01Mechanisms of evolution
Besides selection: genetic drift — random changes in allele frequency, strongest in small populations. Bottleneck effect: disaster shrinks a population (northern elephant seals). Founder effect: a few individuals start a new population (Amish polydactyly).
Gene flow moves alleles between populations, making them more similar. Mutation adds new alleles.
02Why small populations matter
Drift can fix harmful alleles or erase helpful ones by chance. Low genetic diversity reduces a population's ability to adapt (cheetahs).
03Notebook box
Selection = non-random. Drift = random. Gene flow = migration.
Worked example: A storm randomly kills 90% of a beetle population, leaving mostly green beetles. Mechanism? → Bottleneck (genetic drift), not selection.