About 10 years ago we found evidence for coevolution between crossbills (Loxia) and lodgepole pine (Pinus contorta latifolia). Because it was so striking, we began examining other crossbill-conifer systems to (1) see how common coevolution is in the adaptive radition of crossbills and (2) determine the conditions that favor and disfavor coevolutionary arms races. This has taken us across North America, down to Hispaniola and east to the Mediterranean. We have found evidence for coevolution in most crossbill populations that we've studied and are beginning to understand how the presence and absence of preemptive competitors (tree squirrels), resource stability, and habitat area and structure alter the form and strength of phenotypic selection and coevolution. Here are our publications related to this research (many can be downloaded from my "publications" page).
Benkman C. W., T. L. Parchman, and E. T. Mezquida. 2010. Patterns of coevolution in the adaptive radiation of crossbills. Annals of the New York Academy of Sciences 1206:1-16.
Mezquida, E. T., and C. W. Benkman. 2010. Habitat area and structure affect the impact of seed predators and the potential for coevolutionary arms races. Ecology 91:802-814.
Benkman, C. W. 2010. Diversifying coevolution between crossbills and conifers. Evolution: Education and Outreach 3:47-53.
Benkman, C. W., and T. L. Parchman. 2009. Coevolution between crossbills and black pine: the importance of competitors, forest area, and resource stability. Journal of Evolutionary Biology 22:942-953.
Parchman, T. L., and C. W. Benkman. 2008. The geographic selection mosaic for ponderosa pine and crossbills: a tale of two squirrels. Evolution 62:348-360.
Parchman, T. L., C. W. Benkman, and E. T. Mezquida. 2007. Coevolution between Hispaniolan Crossbills and pine: Does more time allow for greater phenotypic escalation at lower latitude? Evolution 61:2142-2153.
Edelaar, P., and C. W. Benkman. 2006. Replicated population divergence caused by localised coevolution? A test of three hypotheses in the red crossbill-lodgepole pine system. Journal of Evolutionary Biology 19:1651-1659.
Siepielski, A. M., and C. W. Benkman. 2005. A role for habitat area in the geographic mosaic of coevolution between Red Crossbills and lodgepole pine. Journal of Evolutionary Biology 18:1042-1049.
Mezquida, E. T., and C. W. Benkman. 2005. The geographic selection mosaic for squirrels, crossbills and Aleppo pine. Journal of Evolutionary Biology 18:348-357.
Siepielski, A. M., and C. W. Benkman. 2004. Interactions among moths, crossbills, squirrels and lodgepole pine in a geographic selection mosaic. Evolution 58:95-101.
Benkman, C. W., T. L. Parchman, A. Favis, and A. M. Siepielski. 2003. Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine. American Naturalist 162: 182-194.
Parchman, T. L., and C. W. Benkman. 2002. Diversifying coevolution between crossbills and black spruce on Newfoundland. Evolution 56:1663-1672.
Benkman, C. W., W. C. Holimon, and J. W. Smith. 2001. The influence of a competitor on the geographic mosaic of coevolution between crossbills and lodgepole pine. Evolution 55:282-294.
Benkman, C. W. 1999. The selection mosaic and diversifying coevolution between crossbills and lodgepole pine. American Naturalist 154:S75-S91.
We have also examined how ecological adaptation as a result of divergent selection has led to reproductive isolation. The South Hills Crossbill has diverged not only in bill size (they are larger than other crossbills in the region) but also in calls and song. We have found very high levels of reproductive isolation between the South Hills Crossbill and the two other Red Crossbill call types that breed in the South Hills. Much of the premating reproductive isolation is the result of South Hills Crossbills depressing seed availability so that other crossbills with smaller (less efficient) bills are unable to persist and breed. In addition, South Hills Crossbills preferentially pair with other South Hills Crossbills. Because the South Hills Crossbills are so divergent from other crossbills and levels of reproductive isolation are so high, we have recently argued that they should be recognized as a new species. Below are some publications related to ecological speciation and mate choice in South Hills Crossbills (some can be downloaded from my "publications" page).
Snowberg, L. K., and C. W. Benkman 2009. Mate choice based on a key ecological performance trait. Journal of Evolutionary Biology 22:762-769.
Benkman, C. W., J. W. Smith, P. C. Keenan, T. L. Parchman, and L. Santisteban. 2009. A new species of Red Crossbill (Fringillidae: Loxia) from Idaho. Condor 111: 169-176.
Keenan, P. C., and C. W. Benkman. 2008. Call imitation and call modification in Red Crossbills. Condor 110:93-101.
Snowberg, L. K., and C. W. Benkman. 2007. The role of marker traits in the assortative mating within Red Crossbills, Loxia curvirostra complex. Journal of Evolutionary Biology 20:1924-1932.
Smith, J. W., and C. W. Benkman. 2007. A coevolutionary arms race causes ecological speciation in crossbills. American Naturalist 169:455-465.
Smith, J. W., C. W. Benkman, and K. Coffey. 1999. The use and mis-use of public information by foraging red crossbills. Behavioral Ecology 10:54-62.
Our interest is in understanding how the presence and absence of a superior preemptive competitor (pine squirrels Tamiasciurus) has affected the evolution of seed dispersal in limber pine (Pinus flexilis) and whitebark pine (P. albicaulis) and how this affects the ecology of the pines. We continue to conduct research on the ecology and evolution of the interactions between Clark's Nutcrackers (Nucifraga columbiana; usually the main seed disperser of these pines), pine squirrels, and limber and whitebark pine. Below are our publications related to this research.
Siepielski, A. M., and C. W. Benkman. 2010. Conflicting selection from an antagonist and a mutualist enhances phenotypic variation in a plant. Evolution 64:1120-1128.
Garcia, R., A. M. Siepielski, and C. W. Benkman. 2009. Cone and seed trait variation in whitebark pine Pinus albicaulis (Pinaceae) and the potential for phenotypic selection. American Journal of Botany 96:1050-1054.
Siepielski, A. M., and C. W. Benkman. 2008. A seed predator drives the evolution of a seed dispersal mutualism. Proceedings of the Royal Society of London Series B. 275:1917-1925.
Siepielski, A. M., and C. W. Benkman. 2008. Seed predation and selection exerted by a seed predator influence subalpine tree densities. Ecology 89:2960-2966.
Siepielski, A. M., and C. W. Benkman. 2007. Extreme environmental variation sharpens selection that drives the evolution of a mutualism. Proceedings of the Royal Society of London Series B 274:1799-1805.
Siepielski, A. M., and C. W. Benkman. 2007. Selection by a pre-dispersal seed predator constrains the evolution of avian seed dispersal in pines. Functional Ecology 21:611-618.
Siepielski, A. M., and C. W. Benkman. 2007. Convergent patterns in the selection mosaic for two North American bird-dispersed pines. Ecological Monographs 77:203-220.
Benkman, C. W. 1995. The impact of tree squirrels (Tamiasciurus) on limber pine seed dispersal adaptations. Evolution 49:585-592.
Benkman, C. W. 1995. Wind dispersal capacity of pine seeds, with comments on the evolution of different seed dispersal modes in pines. Oikos 73: 221-224.
Benkman, C. W., R. P. Balda, and C. C. Smith. 1984. Adaptations for seed dispersal and the compromises due to seed predation in limber pine. Ecology 65:632-642.