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S pecies often occupy a re-stricted geographic range even when there are no geographic barriers limiting further expansion. To understand why such borders exist, two questions need to be addressed. First, what factors and traits are responsible for borders at the ecological level? Second, if a border is associated with a specific trait, what prevents the trait from evolving so that an organism can expand its range? Answers to the first question have been obtained in several ecological studies. Traits responsible for borders have been identified in a few cases, particularly for borders associated with abiotic stresses. In contrast, few