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Accurate delimitation of species is crucial for a stable taxonomy, which provides the foundation for the study of evolutionary biology, ecology, and essentially all biological disciplines. Several approaches toward impartial and repeatable taxonomic practices are available but all existing methods have potentially unacceptable shortcomings. In particular, problems can arise when the underlying model assumptions are violated, for instance, in the presence of reduced gene flow. This is observed in the context of sex-biased dispersal, which is a common but underappreciated feature in many groups of organisms. Previously, simulations have indicated that sex-biased dispersal may lead to erroneous estimations of the true species numbers. However, this phenomenon has never been examined using empirical data. We evaluate the bias introduced by extreme female philopatry on a range of de novo [GMYC, PTP, ABGD, statistical parsimony, trinomial distribution of triplets model (tr2)] and validation (STACEY, iBPP) approaches to species delimitation in the scarab beetle genus Pachypus. Since female philopatry exhibited in this genus in particular can affect mitochondrial gene flow, we compared the results from analyses of single loci, mitochondrial loci, nuclear loci and combined data, as well as the performance of morphometric data as a secondary data source in a fully integrative Bayesian framework. Large overestimation of species numbers was observed across all analyses of combined and mitochondrial DNA data sets, suggesting specimens from nearly every sampling location as separate species. The use of nuclear data resulted in more reasonable estimations of species boundaries, which were largely supported by morphometrics of linear measurements, while geometric morphometrics of body outlines resulted in stronger splitting. Simulations of population divergence with migration, corresponding to the biology of Pachypus, showed that female philopatry strongly increases reciprocal monophyly of mitochondrial markers and may substantially contribute to over-splitting in species delimitation. Robust results recovered using nuclear DNA and morphological data nevertheless enabled us to reach novel conclusions about species boundaries in Pachypus. Our findings suggest that mitochondrial DNA will be less suited to species delimitation in many cases, in particular in the presence of sex-biased dispersal., (© The Author(s) 2018. Published by Oxford University Press, on behalf of the Society of Systematic Biologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

DNA-based species delimitation may be compromised by limited sampling effort and species rarity, including "singleton" representatives of species, which hampers estimates of intra- versus interspecies evolutionary processes. In a case study of southern African chafers (beetles in the family Scarabaeidae), many species and subclades were poorly represented and 48.5% of species were singletons. Using cox1 sequences from >500 specimens and ∼100 species, the Generalized Mixed Yule Coalescent (GMYC) analysis as well as various other approaches for DNA-based species delimitation (Automatic Barcode Gap Discovery (ABGD), Poisson tree processes (PTP), Species Identifier, Statistical Parsimony), frequently produced poor results if analyzing a narrow target group only, but the performance improved when several subclades were combined. Hence, low sampling may be compensated for by "clade addition" of lineages outside of the focal group. Similar findings were obtained in reanalysis of published data sets of taxonomically poorly known species assemblages of insects from Madagascar. The low performance of undersampled trees is not due to high proportions of singletons per se, as shown in simulations (with 13%, 40% and 52% singletons). However, the GMYC method was highly sensitive to variable effective population size ([Formula: see text]), which was exacerbated by variable species abundances in the simulations. Hence, low sampling success and rarity of species affect the power of the GMYC method only if they reflect great differences in [Formula: see text] among species. Potential negative effects of skewed species abundances and prevalence of singletons are ultimately an issue about the variation in [Formula: see text] and the degree to which this is correlated with the census population size and sampling success. Clade addition beyond a limited study group can overcome poor sampling for the GMYC method in particular under variable [Formula: see text] This effect was less pronounced for methods of species delimitation not based on coalescent models., (© The Author(s) 2016. Published by Oxford University Press, on behalf of the Society of Systematic Biologists. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)