The development of an accurate and reproducible approach to measuring the tensile behavior of spider silk has allowed characterizing and comparing the range of mechanical properties exhibited by different spider species with unprecedented detail. The comparison of silks spun by spiders belonging to different phylogenetic groups has revealed that evolution locked in many of the important properties of spider silks very early in the history of orb-web weaving spiders, despite the fact that the silk gland system is relatively isolated in physiological terms from the rest of the organism and should thus mutate quickly. The variations observed between species may be grouped in at least two patterns that are shown not to be related to phylogeny. Beyond the relevance of these results for the evolutionary biology of spiders and silks, the conservation of the basic traits observed in the mechanical behavior of spider silks is likely to set a limit to the range of properties that can be expected from artificial fibers bioinspired in natural silks. © 2009 American Chemical Society.