The evolution of biological materials is a critical, yet poorly understood, component in the generation of biodiversity. For example, the diversification of spiders is correlated with evolutionary changes in the way they use silk, and the material properties of these fibers, such as strength, toughness, extensibility, and stiffness, have profound effects on ecological function. Here, we examine the evolution of the material properties of dragline silk across a phylogenetically diverse sample of species in the Araneomorphae (true spiders). The silks we studied are generally stronger than other biological materials and tougher than most biological or man-made fibers, but their material properties are highly variable; for example, strength and toughness vary more than fourfold among the 21 species we investigated. Furthermore, associations between different properties are complex. Some traits, such as strength and extensibility, seem to evolve independently and show no evidence of correlation or trade-off across species, even though trade-offs between these properties are observed within species. Material properties retain different levels of phylogenetic signal, suggesting that traits such as extensibility and toughness may be subject to different types or intensities of selection in several spider lineages. The picture that emerges is complex, with a mosaic pattern of trait evolution producing a diverse set of materials across spider species. These results show that the properties of biological materials are the target of selection, and that these changes can produce evolutionarily and ecologically important diversity.

Fiddler crabs, genus Uca, are classic examples of how intense sexual selection can produce exaggerated male traits. Throughout the genus the enlarged “major” cheliped (claw) of the male fiddler crab is used both as a signal for attracting females and as a weapon for combat with other males. However, the morphology of the major claw is highly variable across the approximately 100 species within the genus. Here we address variation, scaling, and correlated evolution in the mechanics of the major claw by analyzing the morphology and mechanical properties of the claws of 21 species of fiddler crabs from the Pacific, Gulf and Atlantic coasts of the Americas.

Spider silk is a high-performance biomaterial with exceptional mechanical properties and over half a century of research into its mechanics, structure, and biology. Recent research demonstrates that it is a highly variable class of materials that differs across species and individuals in complex and interesting ways. Here, we review recent literature on mechanical variation and evolution in spider silk. We then present new data on material properties of silk from nine species of spiders in the Mesothelae and Mygalomorphae, the two basal clades of spiders. Silk from spiders in the Araneomorphae (true spiders where most previous research on silk has focused) is significantly stronger and therefore much tougher than the silk produced by spiders in the basal groups. These data support the hypothesis that the success and diversity seen in araneomorph spiders is associated with the evolution of this high-performance fiber. This comparative approach shows promise as a way to understand complex, high-performance biomaterials.