Mitchell B Cruzan and Jennifer M Rhode

Abstract: The metaphor of a fitness landscape of peaks and valleys has been a valuable heuristic tool for understanding the process of adaptive evolution. Wright envisioned landscapes of high relief, with fitness peaks separated by deep troughs and evolution occurring via peak shifts. Recently, Gavrilets extrapolated Wright's model to a multidimensional scale and demonstrated that high-fitness ridges connecting peaks may be a common feature of landscapes. In this view of a "holey adaptive landscape", populations can traverse ridges through mutation-drift processes alone and become reproductively isolated when they come to reside on opposite sides of a fitness "hole". In this paper, we discuss existing empirical evidence and experimental approaches to the assessment of adaptive landscape topography. We provide an example of landscape topography analysis using fitness distributions of parental and hybrid genotypes of herbaceous perennials in the Piriqueta caroliniana complex. In this study, the mean fitness of backcross hybrids under field conditions was relatively low, but some recombinant hybrids had fitnesses equal to or exceeding parental types. While these data are consistent with the presence of a low-fitness "hole" and a high-fitness "ridge" separating the parental types, details of the intervening topography are lacking. We propose that more rigorous analyses that integrate genomic approaches with experimental tests of fitness distributions will lead to a better understanding of fitness topographies and the process of adaptive evolution.

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