1. Isolation

(a) Geographical isolation (Allopatric speciation)
– Involves a geographical barrier
– Physically isolates populations and thus block gene flow (reproduction btw populations)
– Once isolated, allopatric populations accumulate genetic differences due to natural selection, genetic drift, and mutations.

Note: Sympatric speciation which does not involve a geographical barrier can occur in result of non-random mating (i.e. host shift isolates parasite populations reproductively) and polyploidy, e.g. autopolyploidy (doubling of chromosome number in the same species) and allopolyploidy
(Refer to part (iii) hybridisation)

(b) Reproductive isolation

(i) Prezygotic barriers
* Ecological – different habitats
* Temporal – different breeding season
* Behavioural – different mating behaviour
* Mechanical – morphological/structural differences in reproductive organs

(ii) Postzygotic barriers
* Zygote/embryo dies before reaching maturity
* Hybrids are mature but sterile

2. Genetic drift

– Alteration in allele frequency that usually occurs in a small population and results from chance/random processes only, e.g. natural disasters (not from natural selection/mutation/immigration).
– Bottleneck effect
– Founder effect
– Small genetic variation within the small population

3. Hybridisation (Alloploidy)

– Process of combining different varieties or species of organisms to create a hybrid.
– Alloploidy is the result of hybridisation of two different species (usually plants) in the same area.
– The hybrid is sterile, but after doubling of chromosomes, it become fertile but cannot mate with the parental plants.
– Example: Triticum aestivum (also known as bread wheat)

4. Adaptive radiation (divergent evolution)

– Development of new species from a single recent ancestor and are adapted to each new environment
– Selection pressure acts on populations; population which is well adapted to the niche continue to survive and reproduce.
– As competition grow, some finches fly to find new ecological niches which has less competition and this allow them to survive.
– In this new environment, a new selection pressure apply onto the population and thus the most adapted finches survive and continue to contribute their alleles to the gene pool.
– Since finches of different islands rarely interbeed, eventually the alleles in each gene pool become significantly different, result in genetic differences and reproductive barriers.