研究显示，蛇已经进化了一种对致命神经毒素的显著抗性，而且世界各地蛇的这种抗性都相类似。这一发现，大大地增加了适应遗传基础的了解，是了解适应局限性和进化程度的模型。
研究人员发现，在北美、中美、南美及亚洲，那里的蛇能以分泌致命神经毒素的两栖动物为食，这种神经毒素就是河豚毒素（tetrodotoxin ，TTX）。这些蛇的一个关键钠通道基因存在有类似的突变，这使蛇对TTX高度耐受。突变可以避免TTX阻断肌肉钠通道，否则就麻痹蛇的神经和肌肉组织，使蛇不能移动。
一个关键的发现就是：适应性进化受参与进化反应基因的功能特性所限制。虽然有许多可改善适应性的可能突变，在这种抗河豚毒素案例中，许多突变是有代价的，因为突变改变了基因的正常功能。所以，在观察独立适应河豚毒素的多个物种时，会看到一个非常相似且有限的突变。
这一研究结果发表在期刊Proceedings of the National Academy of Sciences上。
生物体如何通过适应进化来应对环境改变了？这种适应进化包含有什么样的遗传机制？这些适应性反应存在什么限制？自然界中许多生物都是我们了解这些的模型。最好的例子就是寄生虫如何获得耐药性。
为深入了解，需要对更多品种的蛇进行研究，并将这些研究推及大量的其他物种，包括捕食蝾螈毒卵的昆虫。这将拓宽对适应性出现的理解。（生物谷bioon.com）
doi:10.1073/pnas.1113468109
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Constraint shapes convergence in tetrodotoxin-resistant sodium channels of snakes
C. R. Feldman, E. D. Brodie, E. D. Brodie, M. E. Pfrender
Natural selection often produces convergent changes in unrelated lineages, but the degree to which such adaptations occur via predictable genetic paths is unknown. If only a limited subset of possible mutations is fixed in independent lineages, then it is clear that constraint in the production or function of molecular variants is an important determinant of adaptation. We demonstrate remarkably constrained convergence during the evolution of resistance to the lethal poison, tetrodotoxin, in six snake species representing three distinct lineages from around the globe. Resistance-conferring amino acid substitutions in a voltage-gated sodium channel, Nav1.4, are clustered in only two regions of the protein, and a majority of the replacements are confined to the same three positions. The observed changes represent only a small fraction of the experimentally validated mutations known to increase Nav1.4 resistance to tetrodotoxin. These results suggest that constraints resulting from functional tradeoffs between ion channel function and toxin resistance led to predictable patterns of evolutionary convergence at the molecular level. Our data are consistent with theoretical predictions and recent microcosm work that suggest a predictable path is followed during an adaptive walk along a mutational landscape, and that natural selection may be frequently constrained to produce similar genetic outcomes even when operating on independent lineages.
