A Frog Population Split by a New Highway
A wetland population of 4,000 green and golden bell frogs is divided when a four-lane highway is built through the middle of the breeding area. Before construction, genetic sampling showed allele frequencies at three microsatellite loci were statistically indistinguishable across the wetland. Ten years later, samples from each side show: allele frequencies diverged significantly at all three loci; heterozygosity fell from 0.62 to 0.48 on the smaller side (now about 600 frogs); and a normally rare allele is now fixed on the smaller side. No morphological differences are visible. A local naturalist argues that because the frogs look identical, they are still one interbreeding population and no management is needed.
1. Evaluate the naturalist's argument using evidence of gene flow, genetic drift and the definition of a species.
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Evaluating the argument. Visible similarity does not establish interbreeding. The genetic data show the opposite: significant divergence at all three loci in ten years means gene flow has effectively ceased, so the two sides are now reproductively isolated in practice.
Mechanisms at work.
- Gene flow: the highway is a barrier; frogs rarely cross four lanes, so alleles are no longer exchanged.
- Genetic drift: the smaller side (600 frogs) has a small effective population size, so random sampling of alleles between generations is strong. Fixation of a previously rare allele is a classic drift signature - it is unlikely to be selection because no morphological or environmental difference is described.
- Heterozygosity loss: the drop from 0.62 to 0.48 on the small side is consistent with drift and inbreeding in a reduced population.
Species question. The biological species concept is about reproductive isolation, not appearance. The populations are now isolated by a physical barrier and are diverging. They may not yet be separate species, but they are separate management units and the small population is at risk of inbreeding depression and loss of adaptive variation.
2. Recommend one management action and predict its effect on the genetic data over the next 20 years.
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Recommendation. Build and maintain wildlife crossings (underpasses or culverts) at the wetland, and/or translocate a small number of individuals between sides each generation.
Predicted effects.
- Restored gene flow would homogenise allele frequencies across the highway, so divergence at the three loci should shrink toward the pre-construction pattern.
- Heterozygosity on the small side should recover, because immigrants introduce alleles and reduce inbreeding.
- The fixed rare allele would no longer be fixed - it would either spread or be diluted depending on the direction of migration.
Caveats. Translocations must be screened for disease (chytrid fungus) and must be large enough to counter drift; a handful of migrants per generation may be sufficient, but one-off translocations will not sustain gene flow. Monitoring allele frequencies and heterozygosity over time provides the test of success.