Racial evolution is about the same as AA mutates to Aa that backcrosses to aa. The force allowing any aa to persist is climatic change, in this case drought resistance. These alleles could belong to a dominant major gene. The higher heat stress, the smaller the mesquite leaf, and it is understood to be under niche directed selection. The major changes in Tiny are short LR & LL with increased PL, a leaf design that conforms to typical microphyllans of the arid Balsas valley.