Data from: Body size evolution in an old insect order: no evidence for Cope’s Rule in spite of fitness benefits of large size

We integrate field data and phylogenetic comparative analyses to investigate causes of body size evolution and stasis in an old insect order: odonates (“dragonflies and damselflies”). Fossil evidence for “Cope’s Rule” in odonates is weak or non-existent since the last major extinction event 65 million years ago, yet selection studies show consistent positive selection for increased body size among adults. In particular, we find that large males in natural populations of the banded demoiselle (Calopteryx splendens) over several generations have consistent fitness benefits both in terms of survival and mating success. Additionally, there was no evidence for stabilizing or conflicting selection between fitness components within the adult life-stage. This lack of stabilizing selection during the adult life-stage was independently supported by a literature survey on different male and female fitness components from several odonate species. We did detect several significant body size shifts among extant taxa using comparative methods and a large new molecular phylogeny for odonates . We suggest that the lack of Cope’s rule in odonates results from conflicting selection between fitness advantages of large adult size and costs of long larval development. We also discuss competing explanations for body size stasis in this insect group.

本研究整合野外调查数据与系统发育比较分析方法，探究古老昆虫类群——蜻蛉目（Odonata，俗称蜻蜓与豆娘）的体型演化与停滞格局的成因。自6500万年前的末次大灭绝事件以来，蜻蛉目中支持科普法则（Cope's Rule）的化石证据十分薄弱，甚至完全缺失；但相关选择研究却显示，成虫体型增大始终受到正向选择作用。具体而言，我们在多代斑翅豆娘（Calopteryx splendens）的自然种群中发现，体型较大的雄性个体始终能获得更高的适合度收益，体现在存活率与交配成功率两方面。此外，成虫阶段内，不同适合度组分之间未发现稳定选择或拮抗选择的证据。针对多种蜻蛉类群雌雄个体不同适合度组分的文献调研，也独立验证了成虫阶段缺乏稳定选择这一结论。我们依托全新的大型蜻蛉目分子系统发育树与比较分析方法，在现生类群中检测到了多起显著的体型演化偏移事件。我们推测，蜻蛉目缺乏科普法则的现象，源于成虫体型增大带来的适合度优势，与幼虫期漫长发育所付出的代价之间存在拮抗选择。本研究同时探讨了该昆虫类群体型停滞格局的其他竞争性解释。