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.
Year 12 · SACE Stage 2
Genetic drift and gene flow
Distinguish random sampling, migration and selection from supplied cases.
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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.