Isolating Mechanisms Describe the mechanisms which isolate individuals

Isolating Mechanisms Describe the mechanisms which isolate individuals of a population and allow speciation to occur.

Geographical Barriers • Shifts in the continental plates – NZ separated from the supercontinent Gondwanaland at a very early stage before the evolution of mammals. • Our birds, reptiles, insects and plants evolved without the pressure of grazing, browsing or predator mammals.

Geographical Barriers • As a result they did not evolve protective adaptations to counter the attacks of mammals later introduced by settlers. • The separation from Gondwanaland prevented gene exchange between NZ and the vast continent.

Geographical Barriers • The only native NZ mammal is a bat thought to have blown over to NZ from Australia. • Barriers include – water – oceans, rivers changing course. – Desserts and canyons. – Mountain ranges.

Ecological Barriers • Populations may have developed genetic differences to cope with different ecological niches or habitats. – E. g. different temperatures.

Reproductive Barriers • Under natural conditions, sympatric populations of different species are prevented from mating by isolating mechanisms. These can be divided into 2 categories: – Prezygotic – Postzygotic

Reproductive Barriers • Premating / Prezygotic These take effect before fertilisation. – Habitat Differences – the 2 species may never meet, e. g. one living on the top of a tree and the other on the ground. One may be nocturnal and the other diurnal.

Reproductive Barriers – Differences in Breeding Times – the species may be sexually receptive at different times of the year. – Mechanical Differences – e. g. the genitalia of different insect species will not fit together so breeding is prevented. Some plants have flowers that can only be pollinated by certain insects.

Reproductive Barriers – Behaviour Patterns – there are many examples of courtship behaviour which takes place in a stepwise ritual, only members of the same species have the behaviour patterns to complete the ritual. e. g. in birds, the songs and displays will seem wrong to a different species.

Postmating Isolation • Postzygotic Isolation • These isolations prevent successful reproduction after fertilisation. – Hybrid Inviability – a zygote is formed but does not develop properly. – Hybrid sterility – a hybrid forms but it is sterile

Postmating Isolation – Hybrid Breakdown – the hybrid offspring are fertile but produce many infertile or non-viable offspring.

Polyploidy • This is the abrupt formation of a new species. It is an important sympatric speciation as it does not involve geographical isolation. • In one generation the parent and the offspring can belong to a different species. • More common in plants.

Isolation by Time • A species that disappeared one million years ago obviously can not interbreed with a species living today.

• Although individuals living close together within each species can interbreed, as distance increases, fertility decreases, indicating increasing genetic divergence as the habitat and climate change. These frogs are now considered to be four different species: 1) Rana pipiens, 2) R. blairi, 3) R. utricularia, and 4) R. berlandieri. The mating calls of each species differ, and defective embryos occur between certain combinations, indicating that both pre-mating and post-mating isolating mechanisms exist.