Atlantean Evolution in Darwin’s Finches—Issues and Perspectives in Species Delimitation using Phenotypic Data

Abstract

Progress in the development and use of methods for species delimitation employing phenotypic data lags behind conceptual and practical advances in molecular genetic approaches. The basic evolutionary model underlying the use of phenotypic data to delimit species assumes random mating and quantitative polygenic traits, so that phenotypic distributions within a species should be approximately normal for individuals of the same sex and age. Accordingly, two or more distinct normal distributions of phenotypic traits suggest the existence of multiple species. In light of this model, we show that analytical approaches employed in taxonomic studies using phenotypic data are often compromised by three issues: (1) reliance on graphical analyses of phenotypic space that do not consider the frequency of phenotypes; (2)exclusion of characters potentially important for species delimitation following reduction of data dimensionality; and (3) use of measures of central tendencies to evaluate phenotypic distinctiveness. We outline approaches to overcome these issues based on statistical developments related to normal mixture models and illustrate them empirically with a reanalysis of morphological data recently used to claim that there are no morphologically distinct species of Darwins ground-finches (Geospiza). We found negligible support for this claim relative to taxonomic hypotheses recognizing multiple species. Although species limits among ground-finches merit further assessments using additional sources of information, our results bear implications for other areas of inquiry including speciation research: because ground-finches have likely speciated and are not trapped in a process of \"Sisyphean\" evolution as recently argued, they remain useful models to understand the evolutionary forces involved in speciation. Our work underscores the importance of statistical approaches grounded on appropriate evolutionary models for species delimitation. Approaches allowing one to fit normal mixture models without a priori information about species limits offer new perspectives in the kind of inferences available to systematists, with significant repercusions on ideas about the structure of biological biodiversity. [morphology; normal mixture model; phenotype; principal components analysis; species limits; variable selection.]