Biomedical subjects

Craig W Benkman

Publications and source records attributed to Craig W Benkman.

6 recordsLinked to original sources

PubMed2006

Patterns of genetic variation in the adaptive radiation of New World crossbills (Aves: Loxia).

Incipient species groups or young adaptive radiations such as crossbills (Aves: Loxia) present the opportunity to investigate directly the processes occurring during speciation. New World crossbills include white-winged crossbills (Loxia leucoptera), Hispaniolan crossbills (Loxia megaplaga), and red crossbills (Loxia curvirostra complex), the last of which is comprised of at least nine morphologically and vocally differentiated forms ('call types') where divergent natural selection for specialization on different conifer resources has been strongly implicated as driving diversification. Here we use amplified fragment length polymorphism (AFLP) markers to investigate patterns of genetic variation across populations, call types, and species of New World crossbills. Tree-based analyses using 440 AFLP loci reveal strongly supported clustering of the formally recognized species, but did not separate individuals from the eight call types in the red crossbill complex, consistent with recent divergence and ongoing gene flow. Analyses of genetic differentiation based on inferred allele frequency variation however, reveal subtle but significant levels of genetic differentiation among the different call types of the complex and indicate that between call-type differentiation is greater than that found among different geographic locations within call types. Interpreted in light of evidence of divergent natural selection and strong premating reproductive isolation, the observed genetic differentiation suggests restricted gene flow among sympatric call types consistent with the early stages of ecological speciation.

Adaptation, Biological↗

PubMed2005

Can selection by an ectoparasite drive a population of red crossbills from its adaptive peak?

The bill structures of different call types of red crossbills (Loxia curvirostra complex) in western North America usually approximate the predicted optima for foraging on single species of conifers. One clear exception is the call type in the South Hills, Idaho, that is coevolving in an evolutionary arms race with Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia). Although South Hills crossbills forage only on the cones of these lodgepole pines, their average bill depth is smaller than that predicted to be optimal. Because preliminary data showed that large-billed males were more likely to exhibit symptoms of ectoparasitic mite (Knemidokoptes jamaicensis) infestation, the goal of our study was to further quantify the incidence of mite infestation and determine whether selection by mites may have favored smaller-billed crossbills and thus driven crossbills away from the foraging optimum. We estimated annual survival of both infected and uninfected South Hills crossbills using program MARK, which allows for auxiliary variables such as bill size and sex to be included in survival analyses. Mite infestation depressed crossbill survival and, especially for males, caused directional selection against larger-billed individuals. Such selection may explain why South Hills crossbills have smaller bills than the optimum and why average bill size for males has decreased from 1998 to 2003. This selection may also explain why the degree of sexual size dimorphism has decreased by nearly 50% since 1998.

Adaptation, Biological↗

PubMed2004

Interactions among moths, crossbills, squirrels, and lodgepole pine in a geographic selection mosaic.

Repeated patterns among biological communities suggest similar evolutionary and ecological forces are acting on the communities. Conversely, the lack of such patterns suggests that similar forces are absent or additional ones are present. Coevolution between a seed predator, the red crossbill (Loxia curvirostra complex), and lodgepole pine (Pinus contorta var. latifolia) exemplifies the ecological and evolutionary predictions for coevolving systems. In the absence of another seed predator and preemptive competitor (pine squirrels Tamiasciurus hudsonicus), natural selection by crossbills results in the evolution of larger cones with thicker distal scales, while relaxation of selection by squirrels results in the evolution of cones with more seeds and a greater ratio of seed mass to cone mass. However, in one range, the Little Rocky Mountains, distal scale thickness has diverged as expected but cone size has not. In these mountains seed predation by lodgepole pine cone borer moths (Eucosma recissoriana) was about 10 times greater than in other ranges lacking squirrels. We quantified moth predation and cone traits and found that moths select for smaller cones with fewer seeds. Thus, selection by moths in the Little Rocky Mountains counters both selection by crossbills for large cone size and relaxation of selection by squirrels favoring more seeds per cone and accounts for the relatively small and few-seeded cones in these mountains. It is also apparent that selection by crossbills changes seed defenses in a manner that favors seed predation by moths, whereas selection by squirrels likely reduces such predation. These results demonstrate the importance of considering the evolutionary consequences of community context in locally evolved (coevolved) traits and interactions.

Animals↗

PubMed2003

Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine.

Few studies have shown both reciprocal selection and reciprocal adaptations for a coevolving system in the wild. The goal of our study was to determine whether the patterns of selection on Rocky Mountain lodgepole pine (Pinus contorta spp. latifolia) and red crossbills (Loxia curvirostra complex) were concordant with earlier published evidence of reciprocal adaptations in lodgepole pine and crossbills on isolated mountain ranges in the absence of red squirrels (Tamiasciurus hudsonicus). We found that selection (directional) by crossbills on lodgepole pine where Tamiasciurus are absent was divergent from the selection (directional) exerted by Tamiasciurus on lodgepole pine. This resulted in divergent selection between areas with and without Tamiasciurus that was congruent with the geographic patterns of cone variation. In the South Hills, Idaho, where Tamiasciurus are absent and red crossbills are thought to be coevolving with lodgepole pine, crossbills experienced stabilizing selection on bill size, with cone structure as the agent of selection. These results show that crossbills and lodgepole pine exhibit reciprocal adaptations in response to reciprocal selection, and they provide insight into the traits mediating and responding to selection in a coevolutionary arms race.

Adaptation, Physiological↗

PubMed2003

Divergent selection drives the adaptive radiation of crossbills.

Knowledge of how phenotype influences fitness is necessary if we are to understand the basis of natural selection and how natural selection contributes to adaptive radiations. Here I quantify selection on a wild population of red crossbills (Loxia curvirostra complex) in the South Hills, Idaho. Bill depth is the target of selection and selection on bill depth is stabilizing. I then show how fitness is related to both bill depth and performance. I use these and previously published relationships to estimate a fitness surface for five species of red crossbills that are part of an ongoing adaptive radiation in western North America. The fitness surface for crossbills has distinct peaks and valleys, with each crossbill species residing on or very near the summits. This work strongly supports a key tenet of the ecological theory of adaptive radiations; namely, divergent selection for utilizing alternative resources is the ultimate cause of adaptive radiations.

Adaptation, Biological↗

PubMed2002

Diversifying coevolution between crossbills and black spruce on Newfoundland.

Coevolution is increasingly recognized as an important process structuring geographic variation in the form of selection for many populations. Here we consider the importance of a geographic mosaic of coevolution to patterns of crossbill (Loxia) diversity in the northern boreal forests of North America. We examine the relationships between geographic variation in cone morphology, bill morphology, and feeding performance to test the hypothesis that, in the absence of red squirrels (Tamiasciurus hudsonicus), black spruce (Picea mariana) has lost seed defenses directed at Tamiasciurus and that red crossbills (L curvirostra) and black spruce have coevolved in an evolutionary arms race. Comparisons of cone morphology and several indirect lines of evidence suggest that black spruce has evolved defenses in response to Tamiasciurus on mainland North America but has lost these defenses on Newfoundland. Cone traits that deter crossbills, including thicker scales that require larger forces to separate, are elevated in black spruce on Newfoundland, and larger billed crossbills have higher feeding performances than smaller billed crossbills on black spruce cones from Newfoundland. These results imply that the large bill of the Newfoundland crossbill (L. c. percna) evolved as an adaptation to the elevated cone defenses on Newfoundland and that crossbills and black spruce coevolved in an evolutionary arms race on Newfoundland during the last 9000 years since glaciers retreated. On the mainland where black spruce is not as well defended against crossbills, the small-billed white-winged crossbill (L leucoptera leucoptera) is more efficient and specializes on seeds in the partially closed cones. Finally, reciprocal adaptations between crossbills and conifers are replicated in black spruce and Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia), with coevolution most pronounced in isolated populations where Tamiasciurus are absent as a competitor. This study further supports the role of Tamiasciurus in determining the selection mosaic for crossbills and suggests that a geographic mosaic of coevolution has been a prominent factor underlying the diversification of North American crossbills.

Animals↗