Abstract
BackgroundSkewed body size distributions and the high relative richness of small-bodied taxa are a fundamental property of a wide range of animal clades. The evolutionary processes responsible for generating these distributions are well described in vertebrate model systems but have yet to be explored in detail for other major terrestrial clades. In this study, we explore the macro-evolutionary patterns of body size variation across families of Hexapoda (insects and their close relatives), using recent advances in phylogenetic understanding, with an aim to investigate the link between size and diversity within this ancient and highly diverse lineage.ResultsThe maximum, minimum and mean-log body lengths of hexapod families are all approximately log-normally distributed, consistent with previous studies at lower taxonomic levels, and contrasting with skewed distributions typical of vertebrate groups. After taking phylogeny and within-tip variation into account, we find no evidence for a negative relationship between diversification rate and body size, suggesting decoupling of the forces controlling these two traits. Likelihood-based modeling of the log-mean body size identifies distinct processes operating within Holometabola and Diptera compared with other hexapod groups, consistent with accelerating rates of size evolution within these clades, while as a whole, hexapod body size evolution is found to be dominated by neutral processes including significant phylogenetic conservatism.ConclusionsBased on our findings we suggest that the use of models derived from well-studied but atypical clades, such as vertebrates may lead to misleading conclusions when applied to other major terrestrial lineages. Our results indicate that within hexapods, and within the limits of current systematic and phylogenetic knowledge, insect diversification is generally unfettered by size-biased macro-evolutionary processes, and that these processes over large timescales tend to converge on apparently neutral evolutionary processes. We also identify limitations on available data within the clade and modeling approaches for the resolution of trees of higher taxa, the resolution of which may collectively enhance our understanding of this key component of terrestrial ecosystems.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0570-3) contains supplementary material, which is available to authorized users.
Highlights
Skewed body size distributions and the high relative richness of small-bodied taxa are a fundamental property of a wide range of animal clades
As Holometabola, the most diverse clade [32] account for the majority of the terminals included in this study (508 out of 775), it is unsurprising that the size distribution of Holometabola mirrors that of hexapods as a whole, with similar average size to the global mean (Hexapoda; mean = 1.946 ln, sd = 0.9491 ln, Holometabola; mean = 1.8032 ln, sd = 0.8078 ln)
When considering the processes that may underlie the evolution of hexapod body size, our analyses identify Holometabola and in particular Diptera, as having undergone divergent evolutionary processes when compared with the remaining Hexapoda
Summary
Skewed body size distributions and the high relative richness of small-bodied taxa are a fundamental property of a wide range of animal clades. Body size impacts on almost every major life history trait including: growth, parental investment, range size, dispersal and degree of host specificity (see [13,14,15], and references therein, for reviews of Hexapoda) Based on these observations a number of size-dependent mechanisms linked to clade diversification have been proposed (reviewed in [3, 4]). These include; hard limits on minimum size, which restrict random character change [16], energetic models emphasizing the relative efficiency of small body sizes in the production of offspring [11, 17], and fractal environmental models, exploring the capacity for small-bodied taxa to more finely subdivide a given environmental landscape [8]. Testing the predictions of these models, e.g. the presence of a relationship between clade richness and body size, as well as more generally exploring the processes that may underlie size evolution, requires that we extend our perspectives to encompass other major lineages that may show differences from our vertebrate model systems [20]
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