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This chapter analyzes how the adaptive landscape concept can be extended from a single population in a single environment to multiple populations in multiple environments. Specifically, different environments produce different fitness peaks and divergent selection then drives different populations toward those different peaks. The chapter outlines methods for inferring adaptive divergence with respect to both phenotypes and fitness. It then turns to a review of empirical data informing several key questions about adaptive divergence in nature, including how prevalent and strong it is, how many peaks adaptive landscapes have, how predictable it is (parallel and convergent evolution), and what the role of sexual selection is in modifying adaptive divergence.

This chapter outlines three basic ways in which humans can alter evolution on adaptive landscapes: through changes in topography, changes in dimensionality, and phenotypic excursions. Changes in topography involve the numbers, positions, gradients, and elevations of surface features on the landscape, such as peaks and valleys. Changes in dimensionality involve the number of at least partially independent traits under selection. Excursions typically involve more or less abrupt changes in the phenotypic position of populations on existing adaptive landscapes, such as through plasticity, hybridization, or genetic manipulation. These different types of change can generate predictions for changes in selection and alterations in evolution — assuming the population can persist through the disturbance. Invasive species can have all of these classes of effects, either for the invasive species or for native species. Climate change will most obviously involve a shift in peak position, such as breeding times under warmer temperatures. Hunting/harvesting will also often involve a shift in peak position, particularly toward smaller and slower growing individuals, and might also decrease phenotypic variance. Habitat loss and fragmentation will influence numbers and positions of adaptive peaks, and can also influence excursions by altering patterns of gene flow in meta-populations. Finally, a decrease in habitat quality can decrease the heights of fitness peaks and cause adaptive landscapes to become smoother. It can also change dimensionality, such as through the introduction of a new contaminant. In conclusion, viewing human-induced environmental change in the framework of changes to adaptive landscapes offers new insights and new perspectives for research.

This chapter looks at empirical methods for quantifying gene flow and inferring its role in adaptive divergence. An important point made therein is that gene flow can sometimes aid adaptation, such as when it enhances the genetic variation on which selection acts. The key questions addressed with empirical data are divided into the potential negative versus positive effects. On the negative side, questions include to what extent gene flow constrains adaptive divergence among environments, and how the resulting maladaptation might cause population declines and limit species' ranges. On the positive side, questions include whether gene flow has a special benefit in the case of antagonistic coevolution, and whether it can save (rescue) populations that would otherwise go extinct.