Notes from the taxonomic disaster zone: Evolutionary drivers of intractable species boundaries in an Australian lizard clade (Scincidae: Ctenotus)

Genomic-scale datasets, sophisticated analytical techniques, and conceptual advances have disproportionately failed to resolve species boundaries in some groups relative to others. To understand the processes that underlie taxonomic intractability, we dissect the speciation history of an Australian lizard clade that arguably represents a "worst-case" scenario for species delimitation within vertebrates: the Ctenotus inornatus species group, a clade beset with decoupled genetic and phenotypic breaks, uncertain geographic ranges, and parallelism in purportedly diagnostic morphological characters. We sampled hundreds of localities to generate a genomic perspective on population divergence, structure, and admixture. Our results revealed rampant paraphyly of nominate taxa in the group, with lineages that are either morphologically cryptic or polytypic. Isolation-by-distance patterns reflect spatially continuous differentiation among certain pairs of putative species, yet genetic and geographic distances are decoupled in other pairs. Comparisons of mitochondrial and nuclear gene trees, tests of nuclear introgression, and historical demographic modelling identified gene flow between divergent candidate species. Levels of admixture are decoupled from phylogenetic relatedness; gene flow is often higher between sympatric species than between parapatric populations of the same species. Such idiosyncratic patterns of introgression contribute to species boundaries that are fuzzy while also varying in fuzziness. Our results suggest that "taxonomic disaster zones" like the C. inornatus species group result from spatial variation in the porosity of species boundaries and the resulting patterns of genetic and phenotypic variation. This study raises questions about the origin and persistence of hybridizing species and highlights the unique insights provided by taxa that have long eluded straightforward taxonomic categorization.

Subspecies at crossroads: the evolutionary significance of genomic and phenotypic variation in a wide-ranging Australian lizard (Ctenotus pantherinus)

Many subspecies were described to capture phenotypic variation in wide-ranging taxa, with some later being found to correspond to divergent genetic lineages. We investigate whether currently recognized subspecies correspond to distinctive and coherent evolutionary lineages in the widespread Australian lizard Ctenotus pantherinus based on morphological, mitochondrial and genome-wide nuclear variation. We find weak and inconsistent correspondence between morphological patterns and the presumed subspecies ranges, with character polymorphism within regions and broad morphological overlap across regions. Phylogenetic analyses suggest paraphyly of populations assignable to each subspecies, mitonuclear discordance and little congruence between subspecies ranges and the distribution of inferred clades. Genotypic clustering supports admixture across regions. These results undermine the presumed phenotypic and genotypic coherence and distinctiveness of C. pantherinus subspecies. Based on our findings, we comment on the operational and conceptual shortcomings of morphologically defined subspecies and discuss practical challenges in applying the general notion of subspecies as incompletely separated population lineages. We conclude by highlighting a historical asymmetry that has implications for ecology, evolution and conservation: subspecies proposed in the past are difficult to falsify even in the face of new data that challenge their coherence and distinctiveness, whereas modern researchers appear hesitant to propose new subspecies.

Mapping species diversification metrics in macroecology: Prospects and challenges

The intersection of macroecology and macroevolution is one of today’s most active research in biology. In the last decade, we have witnessed a steady increment of macroecological studies that use metrics attempting to capture macroevolutionary processes to explain present-day biodiversity patterns. Evolutionary explanations of current species richness gradients are fundamental for understanding how diversity accumulates in a region. Although multiple hypotheses have been proposed to explain the patterns we observe in nature, it is well-known that the present-day diversity patterns result from speciation, extinction, colonization from nearby areas, or a combination of these macroevolutionary processes. Whether these metrics capture macroevolutionary processes across space is unknown. Some tip-rate metrics calculated directly from a phylogenetic tree (e.g., mean root distance -MRD-; mean diversification rate -mDR-) seem to return very similar geographical patterns regardless of how they are estimated (e.g., using branch lengths explicitly or not). Model-based tip-rate metrics —those estimated using macroevolutionary mixtures, e.g., the BAMM approach— seem to provide better net diversification estimates than only speciation rates. We argue that the lack of appropriate estimates of extinction and dispersal rates in phylogenetic trees may strongly limit our inferences about how species richness gradients have emerged at spatial and temporal scales. Here, we present a literature review about this topic and empirical comparisons between select taxa with several of these metrics. We implemented a simple null model approach to evaluate whether mapping of these metrics deviates from a random sampling process. We show that phylogenetic metrics by themselves are relatively poor at capturing speciation, extinction, and dispersal processes across geographical gradients. Furthermore, we provide evidence of how parametric biogeographic methods can improve our inference of past events and, therefore, our conclusions about the evolutionary processes driving biodiversity patterns. We recommend that further studies include several approaches simultaneously (e.g., spatial diversification modeling, parametric biogeographic methods, simulations) to disentangle the relative role of speciation, extinction, and dispersal in the generation and maintenance of species richness gradients at regional and global scales.

Genetic variability and the ecology of geographic range: A test of the central-marginal hypothesis in Australian scincid lizards

For many species, both local abundance and regional occupancy are highest near the centre of their geographic distributions. One hypothesis for this pattern is that niche suitability declines with increasing distance from a species geographic centre, such that populations near range margins are characterized by reduced density and increased patchiness. In these smaller edge populations, genetic drift is more powerful, leading to the loss of genetic diversity. This simple verbal model has been formalized as the central-marginal hypothesis, which predicts that core populations should have greater genetic diversity than edge populations. Here, we tested the central-marginal hypothesis using a genomic data set of 25 species-level taxa of Australian scincid lizards in the genera Ctenotus and Lerista. A majority of taxa in our data set showed range-wide patterns of genetic variation consistent with central-marginal hypothesis, and eight of 25 taxa showed significantly greater genetic diversity in the centre of their range. We then explored biological, historical, and methodological factors that might predict which taxa support the central-marginal hypothesis. We found that taxa with the strongest evidence for range expansion were the least likely to follow predictions of the central-marginal hypothesis. The majority of these taxa had range expansions that originated at the range edge, which led to a gradient of decreasing genetic diversity from the range edge to the core, contrary to the central-marginal hypothesis.