The crucial point that can be demonstrated within the mesquite cline is that adaptation encompassed both environmental plasiticity and genetic changes. Read also Hoffmann et al. (1995), Guntrip & Sibly (1998), Pigliucci et al. (1999) and Pigliucci & Schmitt (1999). Although the phenotypic response to heat load/water stress is a tiny leaf, the identification and verifications of controlling genes is far beyond the present study. López & López Fanjul (1993) have established a parallel case in fruitflies, and the work of Rieseberg (1997) and Kim & Rieseberg (1999) on quantitative traits in sunflowers provides the best model to date.