Selfing rate variation within species is unrelated to life-history traits or geographic range position

Premise: In plants, populations and species vary widely along the
continuum from outcrossing to selfing. Life-history traits and ecological
circumstances influence among-species variation in selfing rates but their
general role in explaining intraspecific variation is unknown. Using a
database of plant species, we test whether life-history traits, geographic
range position, or abundance predict selfing rate variation among
populations.  Methods: We identified species where selfing rates
were estimated in at least three populations at known locations. Two key
life-history traits (generation time and growth form) were used to predict
within-species selfing rate variation. Populations sampled within a
species’ native range were assessed for proximity to the nearest edge and
abundance. Finally, we conducted linear and segmented regressions to
determine functional relationships between selfing rate and geographic
range position within species. Key results: While woody species exhibit
lower variation in selfing rates compared to herbs, this is explained by
the lower average selfing rate of woody species. Relationships between
selfing and peripherality or abundance significantly varied among species
in their direction and magnitude. However, there was no general pattern of
increased selfing towards range edges. A power analysis shows that tests
of this hypothesis require studying many (i.e. 40+) populations.
Conclusions: Intraspecific variation in plant mating systems is often
substantial yet remains difficult to explain. Beyond sampling more
populations, future tests of biogeographic hypotheses will benefit from
phylogeographic information concerning specific range edges, the study of
traits influencing mating system (e.g. herkogamy), and measures of
abundance at local scales (e.g. population density).