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Sewall Wright's shifting balance theory remains controversial in part because it is what would today be called a complex systems model that was never developed beyond that of a metaphor. A key component of this theory, the adaptive landscape, has become an important element in population genetics and evolutionary theory. This chapter explores the original metaphor of the adaptive landscape can be modified to make it a usable concept. It suggests that it is useful to consider the landscape to be a very high dimensional space that is conceptually helpful but of little practical use. The high-dimensional landscape potentially includes all aspects of the genotype and phenotype, including aspects of the social and physical environment that affect the phenotype. This broad conceptual visualization of an adaptive landscape is concordant with the diverse uses of this metaphor in evolutionary theory if we consider adaptive landscapes with only a few dimensions to be projections of the high dimension landscape on to the subset that are being used. This modified view of adaptive landscapes is used to discuss Wright's shifting balance theory and how it can be changed to accommodate modern experimental and theoretical findings.

Our understanding of Wright's shifting balance theory (SBT) has deepened and its empirical predictions have been enriched by the theoretical, field and laboratory experimental research. Whereas models with strictly additive genetic effects provide the basis for much of evolutionary genetic theory, studies of the molecular genetic basis of adaptations find that gene interaction is the norm. The impact of Wright's SBT will not be fully understood until we have a better understanding of how physiological interactions affect patterns of phenotypic variation. And, until we understand the theory, it will not be evident whether or not the adaptive landscape is an adequate representation of the adaptive process that Wright envisioned.

Two giants of evolutionary theory, Sewall Wright and R. A. Fisher, fought bitterly for over thirty years. The Wright–Fisher controversy forms a cornerstone of the history and philosophy of biology. The chapter argues that the standard interpretations of the Wright–Fisher controversy do not accurately represent the ideas and arguments of these two key historical figures. The usual account contrasts the major slogans attached to each name: Wright's adaptive landscape and shifting balance theory of evolution versus Fisher's fundamental theorem of natural selection. These alternative theories are in fact incommensurable. Wright's theory is a detailed dynamical model of evolutionary change in actual populations. Fisher's theory is an abstract invariance and conservation law that, like all physical laws, captures essential features of a system but does not account for all aspects of dynamics in real examples. This key contrast between embodied theories of real cases and abstract laws is missing from prior analyses of Wright versus Fisher. They never argued about this contrast. Instead, the issue at stake in their arguments concerned the actual dynamics of real populations. Both agreed that fluctuations of nonadditive (epistatic) gene combinations play a central role in evolution. Wright emphasized stochastic fluctuations of gene combinations in small, isolated populations. By contrast, Fisher believed that fluctuating selection in large populations was the main cause of fluctuation in nonadditive gene combinations. Close reading shows that widely cited views attributed to Fisher mostly come from what Wright said about Fisher, whereas Fisher's own writings clearly do not support such views.

Mimicry and aposematism are phenomena for which the concept of an adaptive landscape has proven helpful. In mimicry evolution, members of a species become similar in appearance to an aposematic model species and thereby gain increased protection from predation. A traditional suggestion is that mimicry evolves in a two-step process, where a large mutation first achieves approximate similarity to the model, after which smaller changes improve the likeness. In terms of adaptive landscapes, the process entails a mutational leap from the adaptive peak of the mimic-to-be, to somewhere on the slope of a higher, more protective peak of the model, thus crossing a fitness valley, followed by a series of smaller modifications climbing the higher peak. Alternatively, evolutionary forces other than mimicry, including genetic drift, may modify the appearance of mimics-to-be, perhaps exploring different peaks of an adaptive landscape in a shifting balance process, and fortuitously bringing about sufficient resemblance to a model to start off mimicry evolution. This chapter reviews and evaluates these ideas. It emphasizes the possibility that mimicry is initiated by a mutation that causes prey to acquire a trait that is used by predators as a feature to categorize potential prey as unsuitable. The theory that species gain entry to mimicry through feature saltation can help in formulating scenarios of the sequence of events during mimicry evolution and in reconstructing an initial mimetic appearance for important examples of mimicry.

This chapter contains a discussion of the ecological and the genetic basis for Sewall Wright’s Shifting Balance Theory. It also relays discussions of the author with Wright about how to experimentally test his theory using flour beetle metapopulations. The experimental design used by Wade and C. J. Goodnight to test Wright’s theory and to estimate realized group heritabilities is illustrated. The surprizing finding that interdemic selection every-other generation produced a larger response than selection at every generation was interpreted as evidence of non-additive effects on population mean fitness.

Warning colour patterns in Heliconius show great diversity as well as convergence due to mimicry. This chapter considers the origins of such diversity. Heliconius have been proposed as an example of the ‘shifting balance’, an evolutionary model involving a balance between drift and selection. Although alternative mimicry patterns can move around the species range (Phase III), there is only weak evidence for genetic drift causing evolutionary novelty in wing patterns (Phase I). There is also weak evidence for the origin of novel patterns in Pleistocene forest refugia. Sequencing of the genes that control wing pattern diversity has revealed a surprising and complex history. Within-species disjunct populations with similar wing patterns show a common origin, suggesting a complex history of movement of patterning alleles across species ranges. Between species, convergent mimetic forms share alleles, likely due to adaptive introgression. Closely related species therefore evolve mimicry by sharing of alleles rather than independent convergence. These patterns contrast with phylogeographic and phylogenetic relationships inferred from the rest of the genome, and suggest caution in inferring the history of adaptive traits from ‘neutral’ molecular markers.

In a finite population, drift is often more important than selection in removing any initial additive variance. This chapter examines the joint impact of selection, drift, and mutation on the long-term response in a quantitative trait. One key result is the remarkable finding of Robertson that the expected long-term response from any initial additive variance is bounded above by the product of twice the effective population size times the initial response. This result implies that the optimal selection intensity for long-term response it to save half of the population in each generation.