Genotypic and Phenotypic Variation

Sessile organisms cannot escape environmental vicissitudes. One would expect to see very good examples of adjustments to stimuli and extremes among these organisms. Focusing on plants, Givnish (2002) reviews the basic responses in various contexts to spatial and/or temporal variation, summarized below and with elaboration in the subsequent sections.

First, when individuals of a species experience relatively the same environment throughout their lifespan, locally adapted genotypes producing relatively invariant (canalized) phenotypes (ecotypes or races, below) are expected. A classic example is Achillea adapted to different elevations in California (Clausen et al. 1948), noted at the outset of the chapter. Another example is the intensive research over many sites and years on plants tolerant to heavy metals growing on mine tailings (e.g., Antonovics et al. 1971).

Under such pervasive and relatively extreme conditions, plasticity would not be advantageous. Invariant phenotypes (phenotypic generalism) would also be the optimum strategy when the environment varied rapidly relative to the potential response time of an individual, or where a generalist phenotype conferring intermediate fitness across multiple environments outperforms, overall, that of alternative specialist phenotypes (Levins 1968, pp. 21-22; Moran 1992). A somewhat analogous example from microbial ecology pertains to bacteria specialized for growth in various extreme environments (‘extremophiles’ in bacteriology jargon), such as acid mine drainage, which is characterized by high concentrations of toxic metals, heat, and extreme acidity (e.g., with pH < 1.0; Baker and Banfield 2003). These and other Bacteria and Archaea in extreme habitats prosper at the physiological limits of life (Madigan et al. 2015).

Second, where individuals are exposed to an environment that varies principally spatially, phenotypic plasticity is expected. Many excellent examples are found among clonal organisms that encounter patchy habitat horizontally or vertically as they expand (see Chap. 5 and van Kleunen and Fischer 2001; de Kroon et al. 2005).

Third, where the environment varies temporally, such as in the seasonal drying of a pond or onset of winter, plasticity should also be favored. This assumes that the adaptable phenotypes can respond effectively, producing competitive advantages over invariant phenotypes.

Finally, when environments vary significantly in space as well as time—a frequent occurrence—plasticity should be favored over invariance, again subject to the caveat of the ability to effectively track or anticipate environmental change. Phenotypes may vary as a continuous function of the environment, or they may alternate in a switch-like fashion as a critical threshold is reached (note later discussion of dimorphic fungi). Some organisms may deploy multiple phenotypes in the absence of environmental information, thereby increasing the likelihood that at least one is suited to any prevailing environment (this is one form of bet-hedging; see later discussion of this topic and Moran 1992; Gremer et al. 2016). Givnish (2002) emphasizes that the important criterion in assessing the costs and benefits of plasticity is relative competitive superiority over a range of environments, not the absolute extent of plasticity.

His and the related conceptual work of others builds on the foundation of theoretical genetics laid by Levins. In a series of now classic papers and a book in the 1960s, Levins (e.g., 1965, 1968) laid out the genetic and phenotypic basis underpinning population dynamics in fluctuating environments, alluded to at various points in this chapter.