen landing on the stigma can only grow a pollen tube if there is chemical compatibility.
