Evolution of intraspecific floral variation in a generalist-specialist pollination system

Intraspecific processes impact macroevolutionary patterns through
individual variation, selection, and ecological specialisation.
According to the niche variation hypothesis, the broader ecological niche
of generalist species has been proposed to increase in variation among
individuals, either because they are constituted of diversified
specialised individuals each exploiting a fraction of the species’ niche,
or because they are constituted of true generalist individuals that
experience relaxed selection. To test this hypothesis, we
surveyed the individual floral morphology of species of Antillean
Gesneriaceae, a group that has transitioned between
specialisation for hummingbird pollination and generalisation
multiple times throughout its evolutionary history. We
characterised the profiles of corollas using geometric morphometrics and
compared the intraspecific shape variance of specialists and generalists
in a phylogenetic context. We used three approaches that differently
accounted for the high dimensionality of morphological traits, the
ancestral reconstruction of pollination syndromes over time, and the error
associated with the estimation of the intraspecific variance. Our
findings provide partial support for the niche
variation hypothesis. If considering the whole shape in the
analysis corroborated this idea, decomposing the shape into
principal components indicated that not all aspects of the corolla exhibit
the same pattern of variation. Specifically, pollination generalists tend
to display greater intraspecific variation than specialists in
terms of tubularity, but not of curvature. Accounting for the error in the
estimation of the variance also reduced the support for the
hypothesis, suggesting that larger sample sizes may be required to reach
stronger conclusions. This study emphasises the reciprocal influence
between plants and their pollinators on floral morphology at
different biodiversity scales, by affecting intraspecific floral variation
that can result in macroevolutionary patterns.