Data from: Issues and perspectives in species delimitation using phenotypic data: Atlantean evolution in Darwin's finches

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 that convey little information on phenotype
frequencies; (2) exclusion of characters potentially important for species
delimitation following reduction of data dimensionality; and (3) use of
measures of central tendency 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 Darwin’s
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. We discuss how normal mixture models
offer new perspectives in the kind of inferences available to
systematists, with significant repercussions on ideas about the phenotypic
structure of biodiversity.