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Lambert, Shea [1], Wiens, John J. [2].

Evolution of viviparity: a phylogenetic test of the cold-climate hypothesis in phrynosomatid lizards.

The evolution of viviparity is a key life-history transition in vertebrates, with many potential consequences for the morphology, physiology, behavior, and ecology of an organism. However, the selective forces favoring its evolution are not fully understood. With >100 origins of viviparity – more than twice that of all other vertebrate lineages combined – squamate reptiles (lizards and snakes) are ideal for addressing this issue. Some evidence from field and laboratory studies supports the “cold-climate” hypothesis for the evolution of viviparity, wherein behavioral thermoregulation by pregnant females provides an advantage in cold environments by maintaining higher temperatures for developing embryos, thereby speeding development and enhancing survival. Surprisingly, the cold-climate hypothesis has not been tested using both quantitative climatic data and phylogenetic comparative methods. Here, we investigate the evolution of viviparity in the lizard family Phrynosomatidae using GIS-based environmental data, an extensive phylogeny (117 species), and recently developed comparative methods. In agreement with previous studies, we find six independent origins of viviparity in Phrynosomatidae. Using phylogenetic logistic regression and Wright’s threshold model, we find significant relationships between viviparity and lower temperatures during the warmest (egg-laying) season, strongly supporting the cold-climate hypothesis. We also use ancestral reconstructions to help explain two patterns in Phrynosomatidae that seemingly contradict the cold-climate hypothesis: the presence of viviparous species restricted to low elevation tropical regions and the paucity of viviparous species at high latitudes. Ancestral reconstructions of environmental conditions suggest that viviparous species currently restricted to low-elevation, warm environments have adapted to these conditions relatively recently, and their ancestors evolved viviparity in cooler high-elevation environments. Remarkably, we also find that viviparity tends to originate more frequently at tropical latitudes, despite its association with cooler climates, and that the few viviparous species seen at high latitudes represent northward range expansion following origins of viviparity at lower latitudes. We suggest that the tendency for viviparity to evolve at lower latitudes may be connected to patterns of seasonal temperature variation on tropical vs. temperate mountains. Specifically, lower temperature seasonality on tropical mountains might favor the evolution of cooler climate specialists at high elevations, which are able to evolve viviparity in the absence of gene flow from populations adapted to lower-elevation, warmer climates. We also find evidence that viviparous clades have higher speciation rates, which may also be related to their primarily tropical montane distributions. Finally, we consider the possible effects of climate change on oviparous vs. viviparous species.

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1 - University of Arizona, Ecology and Evolutionary Biology, 136 Biological Science West, 1041 E Lowell St., Tucson, AZ, 85721, USA
2 - University of Arizona, Department of Ecology and Evolutionary Biology, Tucson, AZ, 85721-0088, USA

Life history
phylogenetic comparative methods

Presentation Type: Regular Oral Presentation
Session: 27
Location: Cotton B/Snowbird Center
Date: Saturday, June 22nd, 2013
Time: 2:00 PM
Number: 27003
Abstract ID:1226
Candidate for Awards:W.D. Hamilton Award for Outstanding Student Presentation

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