How to translate text using browser tools
1 February 2008 The Geographic Selection Mosaic for Ponderosa Pine and Crossbills: A Tale of Two Squirrels
Thomas L. Parchman, Craig W. Benkman
Author Affiliations +

Recent research demonstrates how the occurrence of a preemptive competitor (Tamiasciurus) gives rise to a geographic mosaic of coevolution for crossbills (Loxia) and conifers. We extend these studies by examining ponderosa pine (Pinus ponderosa), which produces more variable annual seed crops than the conifers in previous studies and often cooccurs with tree squirrels in the genus Sciurus that are less specialized than Tamiasciurus on conifer seed. We found no evidence of seed defenses evolving in response to selection exerted by S. aberti, which was apparently overwhelmed by selection resulting from inner bark feeding that caused many developing cones to be destroyed. In the absence of S. aberti, defenses directed at crossbills increased, favoring larger-billed crossbills and causing stronger reciprocal selection between crossbills and ponderosa pine. However, crossbill nomadism in response to cone crop fluctuations prevents localized reciprocal adaptation by crossbills. In contrast, evolution in response to S. griseus has incidentally defended cones against crossbills, limiting the geographic range of the interaction between crossbills and ponderosa pine. Our results suggest that annual resource variation does not prevent competitors from shaping selection mosaics, although such fluctuations likely prevent fine-scale geographic differentiation in predators that are nomadic in response to resource variability.

Thomas L. Parchman and Craig W. Benkman "The Geographic Selection Mosaic for Ponderosa Pine and Crossbills: A Tale of Two Squirrels," Evolution 62(2), 348-360, (1 February 2008).
Received: 20 June 2007; Accepted: 22 October 2007; Published: 1 February 2008

This article is only available to subscribers.
It is not available for individual sale.

Loxia curvirostra
phenotypic selection
Pinus ponderosa
seed predation
selection mosaic
Get copyright permission
Back to Top