When directional selection reduces geographic variation in traits mediating species interactions

Abstract

Although we often focus on the causes of geographic variation, understanding processes that act to reduce geographic variation is also important. Here, we consider a process whereby adaptive foraging across the landscape and directional selection exerted by a conifer seed predator, the common crossbill (Loxia curvirostra), potentially act to homogenize geographic variation in the defensive traits of its prey. We measured seed predation and phenotypic selection exerted by crossbills on black pine (Pinus nigra) at two sites in the Pindos Mountains, Greece. Seed predation by crossbills was over an order of magnitude higher at the site where cone scale thickness was significantly thinner, which was also the cone trait that was the target of selection at the high predation site. Additional comparisons of selection differentials demonstrate that crossbills exert selection on black pine that is consistent in form across space and time, and increases in strength with increasing seed predation. If predators distribute themselves in relation to the defensive traits of their prey and the strength of selection predators exert is proportional to the amount of predation, then predators may act to homogenize trait variation among populations of their prey in a process analogous to coevolutionary alternation with escalation.

Biotic interaction strength and the intensity of selection

“…). However, when the relationship between fitness and phenotype is consistent, as we find for crossbills and pines (Benkman & Parchman ), then interaction strength should be related to selection intensity. This in particular is the situation when it will be especially worthwhile to understand what causes variation in interaction strength.…”

“…b) and (2) the square root of the opportunity for selection is correlated with selection intensity. I use one data set for crossbills ( Loxia curvirostra L.) preying on European black pine ( Pinus nigra Arnold) seeds at three different sites, including samples from 2 years (2006 and 2009) from one of the sites for a total of four site/year combinations (Benkman & Parchman ); crossbills are seed predators as they eat all the seeds they remove from cones. Similarly, I use one data set for Clark's nutcrackers ( Nucifraga columbiana Wilson) harvesting seeds from whitebark pine ( Pinus albicaulis Engelm.)…”

“…I focus on two cone traits and one gall trait that are the key traits mediating each of these interactions (Craig et al . ; Siepielski & Benkman ; Benkman & Parchman ). I use the mean proportion of the seeds eaten or harvested, respectively, from all trees as a measure of interaction strength for the crossbill and nutcracker interactions.…”

“…Coley & Barone ), and empirical support has been provided by studies on selection exerted on galling insects by parasitoids and birds (Weis et al . ), and for invertebrate and vertebrate seed predation (Benkman & Parchman ; Benkman et al . ; Vanhoenacker et al .…”

Ecological causes of morphological evolution in the three‐spined stickleback

“…One approach to identifying causes of selection is to match spatial variation in phenotype with putative agents of selection in the environment. Understanding the extent of genetic and phenotypic divergence, either as a response to geographic variation or from constraints imposed by gene flow and stabilizing selection, is considered a key question in evolutionary biology (Schluter 2001; Coyne and Orr 2004; Benkman and Parchman 2013). …”