AbstractThe North American tiger salamander species complex, including its flagship species the axolotl, has long been a source of biological fascination. The complex exhibits a wide range of variation in developmental life history strategies, including populations and individuals that undergo metamorphosis and those able to forego metamorphosis and retain a larval, aquatic lifestyle (i.e., paedomorphosis). Such disparate life history strategies are assumed to cause populations to become reproductively isolated, but the degree to which they have actually shaped population- and species-level boundaries is poorly understood. Using a large multi-locus dataset from hundreds of samples across North America, we identified genetic clusters with clear signs of admixture across the geographic range of the tiger salamander complex. Population clusters often contain a mixture of paedomorphic and metamorphic taxa, and we conclude that geography has played a large role in driving lineage divergence relative to obligate paedomorphosis in this system. This conclusion is bolstered by model-based analyses demonstrating gene flow between metamorphic and paedomorphic populations. Even the axolotl, a paedomorphic species with an isolated native range, apparently has a history of gene flow with its neighboring populations. This fine-scale genetic perspective on life-history variation establishes a framework for understanding how plasticity, local adaptation, and gene flow contribute to lineage divergence. The axolotl is currently used as the vertebrate model system in regenerative biology, and our findings chart a course for more informed use of these and other tiger salamander species in experimental and field research, including conservation priorities.Significance StatementPopulation structure and speciation are shaped by a variety of biotic and abiotic factors. In the tiger salamander complex, one factor that may influence diversification is life history: some taxa are obligately paedomorphic–a condition where adults maintain an aquatic, larval phenotype–while others are facultatively paedomorphic or entirely metamorphic. Using a large multi-locus dataset, we found evidence of gene flow and/or panmixia between obligately and facultatively paedomorphic taxa, suggesting that an obligately paedomorphic life history is not a strong driver of speciation in the tiger salamander complex. We also recovered a history of gene flow between the critically endangered axolotl and its neighboring populations, providing important information for its conservation and captive management.
Geography is more important than life history in the recent diversification of the tiger salamander complex
