Multiple generations of hybridization between populations of the intertidal copepod Tigriopus californicus

For the intertidal copepod Tigriopus californicus, outbreeding depression for a variety of fitness measures is typically observed in early-generation interpopulation hybrids. This dissertation is an experimental approach to look at morphological, fitness and molecular outcomes of mixed populations following multiple generations of mating. Each chapter expands upon our understanding of the long-term, multi-generational outcomes of hybrid swarms that were initiated with populations showing different degrees of incompatibility.;Chapters 1 and 2 examined early-generation controlled crosses and long-term hybrid swarms. In Chapter 1, crosses between Royal Palms and San Diego, California, showed that only F2 cohorts exhibited significant declines in fitness compared to midparent values. Chapter 2 utilized more divergent populations from Playa Altamira and Punta Morro in Baja California, Mexico. F1 and F2 hybrids showed large declines in survivorship, while backcrosses produced no offspring. Long-term hybrid swarms in Chapter 1 exhibited early fitness declines followed by rapid recovery and surpassing of midparent fitness. Microsatellites revealed extensive introgression. In contrast, highly divergent populations in Chapter 2 showed fitness recovery that was likely due to genetic swamping by the superior parent.;Chapters 3 and 4 investigated the same populations as Chapter 1 but included two environmental treatments: benign conditions versus salinity stress. Cultures were initiated with equal numbers from each source population and allowed to mate freely while generations were kept discrete. For both survivorship and metamorphosis, early generation heterosis was followed by outbreeding depression and recovery, which occurred up to two generations earlier in the high salinity treatment. Microsatellites and male morphology both indicated that swarms became more RP-like over time. High salinity replicates displayed stronger repeatability for both molecular and fitness character. High salinity hybrids were more fit than benign replicates when exposed to a novel stress. Evidence for nuclear-nuclear and nuclear-cytoplasmic coadaptation was observed at generation seven.;This work supports the hypothesis that hybrid breakdown in early generations may be a temporary phenomenon followed by the persistence of highly fit genotypes. While selection may have a deterministic role in hybrid swarm evolution, it is likely to be hampered by drift resulting from environmental fluctuations that produce frequent bottleneck events.