Inbreeding depression in small isolated populations is a major concern in conservation breeding research. While outbreeding can relieve the symptoms of inbreeding depression in small populations, outbred populations can also experience reduced fitness through negative intrinsic and extrinsic interactions. To understand the impact of outbreeding between divergent populations I used the housefly, Musca domestica, as a model system to simulate captive populations. I evaluated the effects of outbreeding on isolated populations under various situations.

Chapter One examines the impact of outbreeding on isolated populations that had different histories of migration and selection. Results demonstrated significant heterotic effects in the outbred lines compared to parent populations. Later generation outbred lines experienced reduced fitness, suggesting hybrid breakdown. While historical levels of migration and selection did not affect initial parent fitness, initial parental fitness was found to influence offspring fitness. Historical levels of migration and selection also did not directly affect the level of heterosis in outbred populations.

Chapter Two further investigates the consequences outcrossing on populations that have different levels of isolation. Outbred and backcross populations generally had higher or equal fitness than their parental counterparts, indicative of potential heterosis. Possible outbreeding depression was again identified in the F₃ outbred offspring generation, which had lower fitness than previous outbred generations. No clear trends relating length of isolation to hybrid fitness were discovered, but an increase in fitness in outbred populations suggests divergence occurred between parent populations. I also found further support that the fitness of outbred offspring was positively correlated to initial parent fitness.

Finally, Chapter Three explores the long-term evolutionary potential of hybrids. Outbred lines experienced higher fitness, which was maintained for multiple generations. Results also determined that more parent populations, rather than outbred populations, had the tendency to go extinct during the experiment. While both parent and outbred lines adapted to both novel and control environments, outbreeding did not facilitate faster adaptation to a novel environment.