Parasites occur in virtually all environments, and they range in size and complexity from tiny pieces of mobile DNA, to viruses, bacteria, worms and ectoparasites such as ticks, mites and biting flies. Due to the sheer number and diversity of parasites, all living organisms are likely to encounter them at some stage of their life cycle. It is therefore not surprising that biologists are now recognizing that parasites are capable of exerting an influence at virtually every conceivable level of biological organization.

The study of parasites of both plants and animals is of enormous importance to many applied disciplines and human medicine, and in evolutionary biology parasites are arguably most famous for their role in the origin of sex. Parasites as a consequence have been attracting the attention of a growing number of workers in a variety of disciplines.

However, the astonishing fact is that we are still generally ignorant of the importance of parasites as agents of selection and evolution in natural populations. We know even less about the effects of an interaction between parasitism and environmental stress, which in turn severely limits our ability to predict the impact of parasitism and disease on population dynamics in changing environments, a growing concern among ecologists, conservation biologists, and epidemiologists.

A major endeavor of my laboratory has been the study of the ecological and evolutionary consequences of parasitism in natural populations. For many years we have been studying a system involving a drosophilid fly and its ectoparasitic mites in the Sonoran Desert of North America. 

By capitalizing on the unique experimental potential and accessibility of this system, we have been addressing the following questions:

• Parasite distributions: How prevalent, and hence potentially evolutionarily important, are ectoparasites in natural insect communities? How are parasites distributed among individuals hosts, and habitat patches? What are the seasonal and ecological determinants of variation in parasite burden? Can ectoparasites serve as vectors of transmissible parasitic disease?
• Fitness consequences: What are the costs of parasitism, expressed as damage to major host life-history traits? To what extent does parasitism interact with abiotic and other biotic stressors to influence host physiological and fitness responses to parasites?
• Defensive traits: How do hosts defend themselves against ectoparasitic attack? Is resistance general or specific? To what extent does host behavior mediate the outcome of host-parasite interactions?
• Heritability: Are host resistance traits heritable in natural populations? What is the scope for evolutionary response to parasitism and disease outbreaks?
• Ecological immunology: How is genetic variation in host behavioral and immune responses maintained in the face of parasite-mediated selection? Are there trade-offs between host defensive traits and other fitness traits such as fertility, ejaculate quality, life span and tolerance to other agents of disease? Are critical trade-offs modulated by environmental variation?