Out of the tropics: Macroevolutionary size trends in an old insect order are shaped by temperature and predators

Global body size distributions are shaped by selection pressures arising from biotic and abiotic factors such as temperature, predation and parasitism. Here, we investigated the ecological and evolutionary drivers of global latitudinal size gradients in an old insect order (Odonata; dragonflies and damselflies). Phylogenetic comparative analyses revealed that global size variation of extant taxa is negatively influenced by both regional avian diversity and temperature. Interestingly, fossil data show that the relationship between wing size and latitude has shifted: latitudinal size trends had initially negative slopes but became shallower or positive following the emergence of birds 150 MYA. These changing size-latitude trends over geological time were likely driven by bird predation and high dispersal ability of large dragonflies. Our results therefore suggest that latitudinal size gradients were shaped by temperature but also by predators driving the dispersal of large-sized clades out of the tropics and in to the temperate zone.

Methods
The phylogenetic tree that we use in this study was constructed from odonate DNA-sequences downloaded from GenBank, using a traditional morphological taxonomy as our backbone (Waller and Svensson 2017). Our phylogeny comprised 1322 taxa, about a fifth of all known extant odonate species (about 6400 in total) (Schorr and Paulson 2015; Waller et al. 2019). Further and more detailed information about phenotypic data collection and phylogeny reconstruction is provided elsewhere (Waller and Svensson 2017; Waller et al. 2019) and  from our open online resource “The Odonate Phenotypic Database”: http://www.odonatephenotypicdatabase.org/shiny/odonates/.

Size data (wing lengths) from extinct odonate fossil taxa were obtained from Clapham and Karr (2012). Although we use our previously published Odonata phylogeny in the present study (Waller and Svensson 2017) and we have published data on interspecific body size variation from the Odonate Phenotypic Database (Waller et al. 2019), all of the analyses in the present paper are new and have not been published elsewhere. Thus, there is no overlap between the results in the present study and our previous work.

全球体型分布由生物与非生物因素（如温度、捕食与寄生）带来的选择压力所塑造。本研究针对古老昆虫类群蜻蜓目（Odonata，涵盖蜻蜓与豆娘），探究了全球纬度体型梯度的生态与演化驱动因子。系统发育比较分析（phylogenetic comparative analyses）显示，现存类群的全球体型变异同时受到区域鸟类多样性与温度的负面影响。值得注意的是，化石数据表明翅长与纬度的关系已发生改变：纬度体型趋势最初呈负斜率，但在1.5亿年前鸟类出现后，其斜率逐渐变缓甚至转为正值。这种地质时间尺度下体型-纬度趋势的变化，可能由鸟类捕食以及大型蜻蜓较强的扩散能力驱动。本研究结果表明，纬度体型梯度不仅受温度塑造，还受到捕食者的影响——捕食者推动大型演化支从热带向温带区域扩散。

方法
本研究使用的系统发育树基于从基因库（GenBank）下载的蜻蜓目DNA序列构建，以传统形态分类学作为构建框架（Waller与Svensson, 2017）。该系统发育树包含1322个类群，约占已知现存蜻蜓目物种总数（约6400种）的五分之一（Schorr与Paulson, 2015; Waller等, 2019）。关于表型数据采集与系统发育重建的更多详细信息，可参见此前发表的研究（Waller与Svensson, 2017; Waller等, 2019）以及本研究的开放在线资源"The Odonate Phenotypic Database"：http://www.odonatephenotypicdatabase.org/shiny/odonates/。

已灭绝蜻蜓目化石类群的体型数据（翅长）取自Clapham与Karr (2012)的研究。尽管本研究使用了此前发表的蜻蜓目系统发育树（Waller与Svensson, 2017），且已通过蜻蜓目表型数据库发布了种间体型变异的相关数据（Waller等, 2019），但本文中的所有分析均为全新内容，尚未在其他出版物中发表。因此，本研究的结果与此前的工作不存在重叠。