GeMBiD, a 'Geometric Morphometric Approach to the Study of Biological Diversity'
Cardini, Andrea, Elton, Sarah, Endangered Species Update
Defining taxonomic units that are significant in terms of their evolutionary potential and history is crucial for an accurate understanding of biological diversity. To clearly and operationally define what these units are is fundamental in conservation biology as well because they are the basis for any decision about priorities, regardless of the criteria for setting these priorities. Without carefully assessing biological diversity, we will not be able to understand what we have, what can still be saved and what might be most urgent to save.
Assessing biological diversity has been revolutionised by tools such as DNA barcoding, the idea that a short stretch of DNA sequence can be used as a barcode to identify and discover taxa. It is simple, fast, and relatively cheap. However, by definition, it only focuses on the genotype. Relying on a single point of comparison, such as mitochondrial DNA (mtDNA), for assessing taxonomic differences poses serious risk of underestimating biological diversity (e.g., Will & Rubinoff 2004; Desalle et al. 2005; Rubinoff 2006; Elias at al. 2007; Trewick 2008). For example, the short sequence of mtDNA used in barcoding represents a tiny fraction of the whole genome, and analysis of nuclear DNA from the same species may give a very different picture to that obtained from mtDNA, as the two do not share the same mechanisms of inheritance and have different modes of evolution (Rubinoff 2006). However seductive the genomic techniques seem, measuring phenotypic diversity remains essential because the phenotype reflects historical processes and plastic responses to the environment. Heritable variation in phenotypes is the expression of adaptive characters "shaped by gene flow, genetic drift and natural selection" (Crandall et al. 2000) and provides information on the evolutionary processes that lead to different adaptations. Plasticity is non-heritable but still important as it might increase evolvability by accelerating the speed by which populations can move between adaptive peaks (Borenstein et al. 2006). Preserving these processes by conserving phenotypic diversity retains evolutionary potential.
In order to add a phenotypic dimension to the quantitative analysis of biological diversity, we suggest a method that is potentially powerful and easily applicable whenever hard tissues (e.g. mammal and other vertebrate bones) are available for analysis. The method, which we have called the Geometric Morphometric approach to the study of Biological Diversity (GeMBiD), takes advantage of the development over the last couple of decades of techniques that quantify the size and shape of organisms or their organs using Cartesian coordinates of homologous anatomical landmarks (Adams et al. 2004; Sanfilippo et al., 2009). Variation in landmark coordinates due to differences in specimen position during data collection are easily removed whereas differences in size and shape are effectively captured by these methods, and shape variables allow powerful statistical analyses.
For the study of biological diversity and the disparity observed within and between taxa, landmarks can be digitized on pictures of specimens (e.g., skulls or mandibles) taken in standardized conditions. These can be obtained from museums and, especially for rare populations, by cooperating with field scientists. A priori groups are then defined based on traditional taxonomy, geographic distribution or other criteria (which can be refined in the course of the study), and used in cross-validated discriminant analyses of shape variables. The proportions of individuals correctly classified according to groups (hit ratios, HR) are proportional to their differences, and thus provide information about the degree of morphological distinctiveness of these groups. In other words, a low HR implies small differences in form and hence negligible morphological variation. HR for poorly known populations can be compared with HR observed in other phylogenetically related but better studied populations whose taxonomic status has already been confidently assessed. …