Tuesday, September 11, 2012

On the evolution of large insects

Posted by Erik Svensson

Our last lab-meeting contained an interesting discussion about the evolutionary significance of large body size in insects, stimulated by the excellent talk by Yuma Takahashi about his ongoing research on Ischnura-damselflies. I thought we should continue on the theme of body size evolution and its drivers in insects, by reading two recent papers that should hopefully be entertaining and interesting.

Both papers discuss the rise and fall of large insects, such as gigantic dragonflies during the Carboniferous Period, and the biotic and abiotic factors driving selection on both body size and wing size. Among the most discussed (but also controversial) idéas is that atmospheric oxygen levels might have been important, but predation has also been suggested to play a role.

Time and place of lab-meeting as usual: "Argumentet" (2nd floor, Ecology Building) at 10.30 on Tuesday September 18 2012. 

Below, you will find the title of the papers and Abstracts and links that should allow you to download the paper if you are on the Lund University network. You can also download them here and here. You might also be interested in the short comment on the latter paper by Steven Chown, which you can download here. 

Environmental and biotic controls on the evolutionary history of insect body size
Author(s): Clapham, ME (Clapham, Matthew E.)1Karr, JA (Karr, Jered A.)1

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA  Volume: 109   Issue: 27   Pages: 10927-10930   DOI:10.1073/pnas.1204026109   Published: JUL 3 2012
Abstract: Giant insects, with wingspans as large as 70 cm, ruled the Carboniferous and Permian skies. Gigantism has been linked to hyperoxic conditions because oxygen concentration is a key physiological control on body size, particularly in groups like flying insects that have high metabolic oxygen demands. Here we show, using a dataset of more than 10,500 fossil insect wing lengths, that size tracked atmospheric oxygen concentrations only for the first 150 Myr of insect evolution. The data are best explained by a model relating maximum size to atmospheric environmental oxygen concentration (pO(2)) until the end of the Jurassic, and then at constant sizes, independent of oxygen fluctuations, during the Cretaceous and, at a smaller size, the Cenozoic. Maximum insect size decreased even as atmospheric pO(2) rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight. A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds. The decoupling of insect size and atmospheric pO(2) coincident with the radiation of birds suggests that biotic interactions, such as predation and competition, superseded oxygen as the most important constraint on maximum body size of the largest insects.

Atmospheric oxygen level and the evolution of insect body size
Harrison, JF (Harrison, Jon F.)1Kaiser, A (Kaiser, Alexander)2VandenBrooks, JM (VandenBrooks, John M.)1

PROCEEDINGS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES  Volume: 277   Issue: 1690   Pages: 1937-1946   DOI: 10.1098/rspb.2010.0001   Published:JUL 7 2010
Abstract: Insects are small relative to vertebrates, possibly owing to limitations or costs associated with their blind-ended tracheal respiratory system. The giant insects of the late Palaeozoic occurred when atmospheric PO(2) (aPO(2)) was hyperoxic, supporting a role for oxygen in the evolution of insect body size. The paucity of the insect fossil record and the complex interactions between atmospheric oxygen level, organisms and their communities makes it impossible to definitively accept or reject the historical oxygen-size link, and multiple alternative hypotheses exist. However, a variety of recent empirical findings support a link between oxygen and insect size, including: (i) most insects develop smaller body sizes in hypoxia, and some develop and evolve larger sizes in hyperoxia; (ii) insects developmentally and evolutionarily reduce their proportional investment in the tracheal system when living in higher aPO(2), suggesting that there are significant costs associated with tracheal system structure and function; and (iii) larger insects invest more of their body in the tracheal system, potentially leading to greater effects of aPO(2) on larger insects. Together, these provide a wealth of plausible mechanisms by which tracheal oxygen delivery may be centrally involved in setting the relatively small size of insects and for hyperoxia-enabled Palaeozoic gigantism.

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