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 species- specific 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.