The study of adaptation marks a central focus in evolutionary biology, with one major thrust of these endeavours being to determine the relative importance of adaptive evolution in diversification. When putative adaptive traits have a conspicuous effect on reproductive isolation, e.g., traits that affect pollinator preferences (Schemske and Bradshaw 1999) or key innovations such as nectar spurs (Hodges and Arnold 1995), the link between adaptation and speciation is easiest to draw. However, less conspicuous features of organisms that have effects on fitness and are subject to natural selection may also contribute to speciation and diversification of lineages. For example, a number of plant species are known to occur only under specific, typically extreme soil conditions, being excluded from other substrates (Kruckeberg 2002). The range of extreme soils that harbour edaphic endemics includes hyper-saline soils such as vernal pools and alkaline flats, soils rich in heavy metals, such as mine tailings and serpentine soils, and nitrogen-rich sites such as guano deposits. Where edaphic endemics represent derived expansion and specialization onto these substrates, adaptation to specific soil conditions may have played a key role in diversification (Macnair 1987; Macnair and Gardner 1998). The link between edaphic adaptation and reproductive isolation may be achieved through various means, including direct impacts on reproductive isolation, for example via pleiotropy or linkage, and indirect effects, e.g., reinforcement, accumulation of pre- or post-zygotic isolating barriers in allopatry.
We wish to focus attention on the potential significance of edaphic divergence in plant speciation. We begin with a synopsis of some key aspects of the biology of edaphic specialists that contribute to their value as systems for the study of adaptation and speciation. We focus in particular on parallel adaptive shifts because these cases provide key opportunities to study the selective pressures and genetic mechanisms that contribute to the establishment of adaptation. In addition, cases of parallel adaptation provide the chance to study the relationship between adaptation and reproductive isolation with the benefit of natural replication. We then summarize our recent research characterizing edaphic specialization in the Lasthenia californica complex, which we believe is among the best documented cases of parallel evolution of edaphic specialization in plants. Although the Lasthenia work characterizes parallel adaptation in greater detail than has been done in many other systems, it seems clear that parallel shifts are likely a common outcome of edaphic specialization. Whether parallel adaptation is commonly accompanied by parallel reproductive isolation remains unclear, but further research in this area will likely provide valuable insights into the role of natural selection in diversification.
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