Natural selection by pollinators on floral attractive and defensive traits

 by García et al.

Population of common milkweed (Asclepias syriaca)

Animal-pollinated plants often must both attract pollinators but deter herbivores for successful reproduction. Traits that primarily function as attraction or deterrence/defense can impact animal interactions more broadly than the intended target, so considering both types of traits and interactions can be important for understanding patterns of reproduction and ultimately natural selection. However, phenotypic selection studies that incorporate both floral traits and plant defenses with pollinator activity and herbivore damage remain rare. We studied a population of the common milkweed Asclepias syriaca (Apocynaceae) in Fredericton, New Brunswick, Canada. Public awareness of the milkweed plant has increased in recent years because it is a host plant for the iconic Monarch butterfly. We measured the association of ten traits including attractive and defensive traits and nectar rewards with reproductive success (= phenotypic selection), as well as pollinator activity and herbivore damage. We found selection to increase apparency traits such as plant and inflorescence size, but to decrease petal width in the flowers. We also found selection to increase a defensive trait, floral latex. We found evidence for pollinator-mediated selection but not herbivore-mediated selection on milkweed traits, perhaps because our population is at the northern range edge and experiences less herbivory than populations further south. Interestingly, pollinator-meditated selection did not directly translate to selection via offspring, suggesting that pollinator choice is not always driving floral evolution.

 

Read the scientific publication in JPE