Intraindividual, intraspecific, and interspecific variation shapes natural selection and its detection in two convergently-evolved lizard species

Understanding natural selection often assumes traits are stable within individuals, yet this study shows intraindividual variation can strongly influence evolutionary inference. Using multi‑year capture‑mark‑recapture data from two lizard species (Sceloporus cowlesi and Holbrookia maculata) at the White Sands–Chihuahuan Desert ecotone, researchers measured traits at every capture rather than relying on a single value. Selection estimates shifted depending on whether first, last or median measurements were used, demonstrating that within‑individual variation can obscure or reveal selection. A contingency framework clarifies when traits are truly under selection versus when variation complicates detection. Despite convergent dorsal blanching on White Sands, the two species experience different selection regimes within the same habitat, offering new insight into repeated evolution and the dynamics of natural selection in open populations.