Aridity as a Stimulus to Plant Evolution

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Abstract
Theoretical considerations regarding the structure of populations indicate that evolution is likely to be more rapid in plant communities of arid or semiarid regions than in mesic communities. The number of spp. present at any one time is smaller in xeric than in mesic communities, but the turnover of spp. in evolutionary time is larger in the former than in the latter. For this reason one might expect that some spp. of mesic communities were derived from xeric types. Morphological comparison of leaves and reproductive structures in a probable phylogenetic line in the Compositae, tribe Cichorieae suggests that the most common species of Scozonera and Tragopogon arose in this fashion. In the family Leguminosae, studies of the ontogeny of both seedling and adult leaves of spp. of Colutea, Inga, Caesalpinia, Acacia, Bauhinia, and Lupinus suggest that (a) even pinnate and bipinnate leaves of Caesalpinoideae and Mimosoideae are derived from odd pinnate leaves via a process of leaflet reduction, followed by later increase in leaflet size, leaflet number, or both; (b) the bifid leaf of Bauhinia is derived from the odd pinnate type through extreme reduction of leaflets, followed by fusion and expansion of one pair; (c) the digitate leaf of Lupinus is derived from a uni- or trifoliolate leaf through increase in leaflet number. All of these derived types are relatively mesophytic, while their ancestors were more or less extreme xerophytes.

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