Population genetics mechanisms shape the genetic makeup of populations over time. Genetic drift, a random process, can significantly impact allele frequencies, especially in small populations. This mechanism operates independently of natural selection, leading to unpredictable changes in genetic diversity.

Founder effects, population size, and the interplay between genetic drift and natural selection all contribute to evolutionary dynamics. Understanding these mechanisms helps explain genetic variation within and between populations, as well as the potential for rapid evolutionary change.

Population Genetics Mechanisms

Genetic drift and allele frequencies

• Random changes in allele frequencies due to chance events affect all populations, more pronounced in small populations, independent of natural selection
• Sampling error in reproduction and random survival of individuals drive genetic drift mechanisms
• Genetic drift effects lead to fixation or loss of alleles, reduce genetic diversity within populations, increase genetic differences between populations
• Mathematical representation: Variance in allele frequency change $Var(Δp) = p(1-p)/2N_e$ where $p$ is initial allele frequency and $N_e$ is effective population size

Founder effect in new populations

• Special case of genetic drift occurs when small group establishes new population (island colonization)
• Founder populations exhibit reduced genetic diversity, different allele frequencies than source population
• Consequences include increased likelihood of genetic disorders (cystic fibrosis), rapid divergence from original population, potential for unique adaptations
• Examples: Pitcairn Islanders descended from Bounty mutineers, Amish populations in North America

Population size vs genetic drift

• Inverse relationship between population size and genetic drift intensity
• Effective population size ($N_e$) accounts for unequal sex ratios and non-random mating, usually smaller than census size
• Time to fixation or loss of alleles faster in small populations, slower in large populations
• Population bottlenecks temporarily reduce size, increase genetic drift impact (northern elephant seals)

Genetic drift vs natural selection

• Directionality: Genetic drift causes random, non-directional changes; natural selection drives directional changes favoring adaptive traits
• Predictability: Genetic drift leads to unpredictable outcomes; natural selection generally yields predictable results based on fitness advantages
• Population size interaction: Genetic drift stronger in small populations; natural selection effective in both small and large populations
• Genetic variation effect: Genetic drift generally reduces variation within populations; natural selection can reduce or maintain variation (stabilizing, directional, disruptive)
• Time scale: Genetic drift causes rapid changes in small populations; natural selection typically operates over longer periods
• Adaptive value: Genetic drift changes unrelated to environmental adaptation; natural selection promotes adaptation to environmental conditions (antibiotic resistance in bacteria)