Distance for mating between individuals in a population can be considered both geographically and temporally if individuals are variable in their reproductive schedules.
The timing of key life history events, such as migration, breeding and flowering, can greatly influence the mating structure of a population. Considering a flowering plant population whose individuals are genetically variable in flowering time, the composition of the mating pool for a given individual will not only depend on the frequency of each genotypic class in the population but also the flowering schedule of each class. Assuming high heritability for flowering time, we can expect the phenotypic correlation between mates, i.e., the degree of temporal assortment, to increase as genotypic flowering schedules become increasingly disparate. Conversely, as flowering schedules converge or as a particular genotypic class becomes increasingly common in the population, we can expect the degree of assortment to decrease.
In addition to influencing the mating structure a population, differences in reproductive timing have been found to covary with a number of phenotypic traits for flowering plants, such as flower number above-ground biomass and height at flowering. Assuming a genetic basis for such traits, phenotypic differences in reproductive time will restrict gene flow, essentially creating subpopulations between which the accumulation of genetic variation due to either selection or drift may occur.
This reproductive isolation due to phenology is termed ‘isolation-by-time’ (IBT), appropriately named with reference to ‘isolation-by-distance’. IBD is conceptually similar to IBT, as gene flow will be restricted when dispersal is limited, allowing for genetic variation to accumulate. Because flowering plants are sessile and can demonstrate variable flowering schedules, both IBD and IBT may influence mating structure in flowering plant populations.
Previous work by Hendry and Day (2005) modelled isolation-by-time (IBT) for an asexual population and considered two traits: reproductive time and body size, a trait they consider to have variable fitness along a temporal cline. While their work is a foundation to our own, their results reflect weak selection and perfect heritability for body size, which, coupled with asexual reproduction, have limited application to real-life population dynamics. To expand upon their investigation, we are modelling a sexually reproducing flowering plant population that can be manipulated for a number of parameters related to IBT and isolation-by-distance (IBD). With this model, we hope to answer:
- Does IBT between early and late flowering plants amplify temporal genetic structure for the flowering time trait by building linkage disequilibrium among alleles with similar phenotypic effect, i.e., if a plant has an ‘early’ allele at one flowering time locus, is it more likely than by chance to have ‘early’ alleles at the other flowering time loci?
- Does the temporal genetic structure for flowering time interact with limited pollen and seed dispersal to create spatial population genetic structure for flowering time, i.e., over time, do early and late plants cluster over the landscape more tightly than by chance?
- Does the temporal genetic structure amplify linkage disequilibrium among neutral loci and alter the impact of genetic drift?
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