Two cautionary points, one perhaps encouraging and another less so, emerge from our study. First, it is evident that a high mutation rate is not always bene®cial. The adaptive neighbourhood of an adaptive peak can constrain evolvability, despite a high mutation rate. A high rate can lead to a high mutational load, as seen during the evolution by clone B. Thus, evolutionary changes in a population of RNA viruses, whether under clinical treatment or otherwise, should not automatically be interpreted as adaptive to the virus. Second, as had already been noted 12 , if multiple peaks exist, a shift to a new adaptive peak can be triggered by a costly resistance mutation and the ensuing compensatory mutations. If some peaks are higher, drugs should be used with caution not only because resistance can evolve but also because drug treatment can lead to the evolution of viruses that are more ®t rather than less. M Methods Strains and culture conditionsThe RNA bacteriophage f6 used in this study is a laboratory clone descended from the original isolate 15 . Populations A and B were generated in a previous study 7 . They are the end points of populations propagated using bottlenecks of 100 and 33 phage, respectively, to allow ®tness recovery after acquisition of a common deleterious mutation. Pseudomonas syringae sv. phaseolicola, the standard host of f6, was obtained from the American Type Culture Collection (ATCC# 21781), and Pseudomonas pseudocaligenes ERA, an alternate host, was obtained from L. Mindich. Details of diluting, ®ltering, culture and storage of phage and bacteria are published 16,17 . All phage and bacteria were grown in LC medium at 25 8C.

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The White-browed Warbler Basileuterus leucoblepharus, a common bird of the BrazilianAtlantic forest, emits only one distinct song type in the context of territorial defense. Individual or neighbor-stranger recognition may be more difficult when birds share similar songs. In fact, the analysis of songs of different individuals reveals slight differences in the temporal and the frequency domains. Effectively, a careful examination of the signals of different individuals (21) by 5 complementary methods of analysis reveals first, that one or two gaps in frequency occur between two successive notes at different moments of the song, and second, that their temporal and frequency positions are stereotyped for each individual. Playback experiments confirm these findings. By propagation experiments, we show that this individual information can be only transmitted at short range (< 100 m) in the forest. In regard to the size and the repartition of territories, this communication process appears efficient and adaptive.