Cryptic Frog Populations in a Protected Wetland
A 2,000-ha wetland complex contains two frog populations, A and B, separated by a 2-km contact zone. Morphometric measurements overlap almost completely: snout-vent length is 28.4 ± 1.1 mm in A (n = 60) and 28.9 ± 1.2 mm in B (n = 45). Male advertisement calls differ sharply: dominant frequency 2.1 kHz in A versus 3.4 kHz in B, and pulse rate 18/s versus 31/s. A 658-bp COI fragment shows 4.6% uncorrected divergence and reciprocal monophyly; nuclear introns give FST = 0.18. In the contact zone, 42 males were recorded, only two mixed choruses were heard, and 18 experimental crosses produced no hybrid embryos. Population A has about 300 breeding adults in one 12-ha pond; population B has about 90 adults in three small ponds. The manager must decide whether the two populations are separate species and whether to move frogs between ponds to reduce inbreeding in B. A morphospecies survey in 2010 recorded only one species at the site.
1. Apply the biological and phylogenetic species concepts to A and B. What additional evidence would be decisive, and why is the 4.6% COI divergence, by itself, insufficient?
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The biological species concept is supported by the near-absence of mixed choruses and the failure of 18 experimental crosses, which point to prezygotic isolation. However, laboratory crosses are a small sample and captivity can alter mating behaviour. The phylogenetic species concept is supported by reciprocal COI monophyly and nuclear FST = 0.18, which indicate separately evolving lineages. A single mtDNA locus can sort by drift or introgression, and 4.6% uncorrected divergence has no universal species threshold; some sister frog species differ by less, while some populations differ by more. Decisive evidence would be a multi-locus coalescent analysis, genome-wide FST, hybrid-zone cline analysis, and playback experiments showing that females discriminate between the two calls. The COI value is consistent with species-level divergence but does not prove reproductive isolation.
2. Explain how the 2010 morphospecies survey underestimated diversity and how the call and genetic data change the management priority for population B.
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Morphospecies surveys rely on visible traits, and A and B overlap almost completely in snout-vent length, so they would be scored as one species. The call data reveal a prezygotic barrier that morphology misses, and the genetic data show that the two populations are not a single panmictic unit. This matters for B: about 90 adults in three ponds is a small effective population, and if B is a distinct species it has high conservation value and cannot simply be topped up with A. The 2010 survey underestimated alpha diversity and also hid the fragmentation of B. Management should protect all three B ponds, maintain connected habitat, and monitor call frequency, larval recruitment, and adult numbers separately for each pond.
3. Recommend whether to translocate frogs from A to B. Justify your answer with genetic and ecological risks.
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Do not translocate A into B. If B is a distinct species, hybridization could swamp its gene pool; even if the two are conspecific, moving individuals between isolated ponds risks introducing pathogens and disrupting local adaptation. B's small size calls for within-B genetic rescue only if diversity is demonstrably low: test neutral diversity, then exchange a few individuals among the three B ponds, not from A. Improve terrestrial and aquatic habitat around B, reduce road runoff, and monitor adult numbers and larval recruitment. If B continues to decline, consider a captive assurance population using B stock. Translocating A would be justified only if multi-locus data show no species-level divergence and B has no unique alleles.