Example 1
Ten diploid residents carry 8 A copies. Five diploid migrants carry 8 A copies. Find combined A frequency.
Total A=16; total copies =2(15)=30. Frequency =3016≈0.533.
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Year 12 · SACE Stage 2
Genetic drift and gene flow
Distinguish random sampling, migration and selection from supplied cases.
Method and worked examples
Genetic drift is random change in allele frequencies through sampling between generations. Its effects are often stronger in small populations. A bottleneck reduces population size, and the survivors may carry an unrepresentative subset of alleles. A founder effect occurs when a small group establishes a new population. Neither requires the surviving alleles to be better adapted.
Gene flow moves alleles between populations when migrants or their gametes reproduce. It can add variants to a population and reduce differences between connected populations. Selection differs because reproductive success is associated with a heritable trait’s effect in the environment rather than sampling alone. For a migration calculation, add allele copies from residents and migrants before dividing by the new total. Genetic diversity can increase resilience but does not guarantee survival under every disturbance.
Example 1
Example 2
A storm kills individuals without regard to their allele. Surviving frequency differs. Name the process.
The random survival-related shift is consistent with genetic drift through a bottleneck, not necessarily adaptive selection.
Watch out: Random drift and gene flow can change frequency without an adaptive advantage.