The evolutionary theory of adaptive radiation posits that lineages that experience a breadth of available resources in the absence of competition, known as ecological opportunity, should diversify to specialize on aspects of these available resources. The rapid evolution decelerates as niches fill in a static, spatially limited system, resulting in an assemblage of ecologically distinct species. Despite evidence to support this mode of diversification, little attention has been given to how this process unfolds in systems with multiple, ecologically similar colonizing lineages. The primary-colonizing, or incumbent, lineage, through exploiting niches in the absence of competitors, may serve to depress the rates and patterns of species and ecological diversity of subsequent colonists. In this dissertation, I explored four aims that seek to test whether the evolution of two clades of rodents endemic to Luzon Island, Philippines, Chrotomyini and Phloeomyini, exhibited evolution consistent with incumbency effects held by Phloeomyini and placed on secondary-colonizing Chrotomyini. First, I determined whether the rates of lineage diversification of the two Luzon Old Endemic (LOE) clades were consistent with reduced ecological opportunity in secondarily-colonizing Chrotomyini, resulting in lower rates of species accumulation. My results instead indicate that Chrotomyini has experienced a faster rate of diversification inconsistent with incumbency effects. Second, I tested whether the mandible of the LOE rodents, as a proxy for diet, exhibits rates of evolution consistent with lower ecological opportunity for Chrotomyini as well as patterns of diversity consistent with clade-specific partitioning of morphological variation. I found that both LOE clades evolved disparate mandible shapes at a similar rate, apart from outlying genus Rhynchomys, but that the two clades occupy nearly discrete areas of morphospace. Third, I tested whether the shape of the humerus can be used to approximate locomotory niche in a similar way to linear measurements of the ulna, metacarpal, and phalanx, to determine whether the morphology associated with locomotory strategy in the two LOE clades is convergent on shared locomotory mode. I found that although the humerus predicts some aspects of locomotory strategy, a substantial proportion of shape variation is reflected by different adaptations within shared locomotory category, thus providing a complement to, rather than replacement for, distal forelimb measurements. Finally, I tested whether the observed lack of mandibular shape overlap between the two LOE clades is consistent Chrotomyini being limited by Phloeomyini in terms of the area of morphospace it could diversify into and whether the ancestral chrotomyine lineage may have exhibited morphology disparate from Phloeomyini, thus facilitating its colonization and subsequent diversification. I found that the patterns of mandibular shape variation in the two LOE clades are consistent with the establishment of a biotic filter, meaning that Chrotomyini’s success on Luzon was facilitated by persistent ecological distinction from incumbent Phloeomyini. This dissertation illustrates that subfamily-related clades can experience substantial ecological distinction both within and between each clade. This distinction can permit repeated colonization of spatially constrained systems: as long as each colonizing clade remains ecologically distinct, evolution may proceed uninhibited by inter-clade competitive effects. Incumbency effects may thus more strongly influence the community assembly of species in a system than their evolutionary rates.
University of Minnesota Ph.D. dissertation. May 2019. Major: Ecology, Evolution and Behavior. Advisor: Sharon Jansa. 1 computer file (PDF); xiv, 216 pages.
The Roles of Ecological Opportunity and Incumbency Effects in the Macroevolution of the Luzon Island, Philippines “Old Endemic” Murine Rodents.
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