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Published in final edited form as: Trends Ecol Evol. 2014 Apr 11;29(5):252–259. doi: 10.1016/j.tree.2014.03.006
© 2014 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).