Which combination of signs indicates a genetic bottleneck, and what method is commonly used to detect it?

• A. Loss of rare alleles and heterozygosity excess; detected with M-ratio or Garza-Williamson test.
• B. Increase in rare alleles and heterozygosity deficit; detected with Hardy-Weinberg test.
• C. Stable allele frequencies and high Garza-Williamson index; detected with Fst test.
• D. Constant population size with stable diversity; detected with Tajima's D.

Answer: (A)

A bottleneck shrinks the population to a small size, so random drift wipes out genetic variation quickly. Rare alleles are the first to disappear, leading to a clear loss of those rare variants. At the same time, allele sizes don’t collapse as rapidly as the number of alleles, which creates a transient excess of heterozygosity relative to what you would expect in a population that’s at equilibrium after a bottleneck. The M-ratio (Garza-Williamson statistic) specifically captures this pattern by comparing how many alleles are present to the range of allele sizes observed across loci; a reduced M-ratio signals a bottleneck. So the described combination—loss of rare alleles with heterozygosity excess—and its detection by the M-ratio or Garza-Williamson test, is the hallmark of a bottleneck. The other options either describe patterns not typical of bottlenecks or rely on tests not suited to identifying this specific signal.