Digest: Old world climates give rise to "young" lizard skulls.

Shared environmental pressures often give rise to the convergence of morphological characters in unrelated and geographically distinct species. Darwin (1859) wrote that “analogous variation” of traits in different organisms could be explained by similar influences or challenges in their environment. Convergence of traits can be driven by adaptive radiation when animals invade similar ecological feeding niches and consequently converge on similar morphologies, such as broader beak shape in seed-crushing species of Darwin’s finches (Grant, 1999). Climate can also be a factor, as demonstrated by morphological convergence of rodents from distinct arid habitats in the Sonoran (SW United States) and Monte (NW Argentina; Mares, 1976) Deserts. Observed patterns of ecomorphology suggest that overall body size decreases with warmer climates (Bergmann’s Rule), while limb length increases (Allen’s Rule; Schreider, 1951). Despite clear evidence of a relationship between morphology and environment, the mechanisms underlying trait convergence are not always clear. Until recently, it has been difficult to determine if convergent evolution occurs as a result of adaptations to a particular habitat, or if trait evolution is constrained by developmental mechanisms. In this issue, Hipsley and Müller (2017) address this fundamental biological question by investigating ecomorphological convergence in skull shape within a broad family of lacertid lizards that span across Eurasia and Africa, inhabiting vastly different climates over their distribution. Using landmark-based geometric morphometrics, Hipsley and Müller (2017) compared derived lizards adapted for dry climates to their basal ancestors that inhabited moderately moist (“mesic”) habitats. Desert-dwelling species’ skull shape was sig-