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When field experiments yield unexpected results: lessons learned from measuring selection in White Sands lizards.

Hardwick KM, Harmon LJ, Hardwick SD, Rosenblum EB - PLoS ONE (2015)

Bottom Line: Color did not have a significant effect on survival, but we found several unexpected relationships including variation in predation over small spatial and temporal scales.In addition, we detected a marginally significant interaction between sex and color, suggesting selection for substrate matching may be stronger for males than females.We use our results as a case study to examine six major challenges frequently encountered in field-based studies of natural selection, and suggest that insight into the complexities of selection often results when experiments turn out differently than expected.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Idaho, Moscow, Idaho, United States of America.

ABSTRACT
Determining the adaptive significance of phenotypic traits is key for understanding evolution and diversification in natural populations. However, evolutionary biologists have an incomplete understanding of how specific traits affect fitness in most populations. The White Sands system provides an opportunity to study the adaptive significance of traits in an experimental context. Blanched color evolved recently in three species of lizards inhabiting the gypsum dunes of White Sands and is likely an adaptation to avoid predation. To determine whether there is a relationship between color and susceptibility to predation in White Sands lizards, we conducted enclosure experiments, quantifying survivorship of Holbrookia maculate exhibiting substrate-matched and substrate-mismatched phenotypes. Lizards in our study experienced strong predation. Color did not have a significant effect on survival, but we found several unexpected relationships including variation in predation over small spatial and temporal scales. In addition, we detected a marginally significant interaction between sex and color, suggesting selection for substrate matching may be stronger for males than females. We use our results as a case study to examine six major challenges frequently encountered in field-based studies of natural selection, and suggest that insight into the complexities of selection often results when experiments turn out differently than expected.

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Design of the enclosure experiment.Panel A shows the “open” enclosure treatment, which allowed avian predators to enter and exit freely, and panel B shows the “closed” enclosure treatment, which excluded avian predators with chicken wire. The top half of panel C shows color-manipulated H. maculata (with the substrate-matched paint treatment on the left and substrate-mismatched paint treatment on the right), and the bottom half shows the corresponding naturally occurring color phenotypes (with the White Sands phenotype on the left and dark soils phenotype on the right) (photograph courtesy of S. Des Roches). Panel D shows a substrate-matched lizard (left) and a substrate-mismatched lizard (right) after release into an enclosure.
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pone.0118560.g001: Design of the enclosure experiment.Panel A shows the “open” enclosure treatment, which allowed avian predators to enter and exit freely, and panel B shows the “closed” enclosure treatment, which excluded avian predators with chicken wire. The top half of panel C shows color-manipulated H. maculata (with the substrate-matched paint treatment on the left and substrate-mismatched paint treatment on the right), and the bottom half shows the corresponding naturally occurring color phenotypes (with the White Sands phenotype on the left and dark soils phenotype on the right) (photograph courtesy of S. Des Roches). Panel D shows a substrate-matched lizard (left) and a substrate-mismatched lizard (right) after release into an enclosure.

Mentions: We constructed the enclosures using steel flashing (0.5 meters in height with approximately 15 centimeters buried under the surface of the sand). To anchor flashing in place we fastened it to rebar posts (1.2 meters in height with approximately half of each post buried under the surface) using zip ties. We divided each enclosure in half with a 10 meter long piece of flashing, and covered seams using aluminum foil tape. In 2011, one half of each enclosure allowed avian predators to enter freely (the “open” treatment), and we covered the other half with chicken wire to exclude predators (the “closed” treatment) (Fig. 1). To create the closed treatment, we used zip ties to fasten chicken wire to the top edge of the steel flashing for one half of each enclosure. To ensure lizards would have an ongoing supply of food we used chicken wire with holes 2.54 centimeters in diameter, which allowed invertebrates to enter and exit freely. To support the chicken wire we placed a number of rebar posts throughout the closed sides of enclosures, and covered support posts with PVC pipe so that the lizards could not use supports to escape. To make sure lizards had adequate ground cover to seek refuge from biotic and abiotic elements of the environment, we chose enclosure locations where existing vegetation covered approximately 20% of the available space (previous research indicates that White Sands H. maculata prefer habitat where on average 20.5% of the ground is covered by vegetation [13]). We ensured that our enclosure design was sufficient to contain lizards by conducting brief (15 minute) observations of H. maculata within enclosures before initiating trials.


When field experiments yield unexpected results: lessons learned from measuring selection in White Sands lizards.

Hardwick KM, Harmon LJ, Hardwick SD, Rosenblum EB - PLoS ONE (2015)

Design of the enclosure experiment.Panel A shows the “open” enclosure treatment, which allowed avian predators to enter and exit freely, and panel B shows the “closed” enclosure treatment, which excluded avian predators with chicken wire. The top half of panel C shows color-manipulated H. maculata (with the substrate-matched paint treatment on the left and substrate-mismatched paint treatment on the right), and the bottom half shows the corresponding naturally occurring color phenotypes (with the White Sands phenotype on the left and dark soils phenotype on the right) (photograph courtesy of S. Des Roches). Panel D shows a substrate-matched lizard (left) and a substrate-mismatched lizard (right) after release into an enclosure.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4340912&req=5

pone.0118560.g001: Design of the enclosure experiment.Panel A shows the “open” enclosure treatment, which allowed avian predators to enter and exit freely, and panel B shows the “closed” enclosure treatment, which excluded avian predators with chicken wire. The top half of panel C shows color-manipulated H. maculata (with the substrate-matched paint treatment on the left and substrate-mismatched paint treatment on the right), and the bottom half shows the corresponding naturally occurring color phenotypes (with the White Sands phenotype on the left and dark soils phenotype on the right) (photograph courtesy of S. Des Roches). Panel D shows a substrate-matched lizard (left) and a substrate-mismatched lizard (right) after release into an enclosure.
Mentions: We constructed the enclosures using steel flashing (0.5 meters in height with approximately 15 centimeters buried under the surface of the sand). To anchor flashing in place we fastened it to rebar posts (1.2 meters in height with approximately half of each post buried under the surface) using zip ties. We divided each enclosure in half with a 10 meter long piece of flashing, and covered seams using aluminum foil tape. In 2011, one half of each enclosure allowed avian predators to enter freely (the “open” treatment), and we covered the other half with chicken wire to exclude predators (the “closed” treatment) (Fig. 1). To create the closed treatment, we used zip ties to fasten chicken wire to the top edge of the steel flashing for one half of each enclosure. To ensure lizards would have an ongoing supply of food we used chicken wire with holes 2.54 centimeters in diameter, which allowed invertebrates to enter and exit freely. To support the chicken wire we placed a number of rebar posts throughout the closed sides of enclosures, and covered support posts with PVC pipe so that the lizards could not use supports to escape. To make sure lizards had adequate ground cover to seek refuge from biotic and abiotic elements of the environment, we chose enclosure locations where existing vegetation covered approximately 20% of the available space (previous research indicates that White Sands H. maculata prefer habitat where on average 20.5% of the ground is covered by vegetation [13]). We ensured that our enclosure design was sufficient to contain lizards by conducting brief (15 minute) observations of H. maculata within enclosures before initiating trials.

Bottom Line: Color did not have a significant effect on survival, but we found several unexpected relationships including variation in predation over small spatial and temporal scales.In addition, we detected a marginally significant interaction between sex and color, suggesting selection for substrate matching may be stronger for males than females.We use our results as a case study to examine six major challenges frequently encountered in field-based studies of natural selection, and suggest that insight into the complexities of selection often results when experiments turn out differently than expected.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Idaho, Moscow, Idaho, United States of America.

ABSTRACT
Determining the adaptive significance of phenotypic traits is key for understanding evolution and diversification in natural populations. However, evolutionary biologists have an incomplete understanding of how specific traits affect fitness in most populations. The White Sands system provides an opportunity to study the adaptive significance of traits in an experimental context. Blanched color evolved recently in three species of lizards inhabiting the gypsum dunes of White Sands and is likely an adaptation to avoid predation. To determine whether there is a relationship between color and susceptibility to predation in White Sands lizards, we conducted enclosure experiments, quantifying survivorship of Holbrookia maculate exhibiting substrate-matched and substrate-mismatched phenotypes. Lizards in our study experienced strong predation. Color did not have a significant effect on survival, but we found several unexpected relationships including variation in predation over small spatial and temporal scales. In addition, we detected a marginally significant interaction between sex and color, suggesting selection for substrate matching may be stronger for males than females. We use our results as a case study to examine six major challenges frequently encountered in field-based studies of natural selection, and suggest that insight into the complexities of selection often results when experiments turn out differently than expected.

Show MeSH