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ScienceWeek
EVOLUTION: ON MIMICRY
The following points are made by G.D. Ruxton and M.P. Speed (Nature 2005 433:205):
1) Charles Darwin (1809-1882) saw mimicry -- strong visual resemblances between unrelated species -- as an excellent test case for his theories of natural selection[1]. The phenomenon continues to exercise evolutionary biologists today, with the latest salvo coming from Skelhorn and Rowe[2], who find that mimicry can work in an unexpected way to provide safeguards against predators.
2) Mimicry generally occurs in two forms, Batesian and Müllerian. Batesian mimicry is essentially parasitic: a prey species evolves to look like a species that is unattractive to predators (because it is poisonous, for example), and in so doing degrades the effectiveness of the signals used by the inedible species to warn off predators. By contrast, Müllerian mimicry involves two unpalatable species, and is thought to be mutualistic because the two species share the mortality costs incurred when naive predators sample them before learning to avoid the warning signal they both use[3]. Butterflies are among the commonest examples of Müllerian mimicry.
3) Müllerian mimicry has become highly contentious, with a body of mainly theoretical work arguing that systems identified previously as Müllerian might in fact be parasitic rather than mutualistic[4,5]. But this conclusion will need to be re-examined in light of the ingenious experiments reported by Skelhorn and Rowe[2]. An often implicit assumption of previous work on Müllerian mimicry has been that the two species are unappealing to predators because they have the same defense -- the same toxin, say. However, there is no logical or observational foundation for this supposition. Skelhorn and Rowe[2] demonstrate that Müllerian mimicry can provide highly effective protection from predation when the two species concerned have different defenses.
4) The Skelhorn-Rowe experiments involved giving domestic chicks colored food crumbs flavored with aversive chemicals. All the crumbs were colored identically, but some were flavored with quinine, some with an equally bitter solution of Bitrex (a preparation designed to discourage people from biting their nails) and some with a blend of the two. All flavorings produced a learned aversion after repeated exposure. Controlling for the density of crumbs, Skelhorn and Rowe found that chicks learned to avoid colored crumbs more quickly when given heterogeneous populations of both quinine crumbs and Bitrex crumbs than when given homogeneous populations of all quinine, all Bitrex or all blended crumbs. Furthermore, and of equal interest, chicks retained the learned aversion for longer when trained with the heterogeneous population than with any homogeneous population.
References (abridged):
1. Darwin, C. The Life and Letters of Charles Darwin (ed. Darwin, F.) (Murray, London, 1887)
2. Skelhorn, J. & Rowe, C. Proc. R. Soc. Lond. B doi:10.1098/rspb.2004.2953 (2005).
3. Müller, F. Zool. Anz. 1, 54-55 (1878).
4. Speed, M. P. Anim. Behav. 46, 1246-1248 (1993).
5. Kokko, H., Mappes, J. & Lindstrom, L. Ecol. Lett. 6, 1068-1076 (2003)
Nature http://www.nature.com/nature
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Related Material:
MIMICRY IN BACTERIAL PATHOGENS
The following points are made by C.E. Stebbins and J.E. Galan (Nature 2001 412:701):
1) The pressures of survival have engendered a wide spectrum of adaptations in organisms, and various organisms have evolved sophisticated methods to exploit both the surrounding environment and each other. An important mechanism that frequently occurs in adaptation is that of mimicry: many organisms both large and small have found a selective advantage in imitating the appearance or function associated with an otherwise distinct creature or aspect of the natural environment.
2) In many of these cases, organisms imitate or copy the appearance of something else, either for the purpose of concealment (e.g., the African praying mantis and chameleons) or to send a message (e.g., coloration in non-poisonous species to mimic poisonous species). Some of the most interesting examples of mimicry, however, may occur in the microbial world. Recent studies have revealed that many bacterial pathogens mimic the function of host proteins in order to manipulate host physiology and cellular functions for the benefit of the microbe. This is in contrast with strategies used by some pathogens that involve microbial products with activities lacking clear counterparts in eukaryotic cells.
3) In particular, recent structural work has provided unique insights into the mechanisms of host mimicry by bacterial virulence factors. In some cases, these factors are homologous to host proteins whose genes have been incorporated into the genome of the pathogen and subverted for its benefit. In other cases, convergent evolution has produced new effectors that have no obvious relationship to host factors. However, although hidden at the sequence level, determination of the crystal structures of several bacterial factors and bacterial-host protein complexes has revealed the presence of mimicry at the molecular level, and examination of such factors is providing important insights into the interplay between host and pathogen, into the mechanisms underlying eukaryotic functional homologues, and into the nature of the evolutionary dynamics shaping these complex ecologies.
Nature http://www.nature.com/nature
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