A species is a group of organisms that can breed together to produce fertile offspring. New species arise in two ways, hybridisation and polyploidy.
Polyploidy is an increase in the number of sets of chromosomes. Hybridisation is the production of offspring from parents of two different species, this means though that the offspring is not fertile such a mule, a cross between a horse and a donkey, which is why hybridisation can only be explained in plants and not animals.
Allopatric speciation is when speciation occurs as a result of barriers this could be by mountain ranges, glaciers and canyons, continental drift (tectonic plates) or islands. This type of isolation is called geographical isolation. Their allele frequencies will change in different ways due to natural selection pressures. Sympatric speciation is the formation of a new species without geographical isolation. Strong forces of natural selection cause genetic differences between two populations that can in theory interbreed.
Natural selection can lead to a change in allele frequency. It is the process by which an environmental factor affects the survival rate of different phenotypes in a population. Selection can be stabilising, directional or disruptive. Stabilising selection takes place in a stable environment over time it usually favours average' members of a population. The ones on average' are usually best adapted to that environment an example of this is birth mass of humans. Directional selection is when environmental change may produce new selection pressures that favour organisms with an extreme form of a characteristic, it moves phenotype variation away from its modal value for example populations of the myxoma virus in Australia. The virus causes myxomatosis, a disease that rabbits get, and was introduced to kill rabbits that were pests on farms. Now the rabbits are becoming more resistant to it and surviving longer. Disruptive selection is where an...