Table 1.

Evolutionary hypotheses to account for limits to species ranges and predictions (based partly on Hoffmann & Parsons 1997).

	predictions
hypotheses	low ‘neutral’ genetic variation	low h2 in traits limiting borders	clinal variation towards border	genetic interactions restricting evolution
low heritability in traits limiting borders due to directional selection at borders		+	+
low overall genetic variation in border populations	+	+
low heritability in traits limiting borders due to physiological constraints/genetic decay		+
asymmetric gene flow from the central to border populations			+
adaptation restricted by trait interactions				+
changes in independent characters required for adaptation beyond the border			+