Paris, March 12, 2008
The experiments were carried out in the laboratory. After creating microcosms made up of several sources of carbon to generate heterogeneous environments, the scientists monitored the evolutionary diversification of a bacterium, Pseudomonas fluorescens. A single clone of this bacterium was inoculated into each microcosm (in a microplate, each well of which contained a different carbon source), and then the bacteria were free to evolve for more than 500 generations. The researchers also manipulated the movements of bacteria from one well to another on the same microplate, according to well-defined levels (0%, 1%, 10% and 100% of dispersal). Indeed, dispersal is known to constitute a central factor in evolutionary diversification. As in most species, bacteria diversify (adaptive radiation(3)) into new ecological types when they are confronted with a new environment. However, unlike most species, in which radiation occurs over very long periods of time, this can be achieved in bacteria within just a few weeks in vitro. By the end of the experiment, the scientists were able to show that intermediate dispersals (1% and 10%) had allowed evolution towards greater bacterial diversity and increased ecological productivity.
The current erosion of biological diversity requires a rapid diagnosis concerning its role in the functioning of natural ecosystems. It is thus necessary to both propose a conceptual approach to the organization of biodiversity, understand its relationships with the functioning of ecosystems and predict the possible consequences of its decline. Until now, these questions were only addressed from a purely ecological angle: by artificially creating assemblies of species (communities) with increasing diversity, and then measuring their functioning over short periods of time. Although such studies showed that under certain conditions there was a positive relationship between the diversity and productivity of communities, the evolutionary approach had never been taken into account. The experiments on experimental evolution in microcosms carried out by the Franco-British team changes the situation entirely. They have quite simply recreated the conditions for the emergence of biological diversity in the laboratory.
Generally speaking, the work by these scientists suggests that evolution may lead to a much greater complexity of ecosystems, resulting in their improved functioning. This process is maximized when the resources available are heterogeneous and the biological systems suitably connected; conditions which do not correspond to the current trend towards the homogenization of ecosystems as a result of human activity. In the longer term, these results suggest that this homogenization may reduce the capacity of living beings for diversification in the future. The marked homogenization of ecosystems would prevent the appearance of new species… with the functional consequences that these researchers have just demonstrated. There is nonetheless a message of hope: evolution is capable of re-complexifying a simple system and improving its performance.
© Patrick Venail (this image is available from the CNRS photo library, firstname.lastname@example.org)
The bacterium Pseudomonas fluorescens strain SBW25 has been very popular during the past ten years as a biological model for different empirical studies on evolutionary processes and the emergence and maintenance of diversity. This bacterium takes its name from its ability to produce fluorescent pigments under certain culture conditions (foreground). In the background are the Biolog GN2 microplates, made up of wells which each contain a different carbon source and whose violet color depends on the ability of P. fluorescences to exploit these sources.
1) Institut des sciences de l'évolution de Montpellier (CNRS, Université Montpellier 2), Laboratoire des écosystèmes lagunaires (CNRS, Université Montpellier 2, Ifremer)
2) Institut des Sciences de l'Evolution (CNRS UMR5554), Université Montpellier II. NERC Centre for Population Biology, Imperial College London. Laboratoire Ecosystèmes Lagunaires (CNRS UMR5119), Université Montpellier II. School of Biological Sciences, Biosciences Building, University of Liverpool.
3) Evolution of a species variety from a common ancestor.
Functional Diversity and Productivity Peak at Intermediate Levels of Dispersal in Evolving Metacommunities, Venail PA, MacLean RC, Bouvier T, Brockhurst MA, Hochberg ME & Mouquet N, Nature 13 March 2008.
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