Species and Speciation -The process by which species are formed -Depends on groups within a population becoming isolated in some way

2 Main Forms of Isolation Allopatric Speciation 2 Main Forms of Isolation Sympatric Speciation

~ The result of 2 populations becoming geographically isolated ~ Any physical barrier which prevents 2 groups meeting prevents them interbreeding. ~ The effectiveness of any barrier varies from species to species. ~ Barriers include mountains, rivers, etc ~ The environmental conditions on each side of the barrier differ which means that groups on either side adapt to suit their own environment ALLOPATRIC SPECIATION

When organisms inhabiting the same area become reproductively isolated for reasons other than geographical barriers. Sympatric Isolation MECHANICAL ISOLATION – when its physically impossible for a male and female to reproduce. The gametes may be prevented from meeting or the fusion of them may not take place Development of the embryo may not take place or there may be abnormalities during early growth BEHAVIOURAL ISOLATION – The timing and type of courtship behaviour may be wrong

POLYPLOIDY ~ When individuals of different species breed, the sets of chromosomes from each parent are different. This means that the sets can’t pair up during meiosis and so the offspring can’t produce gametes. + Horse (2n = 60) Ass (2n = 66) It is impossible for 63 chromosomes to pair up during meiosis so mules are sterile. Mule (2n = 63)