Evolutionary rescue and the limits to phenotypic plasticity: testing theory in the field

快速的气候变化和栖息地的丧失将导致许多物种在本世纪灭绝，除非它们能够应对不断变化和更极端的生态条件。因此，了解是什么限制了物种的生态耐受性是一个至关重要的科学问题，因为它使我们能够预测正在进行的环境变化对种群和生态群落的影响。生物体应对环境变化的一种常见方式是“可塑性”，即直接根据当地环境改变它们的形态、生理或行为特征(表型)，而不需要快速的进化变化。这种“表型可塑性”缓冲了环境中的变化，可以在物种通常经历的各种环境中保持健康。目前，全球生物多样性对气候变化的响应大多归因于这种表型可塑性，而不是实际的进化变化，这突显了它在维持生态输出方面的重要性。然而，表型可塑性应对环境变化的能力是有限的。保持基因表达的变异不仅可能耗费巨大的能量，而且它的进化也只是为了保持一个物种在其最近过去经历的环境范围内的适应性。因此，在新奇或极端的条件下，一个物种的塑料反应没有理由仍然提高它们的生存和繁殖后代的能力。相反，在以前的环境中适应的塑料反应实际上可能会降低它们在新环境中的适应性。这一观点尤其令人担忧，因为它预测，随着生态变化的继续，可塑性将无法应对，当超过关键环境限制时，将导致人口突然下降。相比之下，其他理论模型预测，塑料反应将能够在新的环境中进化得更快，产生比实验室实验在普通花园条件下预测的更快的进化反应。我们将通过测量两种生态上不同的雷公藤(千里光属)对其海拔位置变化的可塑性反应来检验这些理论预测，这两种植物在西西里岛埃特纳山坡上的普遍分布内外都是如此。这些物种，千里光和菊花，在许多表型特征以及与适应不同海拔相关的关键基因的表达上存在差异。我们将把这两个物种的基因类型移植到一系列的田间条件中，并在两年的时间里监测它们的表现和可塑性，以确定每个基因对其正常(家)环境以外的条件的反应。我们将测量生长发育参数和繁殖参数，以衡量每个基因型的局部适合度，并测试随着海拔变化而预期的适合度下降被表型和基因表达的可塑性变化所抵消的程度。我们预测，尽管观察到的塑料反应将使个体在其物种通常的海拔条件范围内保持健康和生产力，但表型反应将不再适合海拔高度超过这些限制的变化。这一经验性发现将对预测物种对气候变化做出可塑性反应的持续能力具有重要意义。特别是，它将表明，当环境变化的速度超过其地理范围内的普遍条件时，为了防止种群和物种灭绝，快速进化将是必要的。

Rapid climate change and habitat loss will cause many species to become extinct this century unless they can cope with changing and more extreme ecological conditions. Understanding what limits species' ecological tolerances is therefore an issue of critical scientific importance because it allows us to predict the consequences of ongoing rates of environmental change to populations and therefore to ecological communities. A common way that organisms deal with environmental variation is to be 'plastic', i.e. to change their morphological, physiological or behavioural traits (their phenotypes) directly in response to their local environment, without requiring rapid evolutionary change. Such 'phenotypic plasticity' buffers changes in the environment, and can maintain fitness across the range of environments typically experienced by a species. Currently, most of the global responses of biodiversity to climate change have been ascribed to such phenotypic plasticity, rather than to actual evolutionary change, underlining its importance in maintaining ecological outputs.However, the ability of phenotypic plasticity to cope with environmental change has limits. Not only is maintaining variation in gene expression likely to be energetically expensive, it also evolves to maintain fitness only within the range of environments a species experiences in its recent past. In novel or extreme conditions, there is therefore no reason that a species' plastic responses will still improve their ability to survive and produce offspring. Instead, plastic responses that were adaptive in former environments may actually reduce their fitness in new environments. This idea is especially worrying because it predicts that plasticity will be unable to cope as ecological change continues, leading to sudden population declines as critical environmental limits are exceeded. By contrast, other theoretical models predict that plastic responses will be able to evolve more quickly in novel environments, generating faster evolutionary responses than predicted by laboratory experiments under common garden conditions. We will test these theoretical predictions by measuring the plastic responses of two ecologically divergent species of ragwort (genus Senecio) to changes in their altitudinal position, both within and outside their prevailing distributions on the slopes of Mount Etna, Sicily. These species, Senecio aethnensis and S. chrysanthemifolius differ in a number of phenotypic traits, as well as in the expression of key genes that are associated with adaptation to different altitudes. We will transplant genotypes of both species into a range of field conditions and monitor their performance and plasticity over a two-year period in order to determine each genotype's response to conditions outside its normal ('home') environment. We will measure growth and development parameters, and reproductive parameters as a measure of each genotype's local fitness, and test the degree to which the declines in fitness expected with changes in altitude are offset by plastic changes in their phenotype and in gene expression. We predict that although observed plastic responses will keep individuals healthy and productive under their species' usual range of altitudinal conditions, phenotypic responses will no longer be appropriate with altitudinal changes beyond these limits. Such an empirical finding will have important implications for predicting the continued ability of species to respond plastically to climate change. In particular, it will suggest, the rapid evolution will be necessary to prevent population and species' extinction where rates of environmental change exceed prevailing conditions within their geographical range.