Evolution under Extreme Edaphic Conditions

Plant species or distinct populations belonging to certain species can often be distinguished by their faithfulness to particular edaphic conditions (Figure 5). Plants that grow on chemically or physically extreme substrates are often derived from populations found off such substrates, suggesting the role extreme soil conditions can play

Figure 5 An edaphically controlled vegetation boundary at Jasper Ridge Biological Preserve, San Mateo County, California. The yellow-flowered Lasthenia californica (Asteraceae) is restricted to serpentine soils. The sharply demarcated boundary between L. californica and grasses is defined by a serpentinesandstone transition. Credit: Bruce A. Bohm.

Figure 5 An edaphically controlled vegetation boundary at Jasper Ridge Biological Preserve, San Mateo County, California. The yellow-flowered Lasthenia californica (Asteraceae) is restricted to serpentine soils. The sharply demarcated boundary between L. californica and grasses is defined by a serpentinesandstone transition. Credit: Bruce A. Bohm.

in generating plant diversity. Influential work conducted during the mid-twentieth century on the grasses of heavy metal-contaminated mine tailings provides a classic demonstration of the role natural selection plays in maintaining diversity. This work, and subsequent work on many plant species, demonstrate that populations can evolve tolerance to extreme edaphic conditions and that this may lead to reduced gene flow between the ancestral population and the divergent, edaphically specialized, population. Such reproductive isolation, followed by further divergence, sets the foundation for the origin of new plant species. Plants that have either evolved in situ (i.e., neoen-demics) or those that have had broader distribution but are currently restricted to extreme substrates (i.e., paleoendemics) are called edaphic endemics. While most plant species can be found under a range of edaphic habitats, it is these edaphically specialized taxa and their ancestral species that have attracted the attention of plant physiologists and evolutionary ecolo-gists alike.

Edaphic endemics provide a model system to examine the process of plant evolution from adaptation and reproductive isolation to genetic divergence. Closely related species pairs are often distinguished by their distinct edaphic preferences. Such pairs can be found on adjacent yet contrasting soils formed naturally due to variation in parental rocks or by anthropogenic acts such as quarrying, mining, and even depositing of chemical waste in landfills. The process of divergence might proceed as follows: some individuals of a species have genetically determined traits that allow them to successfully survive in adjacent, chemically harsh soils. These individuals could become founders of a distinct population characterized by their tolerance to the extreme edaphic condition. Such a transition to a new habitat, if accompanied by a reduction in gene flow, can bring about full-fledged speciation. Evolution of tolerance to extreme conditions can occur quite rapidly, even within a few generations. Current phylogenetic analyses provide strong support for rapid evolution of edaphic specialists as recently illustrated for the species pair Layia glandulosa-L. discoidea (Asteraceae).

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