When population size is small, allele frequencies can fluctuate between generations and some alleles may become fixed or lost. This stochastic process is called __________.

Introduction:
Not all evolutionary change is driven by natural selection. In small populations, random sampling effects can strongly influence allele frequencies, even in the absence of fitness differences. This question asks you to name that stochastic force—genetic drift.

Given Data / Assumptions:

• Population size is small, so sampling error is large.
• No explicit selective advantage is assumed for the alleles discussed.
• Fixation (frequency = 1) or loss (frequency = 0) can occur by chance.

Concept / Approach:
Genetic drift refers to random fluctuations in allele frequencies due to chance events in reproduction and survival. The magnitude of drift is inversely related to effective population size. In extreme cases (founder effects, bottlenecks), drift can rapidly fix alleles, reduce genetic diversity, and increase the influence of deleterious alleles, independent of selection.

Step-by-Step Solution:

Verification / Alternative check:
Wright–Fisher and Moran models mathematically show variance in allele frequency per generation proportional to p*(1−p)/(2N) or related forms, demonstrating stronger drift at smaller N and eventual fixation probabilities equal to initial allele frequency.

Why Other Options Are Wrong:

• Assortative mating: nonrandom mating pattern affecting genotype frequencies, not inherently causing fixation by chance.
• Inbreeding: increases homozygosity but does not by itself randomly change allele frequencies.
• Heterosis: increased fitness of heterozygotes; not a stochastic force.
• Balancing selection: maintains polymorphism via fitness differences, opposite to drift-driven fixation.

Common Pitfalls:
Equating increased homozygosity (from inbreeding) with drift; while often correlated in small isolated groups, the mechanisms are distinct.

Final Answer:
Genetic drift