This proposal has two research goals. One is to study the genetic changes that may be important in the formation of species and the other is to study the factors that may influence the membership in communities. A related group of yeast species which live in decaying plant parts will be used in the investigation. Genetic studies of the yeasts will include evaluation of chromosome, enzyme and DNA changes, while ecological studies will include investigations on host chemicals, insect relationships and yeast-yeast competitive interactions. This research should be important to basic science because it will help us understand how species are formed and why species live in specific habitats. Indirectly, the research will help clarify insect-microorganism relationships that might aid in controlling pests as well as to help understand how to manage plant pathogens.