The current theories of evolution try to integrate these two types of models: the systems which present few constraints (those for which many ecological niches are available, characterized by rapid and early diversification) and the systems dominated by constraints (those for which a well-defined number of ecological niches is available, leading to convergences). The description of signatures making it possible to distinguish them would be very useful to comparative biologists who study the dynamics of clade diversification.

This is the context for work by Bruno Frédérich, of the Functional and Evolutionary Morphology Laboratory of the University of Liege, on damselfishes (1). The originality of this study lies in the fact that various techniques have been integrated. These techniques, which combine ecology, morphology and phylogeny (genetics), make it possible to distinguish the possible modes of diversification: unique adaptive radiation (early burst) or repeated adaptive radiations (convergence). The researcher explains “The idea was to study the ecological and morphological modes of diversification of coral reef fishes during their evolution. Many people produce phylogenetic trees, but this multiple approach is unique to the University of Liege laboratory!”

The family of Pomacentridae, with its 386 species, represents the third largest group of fish in coral reef ecosystems after the Gobiidae and Labridae families. It is divided into five sub-families: Stegastinae, Lepidozygus (monospecific), Chrominae, Abudefdufinae and Pomacentrinae. These damselfishes diversified into three major trophic groups:
(1) The zooplanktivorous species that eat small planktonic crustaceans, the copepods.
(2) The grazing species that eat filamentous algae. 
(3) The intermediary species that eat zooplankton, small benthic invertebrates and algae in variable proportions. Read the box on Linnaeus classification.

Some grazing species have developed ‘farming’ behavior: they fiercely defend their territory by chasing away any intruders and manage algal assemblages like their own farm. “They cultivate their field by cutting some algae that they do not like and allow others to grow. It is as though they have their own vegetable garden!” declares B. Frédérich. Due to the fact that not all grazing species show this behavior, these ‘farmer’ fish are considered as a group apart in the study.

Phylogenetic time-tree.

In order to determine if the diversification of Pomacentridae occurred following a single adaptive radiation (early burst) or following repeated radiations leading to convergences, the researcher first concentrated on phylogeny, that is to say, the study of relationships of kin between the different species.

(1) Frédérich B, Sorenson L, Santini F, Slater GJ, Alfaro ME (2012). Iterative ecological radiation and convergence during the evolutionary history of damselfishes (Pomacentridae). The American Naturalist, in press.