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The Unionoida, commonly known as freshwater pearly mussels or naiades, is a diverse, ancient order of strictly continental, freshwater bivalves. Most previous discussions of freshwater mussel evolution pre-dated the widespread acceptance of phylogenetic systematics and modern biogeographic theory. As a result, our understanding of the macroevolutionary processes behind the present diversity has been limited to untested narratives. This is unfortunate, as the age, distribution, and diversity of freshwater mussels makes them useful for studying divergences ranging from the Mesozoic to the Quaternary. This dissertation documents four cladistic studies of the Unionoida. Because no single character or taxon set is appropriate across all levels of freshwater mussel phylogeny, four specific topics are addressed using separate but overlapping analyses of morphology and mitochondrial and nuclear DNA. The first analysis (Chapter 2) applies a strictly morphological character set to test the position of the Hyriidae among the Unionoida. Chapters 3 and 4 address the phylogeny and brooding character evolution among the Nearctic Unionidae using a combined (mitochondrial + nuclear) character set. Chapter 5 returns to the Hyriidae of the Australasian and Neotropical regions, using molecular characters to test hypotheses of biogeographic process. Chapter 6, the last analytical chapter, tests the position of the Nearctic genera relative to the global Unionoidea. The seventh chapter reviews the macroevolution of the Unionoida from a phylogenetic perspective based on the results of these four studies.

Several related theories that predict optimal reproductive and survival strategies are examined in detail. First, theories regarding the evolution of sex determination in reptiles are analyzed. Then, various optimal egg size theories and their assumptions are examined, utilizing data from numerous taxa. Tests of a prominent offspring sex allocation theory are also reported. Finally, results of predator-prey experiments are analyzed. Allozyme data are used to test criticisms of two recently proposed theories that purport to explain interspecific patterns of sex determination mode in reptiles. Phylogenetic analyses suggests that change between modes is common, thus the current pattern is not due to simple conservation of ancestral mode. Temperature-dependent differential fitness remains the most plausible hypothesis, although there are few data that are clearly supportive. Longevity is estimated in a small group of geckos (Phelsuma) with both sex determination modes. Contrary to predictions, there is apparently little difference in longevity between species with different modes. Turtles appear to fit the explicit assumptions of optimal egg size models particularly well, yet they do not support the models' predictions. There are numerous implicit assumptions of the models. A strong correlation between body size and egg size is demonstrated using meta-analysis. If this correlation has a genetic basis, selection may not be able to act on egg size without also affecting body size. One prediction from sex-allocation theory is that under certain conditions, females with above-average levels of resources bias their offspring sex ratio towards sons. Results reported here demonstrate that white-tailed deer (Odocoileus virginianus) behave according to this theory, while a fruit fly (Drosophila melanogaster) does not. It is suggested that Drosophila may not have the ability to adjust offspring sex ratio, an ability assumed by the theory. The relationship between tail autotomy, body size, and risk of predation in the gecko Coleonyx variegatus is examined experimentally. Contrary to both predictions based on field observations and the findings of previous researchers, neither factor appeared to influence capture success by the night snake (Hypsiglena torquata).