Brian Sidlauskas' paper in Evolution

Testing for unequal rates of morphological diversification in the absence of a detailed phylogeny: a case study from characiform fishes

Why do the species on some branches of the tree of life have disparate anatomical structures and varied ecologies, while species on other branches are conservative in form and function? NESCent postdoctoral fellow Brian Sidlauskas has published a new study in the journal Evolution that develops the Random Phylogenies Rate Test (RAPRATE), a method that determines whether differences in morphological diversity between clades most likely result from shifts in the underlying rate of morphological change or from chance differences accumulated under equal rates. The test represents a methodological advance because it considers many morphological traits simultaneously and does not require extensive prior phylogenetic knowledge. Instead of using a known phylogeny, the test simulates evolution on many possible phylogenies that match the relative ages of the two clades and the number of species that they are known to contain. Matlab scripts that perform the RAPRATE test are available by request to Brian at

The case study in Brian's paper represents the first quantitative exploration of morphological diversification in two major lineages of South American freshwater fishes, the Anostomoidea and Curimatoidea. The first of these fish groups is morphologically and ecologically diverse, and contains species with jaws that face upwards, downwards, forwards and even backwards. Members of the second group are all detritivores and have morphologically conservative jaws. Application of the RAPRATE method revealed that, even without knowing the phylogeny for these fishes, one can have at least 90% confidence that the rate of morphological change in the skulls of the Anostomoidea was higher than the rate of change in the skulls of the Curimatoidea. As part of his postdoctoral project at NESCent, Brian is using skeletal, phylogenetic and ecological data to determine factors that may have caused the shift in rates and is comparing the diversification of these South American fishes to the diversification of their close relatives from Africa. Forthcoming papers will analyze how and why these South American and African fishes have (and have not) evolved in parallel ever since the separation of the two continents 90 million years ago.

Sidlauskas, Brian L. 2007. Testing for unequal rates of morphological diversification in the absence of a detailed phylogeny: a case study from characiform fishes. Evolution 61 (online early).
[ more ]