In this thesis I investigated selective pressures that could shape the ecology and evolution of the malaria parasite and its mosquito host. I used the rodent malaria parasite Plasmodium chabaudi, laboratory mice and the lab adapted malaria vector Anopheles stephensi. Key theory underpinning much of this work is the virulence trade-off hypothesis, where virulence (harm to host) is thought to be an unavoidable consequence of the parasites effort to optimise fitness. Implicit in this theory is that virulence is an intrinsic parasite trait, so that a virulent parasite would be relatively virulent in all host types. Using four genetically distinct P. chabaudi clones and four distinct host strains I found for the most part, this assumption to be true; the virulence phenotype of each of the four parasites was the same across the four host strains tested. I further used these genetically distinct P. chabaudi clones along with candidate malaria vaccine AMA-l to study 1) vaccine efficacy against single and mixed clone infections and 2) the potential selective effects of vaccination. I found that vaccination reduced parasite density in a strain specific manner and that diversity at loci other than the target antigen influenced vaccine efficacy. In both narve and vaccinated hosts, selection for increased virulence was observed in mixed genotype infections, but in no case did vaccination enhance this selective pressure. As Plasmodium evolution is intricately linked to vector population dynamics, I explored environmental conditions that may influence Anopheles fitness. Specifically, I investigated whether the amount of food Anopheles parents experienced influenced the success of their offspring. I found that the food level of both parents and offspring were important in Anopheles fitness. In particular, daughters from parents reared in low food conditions produced more eggs that daughters from parents reared in high food conditions. In sum, my results indicate that the within-host environment of the malaria parasite can act as a selective agent. In addition, the environmental conditions experienced by Anopheles larvae could impact vector population dynamics. Altering the within-host environment of the parasite or the environment of its vector could dramatically alter the ecology and evolution of the malaria parasite.

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