Mesquites over the last 50 years evince genetic responses to heat stress by the design of a more resistant smaller leaf, based upon a shorter rachis as in the new race Tiny of Guaymas. The shorter the rachis, the drier the niche. Evolution by replacement is reported (Earl, 1998), but not by the fixation of favored mutations, apparent in the present case. The major finding is that the genetic foliar adaptation of hybrids to heat stress including the fixation of macromutations is favored by niche-directed selection. See www.pitt.edu/~super1/lecture/lec10071/index.htm