Pre-conditions for selection
It would also be important to conduct studies that evaluate whether the
pre-conditions for HP receipt to exert natural selection on traits that
minimize HP effects in natural communities are met (i.e. opportunity for
selection). One such condition for instance would be that HP transfer
dynamics are relative stable over time (Ashman & Arceo-Gómez 2013). If
HP transfer dynamics are highly stochastic over the years, or over
shorter periods of time, this would strongly limit the opportunity for
selection on traits that minimize HP effects in a population. To my
knowledge, the only study to date that has evaluated temporal stability
in patterns of HP receipt has found good support for this pre-condition
(Fang et al. 2019). Specifically, they showed constant patterns of HP
receipt (HP load size and diversity) in up to 34 insect-pollinated
species over three consecutive years. This result suggests that HP
receipt may not vary stochastically over time and that HP tolerance or
avoidance strategies could evolve in such communities (Fang et al.
2019). However, further research is necessary to determine whether this
is the case for other communities or if patterns of HP remain constant
over longer periods of time.
Another important test of the pre-conditions for natural selection would
be to evaluate how variance in HP receipt is structured spatially at
different hierarchical levels of biological organization (populations,
plants and individual flowers). For instance, from an evolutionary
perspective we can expect that the fraction of total variance that is
accounted for by within and among population differences in HP receipt
would be more directly related to the potential for natural selection to
act on traits that minimize HP effects (see Herrera 2002, Arceo-Gómez et
al. 2016c). On the other hand, a higher degree of within-plant
variability will greatly reduce the opportunity for selection on such
traits (Herrera 2002), as flowers within the same plant will experience
very different HP transfer regimes. Although a few studies have
evaluated how variance in conspecific pollen deposition is partitioned
among biological levels of organization (e.g. Herrera 2002, Arceo-Gómez
et al. 2016c), to my knowledge no study has evaluated how much of the
variance in HP receipt is accounted for by within- versus among-plant,
and among-population differences. Studies that partition the variability
in HP receipt at the scale of populations and below (individual plants
and flowers) are needed in order to gain a better understanding for the
opportunity of selection in natural communities. Such studies can also
provide insights into the factors underlying variation in HP receipt at
different spatial scales. For instance, greater among-population
variance would suggest that community-level attributes such as
conspecific flower density (e.g. de Waal et al. 2015, Thomson et al.
2019), or changes in co-flowering (e.g. Arceo-Gómez & Ashman 2014a) and
pollinator community composition (e.g. Johnson & Ashman 2019) are key
determinants of HP receipt. Greater among plant variance, on the other
hand, may indicate that within-species variation in intrinsic plant
traits (e.g. flower size; Arceo-Gómez et al. 2016b) or spatial
structuring within a site may play a more important role (Bruckman &
Campbell 2016, Thomson et al. 2019). If greater variance in HP receipt
is observed among flowers within the same plants, then stochastic
pollination events may be more important (Herrera 2002, Fang & Huang
2013, Arceo-Gómez et al. 2016b).