Island lizards have evolved darker coloration as a local adaptation to their volcanic rock habitat (darker = less predation). However, the island receives migrants from a lighter-colored mainland population at rate m = 0.05 per generation, and selection favoring dark coloration has coefficient s = 0.01. What is the expected outcome over many generations?
AThe island will maintain its dark coloration because natural selection always overcomes migration
BThe island lizards will gradually become lighter, matching the mainland, because migration rate exceeds selection strength
CMigration has no effect on the island's allele frequencies because the populations are geographically separate
DThe island will become more genetically diverse than the mainland due to the influx of new alleles
The outcome depends on the relative strength of gene flow and selection. Here m = 0.05 >> s = 0.01, so gene flow is approximately five times stronger than selection. The island population cannot maintain its local adaptation because each generation, migration imports mainland alleles faster than selection can increase the frequency of the locally adaptive dark allele. This is migration-selection balance: local adaptation persists only when selection substantially exceeds migration (s >> m). Option A is the common misconception — selection does NOT always overcome migration.
Question 2 Multiple Choice
A population geneticist compares two populations of the same bird species and finds FST = 0.35 between them. What does this high FST value imply about their evolutionary history?
AThe populations have been exchanging many migrants per generation, maintaining similar allele frequencies
BThe populations have experienced little historical gene flow, allowing substantial genetic differentiation to accumulate
CThe populations have undergone recent bottlenecks that increased heterozygosity within each population
DThe populations are reproductively isolated and therefore constitute separate species
FST measures genetic differentiation between populations: values near 0 indicate high genetic similarity (high gene flow); values near 1 indicate strong differentiation (low gene flow). FST = 0.35 is high and implies that little historical gene flow has occurred between the populations, allowing allele frequencies to diverge through local selection and/or genetic drift. High FST is evidence for isolation, but it does not by itself confirm speciation (option D) — populations can be highly differentiated but still capable of interbreeding.
Question 3 True / False
Even a very small rate of gene flow — a few migrants per generation in a large population — can be sufficient to prevent substantial genetic differentiation between populations.
TTrue
FFalse
Answer: True
This is one of the most counterintuitive results in population genetics. The island model shows that the homogenizing effect of gene flow is proportional to the migration rate m, and even m = 0.01–0.05 (1–5% replacement per generation) is strong enough to keep allele frequencies closely tracking the source population over evolutionary time. Small amounts of gene flow act like a spring constantly pulling the recipient population's allele frequencies toward the source, and selection strengths rarely exceed a few percent per generation, so even modest migration can counteract strong local selection.
Question 4 True / False
Restricting gene flow between two populations is sufficient on its own to cause them to become separate species.
TTrue
FFalse
Answer: False
Restriction of gene flow is necessary but not sufficient for speciation. Gene flow restriction (via geographic, behavioral, or temporal barriers) removes the homogenizing force that keeps populations genetically similar. But divergence into separate species also requires that the isolated populations actually diverge — through natural selection, genetic drift, or sexual selection — and eventually accumulate enough differences to become reproductively isolated even if brought back into contact. Allopatric speciation requires both isolation (reduced gene flow) and divergence; isolation alone, without divergence, produces isolated but genetically similar populations.
Question 5 Short Answer
Explain how migration-selection balance works, and describe the conditions under which a locally adaptive allele can be maintained in a population that receives gene flow from a different-environment source population.
Think about your answer, then reveal below.
Model answer: Migration-selection balance is a tug-of-war between two opposing forces: local selection favors increasing the frequency of a locally adaptive allele, while gene flow keeps importing the non-adaptive allele from the source population. The locally adaptive allele can be maintained when selection is substantially stronger than migration (s >> m). When migration rate exceeds selection strength (m >> s), the influx of non-adapted alleles overwhelms selection and the local adaptation is swamped — the recipient population ends up tracking the source population's allele frequencies instead of adapting to its local environment.
This framework explains why populations across strong environmental gradients (where selection is intense) maintain distinct local adaptations, while populations across gentle gradients (where selection differences are small) tend to look genetically similar despite moderate gene flow. It also explains why conservation geneticists worry about migration from large, genetically different populations into small endangered populations — even 'helpful' gene flow can swamp local adaptations critical to survival in specific environments.