Discussion
Darwin’s Legacy:
The Forms, Function and Sexual Diversity of Flowers
Spencer C.H. Barrett
Department of Ecology and Evolutionary Biology, University of Toronto,
25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada
barrett@eeb.utoronto.ca
RYMER, P.D. (Biological Sciences, Imperial College, London):
Darwin’s theory of natural selection shows a mechanism for speciation through the
survival of the fittest, but it also states that selection is weak and speciation only
happens very slowly over long periods of time. Your example of Sagittaria latifolia
shows variation in the sexual system with two mechanisms (hybridization and male
variation). Does this provide good evidence for rapid speciation?
Response BARRETT, S.C.H. (Ecology & Evolutionary Biology, University of
Toronto)
Populations of Sagittaria latifolia can be either monoecious or dioecious and
controlled crosses of individuals from these populations demonstrate that plants are
fully inter-fertile. In parts of the range (e.g. northern limit) populations contain
mixtures of females, males and hermaphrodites and molecular evidence indicates
hybridization between the two sexual systems. Given these facts I am reluctant to
suggest that sexual system differentiation is associated with rapid speciation as I am
not convinced that populations of the two sexual systems represent two species.
Nevertheless, we have shown that the genetics of sex determination in S. latifolia is
relatively simply inherited [Dorken, M.E. & S.C.H. Barrett. (2004). Sex determination and
the evolution of dioecy from monoecy in Sagittaria latifolia (Alismataceae). Proceeding of
the Royal Society London Ser. B. 271: 213-219] and I certainly would be prepared to
accept that evolutionary transitions between sexual systems in Sagittaria could be
quite rapid and easily achieved given the appropriate selection pressures for combined
versus separate sexes.