Population structure attributable to reproductive time: isolation by time and adaptation by time

Area of Science:

• Evolutionary Biology
• Population Genetics
• Ecology

Background:

• Many populations exhibit temporal variation in reproduction, with reproductive timing often being heritable.
• Gene flow can be restricted between individuals reproducing at different times, analogous to spatial isolation by distance.
• This temporal restriction is termed isolation by time (IBT).

Purpose of the Study:

• To review empirical evidence for isolation by time (IBT) and adaptation by time (ABT) in natural populations.
• To present a theoretical model explaining the drivers of adaptation by time (ABT).

Main Methods:

• Review of empirical studies examining heritability of reproductive time and molecular genetic data.
• Development of a theoretical model to predict conditions favoring adaptation by time (ABT).

Main Results:

• Empirical evidence supports IBT based on heritability and genetic differentiation.
• The theoretical model indicates ABT increases with stronger selection, lower environmental influence on timing, higher heritability, and more uniform reproductive distributions.
• Empirical studies show adaptive temporal clines in phenotypic traits, supporting ABT.

Conclusions:

• Isolation by time (IBT) is a significant factor limiting gene flow in temporally structured populations.
• Adaptation by time (ABT) is driven by seasonal selection acting on heritable variation in reproductive timing.
• Salmonid fishes and flowering plants provide strong current evidence, with potential for broader applicability across taxa.