Principal components analysis was used to determine the degree of covariance among life-history traits in eight populations of the amphipod Hyalella azteca and four populations of the crayfish Orconectes virilis inhabiting Canadian boreal lakes. Assumptions of tightly coupled traits which vary unidimensionally as a single fixed unit or "tactic" are untenable for these organisms. Instead, populations are characterized by an independence among demographic traits which contradict some established beliefs in life-history trade-offs. The degree to which organisms display traits which can vary independently will influence the resulting phenotypic flexibility of life history responses to varying environmental conditions and may dictate ranges of latitudinal distribution.
Key words: Amphipod Hyalella azteca, crayfish Orconectes virilis, life-history traits, independent variation.
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