An Essay: The Stimulus of Unusual Geologies for Plant Speciation

Though a region's climate sets the limits for a biota, geology enriches local discon- tinuity and habitat diversity. When slope, exposure, and physical and chemical properties of rock and soil are arrayed discontinuously, the opportunities for events leading to speciation can occur. Several scenarios can account for evolution in geologically diverse landscapes. They range from ecotypic differentiation and disruptive selection to saltational speciation. I draw upon evidence from microevolutionary response to heavy metals and from serpentine endemism. The western North American genus Streptanthus (Cruciferae), with a number of serpentine endemics, illustrates the possible modes of evolutionary diversification on this demanding substrate. Imagine a world of life without mountains, valleys, rivers, and all the other topographic intricacies of interesting landscapes. Further, conjure up a world without a mosaic of lithol- ogies-an absence of a mix of sedimentary, metamorphic, and igneous rocks, rich in vari- ant chemical and physical attributes. Without a wealth of geological diversity, the only phys- ical variable on the planet might be the contin- uous change of climate from poles to equator. Life would then have evolved a dull contin- uum of response in its tracking of gradual cli- matic change across a monotonous landscape. But the real biosphere is rich in organic diver- sity, and much of that diversity, I contend, comes form the wealth of geologic phenomena, expressed through time and space. The ingredients of the variables fashioned out of geology run the gamut from global events like plate tectonics and the drift of con- tinents to regional and local diversity created out of variant topographies and lithologies. Crucial to the argument that geological diver- sity begets biological diversity, is the realiza- tion that geological events and materials are often arrayed discontinuously. It is this discon- tinuity that sets the stage for speciation; with- out isolation, much of the world's discrete bio- ta-as species-could not have come into being. The diversification of habitats that can flow from differences in land forms and rock types then becomes the stimulus for evolutionary di- versification by speciation. This notion can be conceptualized by paraphrasing Hans Jenny's (1941) factorial equation for soil formation: s =