Phenotypic selection on floral traits in divergent populations of the moth-pollinated orchid Platanthera bifolia

• Among-population differences in flower morphology is commonly attributed to divergent natural selection mediated by pollinators, but comparisons of current selection in populations growing in contrasting environments are scarce. In the moth-pollinated orchid Platanthera bifolia, floral morphology and flowering phenology differ between grassland and woodland populations on the island Öland, SE Sweden. To examine whether among-population variation in floral morphology is consistent with differences in current selection, we documented phenotypic selection through female and male function on flowering phenology, plant height, flower production and spur length in three populations. One grassland and one woodland population were studied in three years, and one population bimodal for spur length and occurring in a mixed habitat was studied in one year. There was evidence of disruptive selection on spur length in the population in the mixed habitat. In one of three years, there was evidence of divergent selection on plant height between the grassland and woodland population.  In all three years selection favoured longer nectar spurs; selection gradients were statistically significant in the short-spurred grassland population, but not in the long-spurred woodland population. In both populations, there was directional selection for earlier flowering and more flowers through female function. Seed production was pollen limited in the grassland population. The results support the hypothesis that disruptive selection through female reproductive success contributes to the maintenance of a bimodal distribution of spur length in P. bifolia in mixed habitats, but provides very limited evidence of divergent selection on plant morphology and flowering phenology in grassland and woodland populations of P. bifolia. The results are discussed in relation to the evolution of floral traits and the challenges associated with detecting stabilizing selection in natural populations.

Subject headings

NATURVETENSKAP  -- Biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences (hsv//eng)

Keyword

Disruptive selection

divergent selection

flower morphology

flowering phenology

selection analysis

seed output

pollen removal

Biology

Biologi

Publication and Content Type

vet (subject category)

ovr (subject category)