Size correction in biology: how reliable are approaches based on (common) principal component analysis?
ABSTRACT Morphological traits typically scale with the overall body size of an organism. A meaningful comparison of trait values among individuals or populations that differ in size therefore requires size correction. A frequently applied size correction method involves subjecting the set of n morphological traits of interest to (common) principal component analysis [(C)PCA], and treating the first principal component [(C)PC1] as a latent size variable. The remaining variation (PC2-PCn) is considered size-independent and interpreted biologically. I here analyze simulated data and natural datasets to demonstrate that this (C)PCA-based size correction generates systematic statistical artifacts. Artifacts arise even when all traits are tightly correlated with overall size, and they are particularly strong when the magnitude of variance is heterogeneous among the traits, and when the traits under study are few. (C)PCA-based approaches are therefore inappropriate for size correction and should be abandoned in favor of methods using univariate general linear models with an adequate independent body size metric as covariate. As I demonstrate, (C)PC1 extracted from a subset of traits, not themselves subjected to size correction, can provide such a size metric.
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ABSTRACT: Migratory divides, the boundary between adjacent bird populations that migrate in different directions, are of considerable interest to evolutionary biologists because of their alleged role in speciation of migratory birds. However, the small size of many passerines has traditionally limited the tools available to track populations and as a result, restricted our ability to study how reproductive isolation might occur across a divide. Here, we integrate multiple approaches by using genetic, geolocator, and morphological data to investigate a migratory divide in hermit thrushes (Catharus guttatus). First, high genetic divergence between migratory groups indicates the divide is a region of secondary contact between historically isolated populations. Second, despite low sample sizes, geolocators reveal dramatic differences in overwintering locations and migratory distance of individuals from either side of the divide. Third, a diagnostic genetic marker that proved useful for tracking a key population suggests a likely intermediate nonbreeding location of birds from the hybrid zone. This finding, combined with lower return rates from this region, is consistent with comparatively lower fitness of hybrids, which is possibly due to this intermediate migration pattern. We discuss our results in the context of reproductive isolating mechanisms associated with migration patterns that have long been hypothesized to promote divergence across migratory divides.Ecology and Evolution 09/2014; 4(17). DOI:10.1002/ece3.1205 · 1.66 Impact Factor
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ABSTRACT: The land snail genus Rumina shows much intra- and interspecific variation in shell morphology. Three morphospecies are commonly recognized: R. decollata, R. saharica and R. paivae. The descriptions of these three morphospecies were based on differences in shell and genital characters for R. saharica and R. decollata, and shell size for R. paivae. However, recently DNA-sequence data have suggested that these morphospecies comprise at least seven phylogenetic species (R. decollata molecular operational taxonomic units (MOTUs) A–F and R. saharica). The present study explores to what extent these phylogenetic species are morphologically diagnosable and/or can be reconciled with the three currently recognized morphospecies. It shows that: (1) R. saharica, and to a lesser extent R. decollata MOTUs A and Eb, are significantly differentiated from the other phylogenetic species by both genital and shell characters, and can be regarded as three diagnosable biological and phylogenetic species; (2) there are no diagnostic genital features for R. paivae, so that this taxon should not be treated as a separate species; (3) none of the other DNA-based R. decollata MOTUs could be consistently differentiated by the shell and genital characters analysed in this study, so that their status needs further scrutiny.Journal of Molluscan Studies 12/2014; 81(2). DOI:10.1093/mollus/eyu080 · 1.50 Impact Factor
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ABSTRACT: The snail-feeding carabid beetle Damaster blaptoides exhibits diverse head and thorax morphologies, and these morphotypes are linked with two alternative feeding behaviors. Stout-shaped beetles feed on snails by crushing the shells, whereas slender-shaped beetles consume snails by inserting their heads into the shells. A trade-off exists between these feeding strategies. Because intermediate-shaped beetles are less proficient in these two behaviors, stout-slender morphological divergence occurs between related species feeding on land snails. To examine the genetic basis of these morphotypes, we conducted morphological analyses and quantitative trait locus (QTL) mapping using backcross offspring between the stout and slender subspecies. The morphological analyses showed that the width and length of the beetle body parts were correlated with each other; in particular, the head width and thorax length were strongly negatively correlated. QTL mapping showed that QTLs for head width and thorax length are located in close proximity to one another on the longest linkage group, and that they have positive and negative additive genetic effects. Our results suggest that the adaptive phenotypic sets of a wide head and short thorax and a narrow head and long thorax are based on the closeness of these QTLs. Morphological integration between the head and thorax may play an important role in the adaptive divergence of these beetles.This article is protected by copyright. All rights reserved.Molecular Ecology 10/2014; DOI:10.1111/mec.12976 · 5.84 Impact Factor