Two Lizard Populations on Either Side of a Road
A population of small ground-dwelling lizards lives in scrub on both sides of a four-lane highway built 40 years ago. On the north side the population is large and genetically diverse. On the south side the population is small, and sampling shows reduced heterozygosity and a higher frequency of a rare recessive allele that causes a mild tail deformity. No lizards have been recorded crossing the road in three years of surveys, though the species can move several hundred metres per season. The south side also has more bare ground and fewer shrubs than the north. A proposal suggests moving 20 lizards from north to south to 'boost the gene pool'.
1. Explain why the south population has reduced heterozygosity and a higher frequency of the rare recessive allele, using more than one mechanism.
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Two processes are likely acting together. First, the road fragments the habitat and prevents gene flow, so the south population is effectively isolated and small. In a small population, genetic drift causes random changes in allele frequencies, and rare alleles can rise in frequency by chance rather than by selection. Second, inbreeding in a small closed population increases homozygosity, so a rare recessive allele that was mostly hidden in heterozygotes now appears in homozygotes and is expressed as the tail deformity. The higher frequency of the rare allele on the south side is best explained by drift plus founder effects if the south side was colonised by few individuals; selection is unlikely to favour a deformity allele. Reduced heterozygosity reflects both drift and inbreeding.
2. Evaluate the translocation proposal. What could go wrong, and what would you check first?
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Translocation can reduce inbreeding and restore diversity, but it carries risks. Moving northern lizards into a habitat with more bare ground and fewer shrubs may expose them to conditions they are not adapted to, and if the two populations have diverged over 40 years they may have different local adaptations; mixing can break up co-adapted gene combinations (outbreeding depression). There is also a disease and parasite transfer risk. Before moving anything, check: (1) whether the south population is actually declining or merely small and stable; (2) whether the habitat difference, not genetics, is the main problem, in which case restoring shrubs may matter more; (3) the genetic distance between populations; (4) whether the deformity allele causes real fitness loss. A better first step may be habitat restoration plus a small, monitored pilot translocation rather than a bulk move.
3. If the goal is long-term viability, which is more urgent: adding lizards or restoring habitat? Defend your answer.
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Restoring habitat is the higher priority. A small population in poor habitat will remain small no matter how many individuals are added, because the carrying capacity is set by resources and cover. The south side has more bare ground and fewer shrubs, which for a ground-dwelling lizard means more predation risk and less thermal cover, so adding 20 lizards may simply raise mortality. Genetic rescue buys time but does not fix a habitat that cannot support a larger population. The logical sequence is: restore shrub cover and connectivity, confirm the population responds, and use translocation only if genetic problems persist once habitat is adequate. Connectivity (e.g., a wildlife crossing) would also address the root cause of isolation.