An Essay: The Stimulus of Unusual Geologies for Plant Speciation

Though a region's climate sets the limits for a biota, geology enriches local discontinuity 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 lithologies-an absence of a mix of sedimentary, metamorphic, and igneous rocks, rich in variant chemical and physical attributes. Without a wealth of geological diversity, the only physical variable on the planet might be the continuous change of climate from poles to equator. Life would then have evolved a dull continuum of response in its tracking of gradual climatic change across a monotonous landscape. But the real biosphere is rich in organic diversity, 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 continents to regional and local diversity created out of variant topographies and lithologies. Crucial to the argument that geological diversity begets biological diversity, is the realization that geological events and materials are often arrayed discontinuously. It is this discontinuity that sets the stage for speciation; without isolation, much of the world's discrete biota-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 diversification by speciation. This notion can be conceptualized by paraphrasing Hans Jenny's (1941) factorial equation for soil formation: s = f(cl, o, r, p, t). Soil formation is taken as a function of the five independent variables: climate, organisms, topography, parent materials (rocks), and time. Jack Major (1951) adapted the formulation to account for the genesis of vegetation, v = f(cl, o, r, p, t). I propose taking Jenny's functional, factorial approach one step farther, to account for plant species diversity in any region. The factorial equation for botanical (or biological) diversity now would read, B.D. = f(cl, o, r, p, t). Of course, adapting the Jenny equation to biotic diversity has its pitfalls. The most serious omission is the genetic factor. Without the generation of continuity and variety by genes, the biotic diversity, though stimulated by environmental variables, cannot be realized, let alone maintained. For biotic diversity, the equation may call for another independent variable, heredity. Whether applied to soils, vegetation, or to species diversity, the formulation depends on the action and interaction of the five environmental variables, all of which are under the influence of geological processes and states. Though it is obvious that parent material (p, or rock type) and topography (r) are geological in origin, it can also be argued that the other variables are under the influence of geology. Local climates, though limited by the regional climate, are generated by diverse topographies. And of course, microclimates are the direct