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

Abstract

Many populations are composed of a mixture of individuals that reproduce at different times, and these times are often heritable. Under these conditions, gene flow should be limited between early and late reproducers, even within populations having a unimodal temporal distribution of reproductive activity. This temporal restriction on gene flow might be called 'isolation by time' (IBT) to acknowledge its analogy with isolation by distance (IBD). IBD and IBT are not exactly equivalent, however, owing to differences between dispersal in space and dispersal in time. We review empirical studies of natural populations that provide evidence for IBT based on heritabilities of reproductive time and on molecular genetic differences associated with reproductive time. When IBT is present, variation in selection through the reproductive season may lead to adaptive temporal variation in phenotypic traits [adaptation by time (ABT)]. We introduce a novel theoretical model that shows how ABT increases as (i) selection on the trait increases; (ii) environmental influences on reproductive time decrease; (iii) the heritability of reproductive time increases; and (iv) the temporal distribution of reproductive activity becomes increasingly uniform. We then review empirical studies of natural populations that provide evidence for ABT by documenting adaptive temporal clines in phenotypic traits. The best evidence for IBT and ABT currently comes from salmonid fishes and flowering plants, but we expect that future work will show these processes are more widespread.

Signatures of selection in mammalian clock genes with coding trinucleotide repeats: Implications for studying the genomics of high‐pace adaptation

“…Further, in the deer mouse, deviations from HWE and an excess of observed homozygotes at the most prevalent PER1 alleles in both large‐ and small‐scale analyses of sites within and surrounding Algonquin Provincial Park suggest disruptive selection in this area, where small‐scale changes in environmental features (e.g., microhabitats) may drive the selection of particular alleles in slightly different environments. These results would be predicted if there were mice with predispositions to breeding at different times of the year, essentially “isolation by time” (Hendry & Day, 2005). Pairwise estimates of genetic differentiation at our set of neutral microsatellites and the PER1 locus showed contrasting patterns for white‐footed and deer mouse, indicating that diverse processes may be influencing the two closely related species differently.…”

Rapid genetic divergence in response to 15 years of simulated climate change

“…Finally, recent studies of floral phenology suggest substantial overlaps in flowering time between subpopulations occupying different climate treatments at BCCIL (S. Buckland, unpublished data). Thus, we have no reason to believe that any barrier to gene flow has arisen through asynchrony in floral timing, which could otherwise have facilitated genetic drift (Fox, 2003; Hendry & Day, 2005). We conclude that it is extremely unlikely that climatic subpopulations at BCCIL have become demographically isolated from each other.…”

Incipient allochronic speciation due to non-selective assortative mating by flowering time, mutation and genetic drift

“…With baseline parameter values, the duration of clusters was increased up to 200 per cent when including days with single open flowers in the disjunctions between clusters. Furthermore, our model did not allow the joint evolution of other isolating mechanisms, although some can occur within the duration of the clusters we observed ( Turelli et al 2001;Ramsey et al 2003;Primack et al 2004;Hendry & Day 2005;Savolainen et al 2006;Franks et al 2007). For example, temporal isolation can arise from pollinator shifts or preferences for specific floral traits (Coyne & Orr 2004;Johnson 2006) or coevolution of pollinators and plants (Bhattacharyay & Drossel 2005;Sargent & Otto 2006).…”