Coevolution Insights: Crossbills, Pines, and Squirrels Analysis

Coevolution of Crossbills, Pines and Squirrels Your name: Kamal Mahatara Documenting coevolution in nature requires satisfying two criteria: that the two species interact and that the activity of one species directly causes the differential survival of individuals of another and vice versa. Imagine you are a researcher studying the interaction among crossbills, squirrels and pines and are focused on one aspect of the interaction. Evaluate the results of your scientific study with regard to whether it satisfies the criteria for coevolution. Evidence 1 - Graph showing the dependence of survival on bill depth for a population of birds that feed on lodgepole pine cones. Individuals that survived or died are assigned values of 1 or 0, respectively. The expected probability of survival (the solid line) was estimated using a statistical regression method. 1. What is this evidence telling you about co-evolution? This evidence suggests a specific coevolutionary relationship between crossbills and lodgepole pines. Crossbills have specialized bills adapted to extracting seeds from pinecones. The graph showing the dependence of survival on bill depth indicates that crossbills with particular bill depths have a higher probability of survival when feeding on lodgepole pinecones. This implies a strong interaction where the adaptation of crossbills (bill depth) is directly influencing their survival by enabling them to efficiently obtain food from pinecones. The dependence of crossbills on pine cones for survival, and the corresponding

adaptation in bill depth, illustrates a classic case of coevolution between these species 2. How strongly does this evidence for co-evolution? The evidence presented in the form of a graph, coupled with the use of statistical regression to estimate survival probabilities, provides robust support for coevolution. The graph clearly demonstrates the interaction between crossbills and lodgepole pines. Crossbills' bill depth directly influences their ability to extract seeds from pinecones, affecting their survival. The statistical analysis further strengthens the evidence by quantifying this relationship. If the regression analysis shows a significant correlation between bill depth and survival, it indicates a strong and direct causal link, affirming the coevolutionary dynamics between crossbills and lodgepole pines.

Evidence 2 – Comparison of the traits of in a population of lodgepole pines for trees that were subject to predation by crossbills and trees that were not predated on by crossbills. The rs statistic is an indication of the strength of the effect (larger numbers indicate larger effect) and the sign (plus or minus) indicates whether the trait was larger (positive sign) or smaller (negative sign) for trees that were subject to predation than those that were not subject to predation. The P value is an indication of whether the effect is significant or not, with values greater than 0.01 (or 0.05) indicating a lack of effect. 3. What is this evidence telling you about co-evolution? The comparison of lodgepole pine traits between trees subject to crossbill predation and those that did not provide crucial insights into the coevolutionary relationship between these species. A positive rs value, indicating larger traits in trees predated upon, suggests an evolutionary response in pine trees to counter crossbill predation, indicating an ongoing adaptive process. Conversely, a negative rs value would imply adaptations in crossbills to overcome the pine trees' defenses, illustrating an arms race. The significance of these traits, denoted by the P value, is fundamental. If the P value is less than 0.01 or 0.05, it underscores the strong selection pressure imposed by crossbills, substantiating a significant coevolutionary interaction. 4. How strongly does this evidence for co-evolution? The strength of this evidence for coevolution hinges on the magnitude of the rs statistic and the associated P value. A large absolute rs value signifies a potent effect, indicating substantial evolutionary changes in response to

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predation pressure. If these rs values persistently demonstrate statistical significance with P values below 0.01 or 0.05 across multiple traits, it robustly supports the coevolutionary dynamics between crossbills and lodgepole pines. Such consistent, significant results highlight a compelling case for ongoing reciprocal adaptations, affirming the intensity of the coevolutionary relationship between these species.

Evidence 3 - Two pine cone traits in areas with and without squirrels. The circles above the shape bars indicate the general shape (more spherical or more oblong). The data were collected by sampling adult trees of the same species in an area with squirrels and in an area without squirrels. 5. What is this evidence telling you about co-evolution? 6. How strongly does this evidence for co-evolution?

7. Based on these data, what is the strength of co-evolutionary pressures between each of the following species pairs: Squirrel and lodgepole: Lodgepole and crossbill: Crossbill and squirrel: 8. Which of the pairs can you confidently claim are in a co-evolutionary relationship? Do they meet both necessary criteria?

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