Using ecological context to interpret spatiotemporal variation in natural selection

Spatiotemporal variation in natural selection is expected, but difficult to estimate. Pollinator-mediated selection on floral traits provides a good system for understanding and linking variation in selection to differences in ecological context. We studied pollinator-mediated selection in five populations of Dalechampia scandens (Euphorbiaceae) in Costa Rica and Mexico. Using a nonlinear path-analytical approach, we assessed several functional components of selection, and linked variation in pollinator-mediated selection across time and space to variation in pollinator assemblages. After correcting for estimation error, we detected moderate variation in net selection on two of four blossom traits. Both the opportunity for selection and the mean strength of selection decreased with increasing reliability of cross-pollination. Selection for pollinator attraction was consistently positive and stronger on advertisement than reward traits. Selection on traits affecting pollen transfer from the pollinator to the stigmas was strong only when there was a mismatch between pollinator and blossom size under unreliable cross-pollination. These results illustrate how consideration of trait function and ecological context can facilitate both the detection and the causal understanding of spatiotemporal variation in natural selection.

Methods
We studied phenotypic selection on the blossom traits in five populations, three in Costa Rica (Palo Verde, Puente la Amistad and Horizontes) and two in Mexico (La Mancha and Puerto Morelos). Both Mexican populations and one Costa Rican population were studied in two consecutive years. The data from the La Mancha population in 2007 were analyzed by Pérez-Barrales et al. (2013). In each population, we marked distinct patches of one to several intertwined individuals. Plants flower for an extended period, and we selected multiple blossoms per patch as they came into flower.

We followed each focal blossom throughout the female phase and for the first day of the bisexual phase. Each day, we recorded the number of pollen grains on the three stigmas with the aid of a LED light and a 10× hand lens, and whether resin had been collected. On the first day of the bisexual phase, when the first male flower was open, we counted pollen on the stigmas one last time, and measured gland-stigma distance (GSD), anther-stigma distance (ASD), gland area (GA), and upper bract area (UBA). All distance traits were measured in mm using digital calipers. For the Costa Rican populations, we also measured the height of the blossom above the ground. After completing the measurements, we marked the blossoms with a small tag tied around the peduncle. We collected the marked blossoms 3-4 weeks later and recorded the number of seeds set (seed set).