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).