The study of morphological integration can yield important insights into the interplay between intrinsic and extrinsic factors in evolution. In this dissertation I study the role of internal constraints, as reflected by morphological integration, in determining ruminant diversification. In the last three decades, the evolutionary implications of morphological integration have been commonly studied from the microevolutionary perspective of quantitative genetics. Predictions and generalizations based on this framework are complicated by two main issues: the evolution of the covariation structure itself, and the idiosyncratic history of selection. Despite the importance of integration in bridging the micro- and macro- evolutionary levels, both the issue of its stability and the issue of its long-term evolutionary implications remain largely unresolved. My dissertation addresses these problems from a macroevolutionary perspective. I examine the evolution of integration pattern across the phylogeny of ruminants, using the integration space as the basis for phylogenetic comparative methods. Using a phylogenetic decomposition of the variation contained in these axes, as well as evolutionary model-fitting procedures, I show that closely related taxa differ greatly from each other in their pattern of integration---more than expected from their phylogenetic distance---which suggests that integration structure can respond to localized selective pressures. At the same time, variation among higher-level clades still reflects their history of common descent, indicating that the effect of selection is limited. I develop an interpretation of evolvability as the ability of the population to respond to a wide range of selective pressures, measured by the strength of integration, and I explore the utility of this approach in making long-term predictions regarding the association of integration and evolution in ruminants. The advantage of this approach is that it relies on the integration level, rather than the pattern, which is easier to reconstruct and more likely to be stable because it is a univariate rather than a multivariate quantity. Moreover, its effect on the long-term potential of populations to evolve and diversify depends on the heterogeneity of selection vectors throughout the history of a taxon, which should be easier to estimate than the exact direction of selection at any given time. Using phylogenetic comparative methods I reconstruct the ruminant mode and rate of morphological diversification as well as species-level anagenetic rates, and I show a non-random, albeit complex, relationship between these macroevolutionary metrics and integration level. Thus, my results suggest that the level of integration is the relevant property of covariation structures for long-term evolution, and has the potential to facilitate our understanding of the interplay between intrinsic and extrinsic factors. This dissertation includes two supplemental files. One is a list of specimen information, including specimen catalogue number, sex, and taxonomic identification. The second file includes the three dimensional landmark configurations digitized from each of the specimens included in the first file.
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