Résumé
Chaotic genetic patchiness denotes unexpected patterns of genetic
differentiation that are observed at a fine scale and are not stable in
time. These patterns have been described in marine species with
free-living larvae, but are unexpected because they occur at a scale below
the dispersal range of pelagic larvae. At the scale where most larvae are
immigrants, theory predicts spatially homogeneous, temporally stable
genetic variation. Empirical studies have suggested that genetic drift
interacts with complex dispersal patterns to create chaotic genetic
patchiness. Here we use a coancestry model and individual-based
simulations to test this idea. We found that chaotic genetic patterns
(qualified by global FST and spatio-temporal variation in FST's
between pairs of samples) arise from the combined effects of (1) genetic
drift created by the small local effective population sizes of the sessile
phase and variance in contribution among breeding groups and (2)
collective dispersal of related individuals in the larval phase.
Simulations show that patchiness levels qualitatively comparable to
empirical results can be produced by a combination of strong variance in
reproductive success and mild collective dispersal. These results call for
empirical studies of the effective number of breeders producing larval
cohorts, and population genetics at the larval stage.