Free Geographical and Reproductive Isolation OCR A Level Biology revision notes – covering specification point 6.1.2(g).
Isolating mechanisms
Factors that prevent gene flow between populations are called isolating mechanisms.
Isolating mechanisms allow allele frequencies in separate populations to diverge over time and (potentially) give rise to a new species.
The isolating mechanisms which can give rise to a new species include geographical and reproductive isolation:
- Geographical isolation prevents individuals from reproducing due to a physical barrier between populations, resulting in allopatric speciation.
- Reproductive isolation prevents individuals in the same geographic area from interbreeding due to biological or behavioural differences, resulting in sympatric speciation.
If the two populations experience different selection pressures, then the allele frequencies within their gene pools will change over generations and may evolve into separate species.
Allopatric speciation is when two new species evolve from one original species due to geographical isolation.
Sympatric speciation is when two new species evolve from one original species due to reproductive isolation.
Reproductive isolation can arise through several distinct mechanisms:
- Temporal isolation occurs when populations reproduce at different times of day or year, preventing individuals from the two groups from meeting to mate.
- Chromosome number changes prevent gene flow when a change in chromosome number means gamete fusion is prevented, zygotes are not viable, or hybrid offspring have an odd number of chromosomes, which makes them infertile.
- Behavioural isolation occurs when divergence in courtship displays, mating calls or other behaviours means individuals from the two groups do not recognise one another as suitable mates.
- Structural isolation occurs when differences in the morphology of animal genitalia or plant flower structure prevent successful mating or pollination.