Le site de vulgarisation scientifique de l’Université de Liège. ULg, Université de Liège

The evolution of damselfishes
1/4/13

The coral reefs shelter one of the most spectacular populations of fish on our planet and more than half of the species found in the oceans live there. Until recently, however, the fish that live in these reefs were the subject of very few evolutionary studies and there is still only limited knowledge about their modes of diversification. The Functional and Evolutionary Morphology Laboratory of the University of Liege has devoted particular attention to the subject and the recent work on damselfishes, the Pomacentridae family, has allowed the improvement of the evolutionary concepts that are traditionally accepted by scientists specializing in evolutionary biology.

(EN)pinson-darwinSince Charles Darwin published his book On the origin of the species in 1859, evolutionary theories have also evolved. Darwin’s theory is based on the mechanism of natural selection which explains the adaptation of species to environments over the course of generations. By selecting the most adapted individuals, the traits that favor survival and reproduction develop with greater frequency from one generation to another because they are hereditary. Darwin’s work on the finches of the Galapagos Islands illustrates how several species with different kinds of beaks developed from an original species by adaptive radiation: these traits were acquired through adaptation to various diets and colonization of various ecological niches.

In the 1940’s, the Synthetic Theory of Evolution was born by combining Darwinian selection with Mendelian Genetics. The discovery of DNA and molecular biology has greatly enhanced this scientific edifice. Since then, the biology of evolution has been integrated into all disciplines of biology and more recently, the study of evolution has benefitted from the development of information technology and progress in the realm of molecular biology, notably genome sequencing which has enabled the development of phylogeny by contributing very important data.

It is generally accepted among the scientific community that adaptive radiation implies early and rapid diversification of several lineages from a common ancestor due to ecological opportunity: the colonization of new regions, the extinction of competitors or the development of traits representing major innovations. According to this scenario, the rate of diversification of species is initially very high and then diminishes with time (early burst). On the other hand, it is expected that the subclade resulting from this radiation will show a varied ecological and phenotypic diversity.

But other kinds of evolutionary radiation are possible such as repeated radiations leading to evolutionary convergence. This evolutionary mechanism explains the morphological and sometimes behavioral resemblances between species that have been subjected to the same environmental conditions. These species show similar characteristics but these have not been inherited from a common ancestor. In certain cases, the differences between two convergent species can appear small at first glance, and lead to errors of phylogenetic classification. It was not until 1693, that the naturalist John Ray established that the cetaceans actually belong to the class of mammals despite a strong resemblance to fish due to convergent evolutions that led, in both cases, to adaptation to an aquatic environment. According to the scenario of repeated radiations, no early burst in the diversification of the species followed by a slow down of diversification rate over time is expected unlike a model of only one adaptive radiation event.

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