Two Galaxias Morphs and a Removed Culvert
Two morphs of Galaxias in a coastal lake differ in gill raker number (mean 9.2 versus 13.8), body depth, and spawning depth (littoral versus 6 m). They spawn at the same time, and 8% of nests contain mixed parentage. Twelve microsatellite loci give FST = 0.06 (P < 0.001) with no fixed differences; mtDNA is reciprocally monophyletic. F1 hybrids survive to adulthood at 0.7 of the parental rate and backcrosses at 0.4. Body-size assortative mating is strong (r = 0.62), and a culvert removed 80 years ago had connected the two spawning habitats until recently, when it was cleared. The regional council must decide whether the morphs are separate conservation units under the threat classification system, which determines whether a proposed water take can proceed.
1. FST = 0.06 is low, yet mtDNA is reciprocally monophyletic. Explain the disagreement and decide which estimate is more informative about current gene flow.
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FST = 0.06 means 6% of microsatellite variation lies between morphs and 94% within - low differentiation consistent with substantial ongoing gene flow. Under a simple island model, Nm ≈ (1 − FST)/(4FST) ≈ 3.9 migrants per generation.
mtDNA reciprocal monophyly looks far stronger, but it is a single maternally inherited locus with an effective population size roughly a quarter that of nuclear loci, so it sorts to monophyly faster by genetic drift alone - monophyly can arise without nuclear divergence. More importantly, if females return to their natal spawning depth (littoral versus 6 m) while males disperse, mtDNA diverges while nuclear genes continue to flow. That is precisely the pattern expected from sex-biased dispersal and spawning-site fidelity, and it is consistent with strong size-assortative mating (r = 0.62).
For current nuclear gene flow, the 12-locus estimate is the more informative; mtDNA is informative about historical separation and female behaviour. The disagreement is not a contradiction - it is a signal about mating structure, and it should be tested with sex-specific dispersal data.
2. Apply the biological, phylogenetic and genotypic-cluster species concepts to decide the status of the two morphs. What evidence would change your conclusion?
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Biological species concept: 8% mixed parentage means they are not reproductively isolated, so they fail BSC as full species.
Phylogenetic/lineage concept: reciprocal mtDNA monophyly plus distinct trophic morphology (gill rakers 9.2 versus 13.8) and spawning depth indicates separately evolving lineages.
Genotypic-cluster concept: FST = 0.06 is small but significant (P < 0.001), so they are distinguishable clusters - weakly.
The decisive data are the hybrid fitnesses: F1 at 0.7 and backcross at 0.4 of parental survival is a genuine postzygotic barrier. Selection against hybrids can maintain two forms in a tension zone despite 8% hybridisation. The defensible description is therefore partially isolated lineages in a hybrid zone - neither 'one variable species' nor 'two good species'.
What would change it: fixed diagnostic nuclear differences would support species status; hybrid fitness near 1.0 with unrestricted gene flow would support a single unit; genomic clines showing the zone is moving would indicate breakdown rather than stability.
3. How should the culvert history affect the management decision and the water-take application?
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The culvert defines the baseline. If the two spawning habitats were naturally separate before the culvert and it then connected them, present hybridisation may already be elevated above the natural rate and the hybrid zone may be a human artefact. In that case, maintaining separation (or restoring it) is defensible and the distinct morphs deserve separate management. If the habitats were naturally connected and the culvert only briefly interrupted gene flow, hybridisation is a natural process, and blocking it would preserve an artefact of the barrier rather than a natural state.
Conservation-unit decision: until the historical baseline is resolved, treat the morphs as two separate management units. The asymmetry of error matters - wrongly splitting loses little, wrongly lumping risks losing a distinct lineage.
Water take: decline it if it would reduce flow or spawning habitat in either spawning area, since both morphs depend on the lake's hydrological regime and the hybrid zone is sensitive to habitat change. Decisive additional data: historical records or sediment/otolith evidence of prior connection, and genomic clines across the contact zone to test whether hybridisation is increasing.