Pirates of the Caribbean (and elsewhere): three-legged lizards and the study of evolutionary adaptation

Natural selection is widely considered responsible for the fit between organisms and their environment. Lizard limb length variation is a paradigmatic example: studies have shown that limb length differences tightly correlate with habitat use among species, while small differences in limb length between individuals can affect biomechanical function, fitness, and survival within populations. It has therefore been surprising for many field biologists to find otherwise-healthy wild lizards with damaged or missing limbs, appearing to challenge associated expectations of substantial fitness costs. We document limb loss (from a foot to an entire limb) in 58 lizard species, with all cases showing healed limbs and many lizards appearing robust and healthy. Data indicate that limb-deficient lizards typically comprise less than 1% of populations and often exhibit body condition, sprint speed performance, and survival comparable to limb-intact individuals. We discuss the implications of these findings for how evolutionary adaptation is studied and understood in natural populations and provide a perspective on conventional assumptions about the strength and ubiquity of selection pressures on seemingly critical traits. Is natural selection always as omnipresent as Darwin envisioned it to be?