Species Delimitation: Inferring Gaps in Morphology across Geography

Abstract

Species are commonly delimited on the basis of gaps in patterns of morphological variation, but there seems to be little recent work on methods to objectively assess such gaps. Here, we introduce a statistical approach that uses measurements of continuous morphological characters and geographic variation in those characters to (i) measure the strength of the evidence for the existence of a gap in morphological variation between two hypothesized species and (ii) examine if a gap in morphological variation between two hypothesized species can be explained by an alternative hypothesis of geographic variation within a species. This approach is based on recent developments in analyses of multivariate normal mixtures, estimates of multivariate tolerance regions, and principal coordinates of neighboring matrices. We demonstrate the application of the approach by examining previously proposed hypotheses of species limits in the plant genus Escallonia. We discuss the main features of the method, including potential limitations, in relation to other approaches that use gaps in morphological variation as a criterion for species delimitation. The method we propose can help strengthen the link between the theory and practice of species delimitation by increasing the transparency and consistency of taxonomic decisions based on morphology, thus contributing to integrative approaches for species delimitation that consider morphological and geographic data on an equal footing with other kinds of information.

Species detection and individual assignment in species delimitation: can integrative data increase efficacy?

“…Broad-scale environmental variables, used here, may not adequately distinguish between sympatric species occupying differential microhabitats for which finer-scale microhabitat data may be more appropriate [45]. Morphological data, as demonstrated in our study (see also [24]), represent a valuable resource for species detection and individual assignment but should be informative with respect to phenotypic divergences among species. Collectively, these results highlight the importance of consistent sampling strategies and informative data in species delimitation studies.…”

“…We instead focus on individual assignment and species detection, which must be defined a priori in coalescent-based species delimitation and have received nominal attention for methodological advancement. Methods to detect species and assign individuals prior to coalescent-based species delimitation include clustering using genetic allele frequency data [23], genetic distances [4] and morphological data [24]. These statistical, rather than qualitative approaches, provide much-needed objectivity.…”

Different roads lead to Rome: Integrative taxonomic approaches lead to the discovery of two new lizard lineages in theLiolaemus montanusgroup (Squamata: Liolaemidae)

“…We inferred gaps in morphology for the three types of data (morphometric, meristic and head shape) as described in Zapata & Jimenez (2012). This method uses the multivariate morphological space derived by a PCA to estimate a ridgeline manifold, the corresponding probability density function (PDF), and ellipsoids of tolerance regions for each pair of samples to test for discontinuities in phenotypic values.…”

“…A mixed distribution is used to model the multivariate data in a set of two components (two groups of samples) that might have more than one mode. If the ridgeline manifold of a PDF suggests that there is more than one peak for different values of a variable a (which varies from 0 at the multivariate mean of one component, to 1 at the multivariate mean of the other), then one can infer two modes and a gap in morphological space (Zapata & Jimenez, 2012).…”

Morphological variation and taxonomy of Lepidocolaptes angustirostris (Vieillot, 1818) (Passeriformes: Furnariidae)

“…So, how to explain the high colour-polymorphism found in the populations of The delimitation of species and subspecies is a topic highly debated, and theoretical and methodological approaches have been proposed (Amadon, 1949;Zink, 2006;Alström et al, 2008;Cicero, 2010;Marantz & Patten, 2010;Tobias et al, 2010;Zapata & Jiménez, 2012;Camargo & Sites, 2013). However, there is still no consensus about concepts and methods applied to different taxa (Agapow et al, 2004;de Queiroz, 2007;Gill, 2014;Sangster, 2014).…”