The costs of specialization
Adaptation to a single niche limits the ability of an organism to diversify further | By Andrea Rinaldi
As an organism becomes specialized to a specific ecological niche, its fitness to survive in an alternative niche in the same environment decreases. This long-recognized trend is key to the dynamics of adaptive radiations—concentrated bursts of evolution during which new species or variants rapidly form—diverging from a common ancestor in ecology and phenotype. In the December 19 Science, Angus Buckling and colleagues at the University of Bath focus on the constraints regulating this process and report that niche adaptation itself may limit a population's ability to subsequently diversify into other niches (Science, 302:2107-2109, December 19, 2003). Buckling et al. explored the adaptation–diversification ability of the common plant-colonizing bacterium Pseudomonas fluorescens in “rugged fitness landscapes”—environments in which several distinct ecological niches coexist. They generated a heterogeneous environment comprising a static glass culture bottle containing nutrient-rich medium, with the niches defined by microenvironmental conditions found in the main liquid phase, at the air–broth interface, or at the less aerobic conditions at the bottom of the flask. When P. fluorescens is grown under these culture conditions, it typically rapidly diversifies, generating numerous niche specialist genotypes that are readily distinguished by their colony morphology when transferred to agar plates. Buckling et al. propagated six replicate populations of P. fluorescens in a weekly batch-transfer protocol for 5 weeks, isolating a single cell of the dominant phenotype and using it to seed the next population. Within the same culture, the fitness of the dominant phenotype increased with time, signaling that this best-adapted population was further specializing to a given niche. However, when replicated, the same phenotype showed a net and progressive decreased ability to genetically diversify and to produce de novo morphs adapted to the empty niches found in the new environment. The authors suggest that once an organism has ascended a “fitness peak” in its environment (specializing to a ecological niche), the costs of descending and adapting to other conditions are be too high, and all mutations in this sense removed by selection. “These results are therefore likely to be generally relevant and may help to explain patterns of diversity over both micro- and macroevolutionary time scales,” conclude the authors. The possibility that the observed pattern was rather due to the evolution of generalists, or to an intrinsic reduction in mutation rates of bacteria, was excluded by performing experiments with a slightly modified approach. “We predict that in environments that can potentially support similar levels of diversity, diversification is more likely to occur immediately following colonization of the environment than through expansion into new niches within the environment after an extinction event,” say the authors. Thus, following an extinction, generalists would likely diversify rapidly to occupy the newly available niches and colonize the environment, whereas specialists would remain confined to their “peaks,” contributing little to environment repopulation. In a related Perspectives article, Santiago Elena from the Institute for Molecular and Cellular Plant Biology and Rafael Sanjuán at the University of València, underline that the findings of Buckling et al. depend strongly on the association between phenotypes and genotypes, which is well established in some cases. “However, there is no evidence that the other phenotypes observed by Buckling and co-workers are associated with particular genotypes; they may simply reflect random asymmetries during the process of colony growth on a solid surface,” they suggest. Elena and Sanjuán also recall that other evolution experiments with microbes have yielded generalists instead of specialists, demonstrating that “generalists can also evolve without paying a significant fitness cost.” Links for this article B.C. Emerson, “Evolution on oceanic islands: molecular phylogenetic approaches to understanding pattern and process,” Molecular Ecology, 11:951-966, November 2002. [ PubMed Abstract]
A. Buckling et al., “Adaptation limits diversification of experimental bacterial populations,” Science, 302:2107-2109, December 19, 2003. http://www.sciencemag.org
University of Bath http://www.bath.ac.uk/
P.B. Rainey, M. Travisano, “Adaptive radiation in a heterogeneous environment,” Nature, 394:69-72, July 2, 1998. [ PubMed Abstract]
S.F. Elena, R. Sanjuán, “Climb every rugged peak or just one?” Science, 302, December 19, 2003. http://www.sciencemag.org
Institute for Molecular and Cellular Plant Biology http://www.ibmcp.upv.es/
University of València http://www.uv.es/~webuv/
S.F. Elena, R.E. Lenski, “Evolution experiments with microorganisms: the dynamics and genetic bases of adaptation,” Nature Reviews Genetics, 4:457-469, June 2003. [ PubMed Abstract]
|