Although changes in magnitude of single traits responding to selective agents have been studied intensively, little is known about selection shaping networks of traits and their patterns of covariation. However, this is central for our understanding of phenotypic evolution as traits are embedded in a multivariate environment with selection affecting a multitude of traits simultaneously rather than individually. Here, we investigate inter-and intraspecific patterns of trait integration (trait correlations) in the larval abdomen of dragonflies as a response to a change in predator selection. Species of the dragonfly genus Leucorrhinia underwent a larval habitat shift from predatory fish to predatory dragonfly-dominated lakes with an associated relaxation in selection pressure from fish predation. Our results indicate that the habitat-shift-induced relaxed selection pressure caused phenotypic integration of abdominal traits to be reduced. Intraspecific findings matched patterns comparing species from both habitats with higher abdominal integration in response to predatory fish. This higher integration is probably a result of faster burst swimming speed. The abdomen holds the necessary morphological machinery to successfully evade predatory fish via burst swimming. Hence, abdominal traits have to function in a tight coordinated manner, as maladaptive variation and consequently nonoptimal burst swimming would cause increased mortality. In predatory dragonfly-dominated lakes, no such strong link between burst swimming and mortality is present. Our findings highlight the importance of studying multivariate trait relationships as a response to selection for understanding patterns of phenotypic diversification.
Burst escape speed is an effective and widely used behaviour for evading predators, with burst escape speed relying on several different morphological features. However, we know little about how behavioural and underlying morphological attributes change in concert as a response to changes in selective predation regime. We studied intercorrelated trait differentiation of body shape and burst-swim-mediating morphology in response to a habitat shift-related reduction in burst escape speed using larvae of the dragonfly genus Leucorrhinia. Species in this genus underwent a well-known habitat shift from predatory fish lakes (fish lakes) to predatory fish-free lakes dominated by large predatory dragonflies (dragonfly lakes) accompanied by relaxed selection on escape burst speed. Results revealed that species from fish lakes that possess faster burst speed have evolved a suite of functionally intercorrelated traits, expressing a wider abdomen, a higher abdominal muscles mass and a larger branchial chamber compared with species from dragonfly lakes. In contrast, populations within species did not show significant differences in muscle mass and branchial chamber size between lake types in three of the species. High multicollinearity among variables suggests that traits have evolved in concert rather than independently when Leucorrhinia shifted from fish lakes to dragonfly lakes. Thus, relaxed selection on burst escape speed in dragonfly-lake species resulted in a correlated reduction of abdominal muscles and a smaller branchial chamber, likely to save production and/or maintenance costs. Our results highlight the importance of studying integrated behavioural and morphological traits to fully understand the evolution of complex phenotypes.
1. Predator species are separated along habitat gradients, with predation known to play an essential role in species' trait diversification. Because predator species differ dramatically in their hunting style and mode, a change in predator species will alter the mean and variance of prey's antipredator traits. Population trait variation has an impact on community ecology. It influences species' niche width and interactions in the food web. However, empirical studies on variance change by predation are scarce.2. In this study, we collected large numbers of the last instar Leucorrhinia pectoralis exuviae from lakes with predatory fish (fish lakes) and lakes with large invertebrate predators (dragonfly lakes) and compared their two antipredator traits (spines and body size).3. We found that individuals in dragonfly lakes grew shorter spines than individuals in fish lakes. Body size showed no significant difference between dragonfly-lake individuals and fish-lake individuals. Moreover, populations from dragonfly lakes showed a smaller variance of spine length than populations from fish lakes; while populations from dragonfly lakes had a larger variance in body size than populations from fish lakes. 4. These results indicate that trait variance, as well as mean, is strongly modified by different predation regimes. Studying the mean and variance of traits can help to define the mode of selection forces (directional selection and stabilising selection) in nature. Moreover, dragonfly larvae might be ideal organisms for the study of phenotypic selection on quantitative traits in the wild.
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