Questions: Gene Flow and Selection: Opposing Forces

Question 1

Two plant populations live 100 km apart — one on dry soil, one on wet soil. Despite dramatically different environments, they show nearly identical allele frequencies at drought-tolerance loci. The most likely explanation is:

• A. Selection pressures are actually similar in both environments
• B. Drought tolerance is a neutral trait, so selection doesn't act on it
• C. Migration rate is high relative to selection strength, so gene flow prevents local adaptation from building up
• D. The populations recently diverged and have not yet had time for selection to differentiate them

Answer: (C)

When migration rate (m) is high relative to the selection coefficient (s) for local adaptation, gene flow continuously imports maladapted alleles that dilute locally favored variants. Even strong environmental differences cannot produce genetic differentiation if migration swamps selection. This is the core insight of the gene flow–selection balance: genetic architecture reflects both the strength of selection *and* the geography of migration. Similar allele frequencies across contrasting environments is strong evidence that m >> s, not that the environments are similar or selection is absent.

Question 2

In a system where the selection coefficient strongly favoring a local allele is s = 0.20 and the migration rate introducing the alternative allele is m = 0.01, what outcome is predicted?

• A. Gene flow erases local adaptation because migration is a constant homogenizing pressure
• B. Local adaptation is maintained because selection (s = 0.20) greatly exceeds migration (m = 0.01)
• C. The populations will immediately speciate due to the strong selection differential
• D. The outcome cannot be predicted without knowing population size

Answer: (B)

When s >> m, selection is strong enough to overcome the homogenizing effect of gene flow. The locally adapted allele is strongly favored in the local environment, and even though immigrants arrive carrying the alternative allele, most of those immigrants or their offspring have reduced fitness in the local environment. The population can therefore maintain a distinct allele frequency. This requires s to substantially exceed m — when they are of similar magnitude, partial local adaptation occurs but is never complete.

Question 3

Populations living in starkly different environments will generally show strong genetic differentiation, because natural selection is expected to eventually overcome gene flow.

• T. True
• F. False

Answer: False

This is false. If migration rate (m) consistently exceeds the selection coefficient (s), gene flow wins and populations behave as a single panmictic unit regardless of environmental differences. 'Eventually' is misleading — in a migration-selection balance, the equilibrium allele frequency is set by the ratio m/s, and if that ratio favors homogenization, no amount of time changes this. Strong selection is necessary but not sufficient for differentiation; sufficiently low migration is also required. This explains why some populations in very different environments show surprisingly little genetic differentiation.

Question 4

Speciation often requires either geographic isolation reducing gene flow or selection strong enough to overcome ongoing gene flow — the gene flow–selection balance is therefore central to understanding how species diverge.

• T. True
• F. False

Answer: True

This is the direct application of the gene flow–selection balance to macroevolution. Allopatric speciation solves the problem by reducing m toward zero (a barrier eliminates gene flow entirely). Sympatric or parapatric speciation requires s to be large enough to drive divergence despite ongoing gene flow. In both cases, the ratio s/m determines whether populations can diverge. Understanding this balance explains why geographic context matters so much for speciation: it is not just selection strength but migration structure that determines the genetic fate of diverging populations.

Question 5

Explain why the ratio of selection coefficient to migration rate (s/m) is more informative for predicting local adaptation than either value alone.

Model answer: Local adaptation depends on whether selection can maintain a locally favored allele despite immigration of the alternative allele. If s is large but m is also large, immigrants continuously dilute local adaptations, and neither selection nor migration 'wins' cleanly. If s is small but m is also small, even weak selection can build local differentiation. What matters is the relative magnitudes: s/m >> 1 allows local adaptation; s/m << 1 prevents it. Knowing only that selection is strong (s = 0.10) or only that migration is low (m = 0.001) is insufficient — you need both to predict the outcome.

This ratio logic generalizes beyond simple two-population models. In clines and continuous populations, the width of a genetic transition zone across an environmental boundary is proportional to the dispersal distance divided by the square root of the selection coefficient — again showing that geographic dispersal and selection strength jointly determine genetic architecture. The gene flow–selection balance is one of the foundational principles of population genetics and speciation biology.