Abstract
The climatic cycles of the Quaternary, during which global mean annual temperatures have regularly changed by 5–10°C, provide a special opportunity for studying the rate, magnitude, and effects of geographic responses to changing climates. During the Quaternary, high- and mid-latitude species were extirpated from regions that were covered by ice or otherwise became unsuitable, persisting in refugial retreats where the environment was compatible with their tolerances. In this study we combine modern geographic range data, phylogeny, Pleistocene paleoclimatic models, and isotopic records of changes in global mean annual temperature, to produce a temporally continuous model of geographic changes in potential habitat for 59 species of North American turtles over the past 320 Ka (three full glacial-interglacial cycles). These paleophylogeographic models indicate the areas where past climates were compatible with the modern ranges of the species and serve as hypotheses for how their geographic ranges would have changed in response to Quaternary climate cycles. We test these hypotheses against physiological, genetic, taxonomic and fossil evidence, and we then use them to measure the effects of Quaternary climate cycles on species distributions. Patterns of range expansion, contraction, and fragmentation in the models are strongly congruent with (i) phylogeographic differentiation; (ii) morphological variation; (iii) physiological tolerances; and (iv) intraspecific genetic variability. Modern species with significant interspecific differentiation have geographic ranges that strongly fluctuated and repeatedly fragmented throughout the Quaternary. Modern species with low genetic diversity have geographic distributions that were highly variable and at times exceedingly small in the past. Our results reveal the potential for paleophylogeographic models to (i) reconstruct past geographic range modifications, (ii) identify geographic processes that result in genetic bottlenecks; and (iii) predict threats due to anthropogenic climate change in the future.
Highlights
Quaternary glacial-interglacial cycles have caused dramatic range shifts of species’ distributions [1,2,3,4,5]
Range fragmentation has led to allopatric genetic differentiation, and range expansions have led to the formation of tension hybrid zones, processes that lead to genetic diversity [8,12
Many studies have addressed geographic range tracking in relation to past and future climates [20,21,22,23,24,25], but only a few have attempted to evaluate the rates and magnitudes by which species respond by adapting to climate change (e.g. [26,27,28,29])
Summary
Quaternary glacial-interglacial cycles have caused dramatic range shifts of species’ distributions [1,2,3,4,5]. Many species in highand mid-latitudes have been extirpated from northern and high altitude areas, often accompanied by range fragmentation and persistence in so-called glacial refugia [6,7,8,9]. These geographic processes have led to phylogeographic differentiation at the population, subspecies and species levels in both plants and animals [10,11]. Many studies have addressed geographic range tracking in relation to past and future climates [20,21,22,23,24,25], but only a few have attempted to evaluate the rates and magnitudes by which species respond by adapting to climate change This area is in urgent need of investigation because it is highly relevant to assessing the impact of rapidly changing climate
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