Fossils and mutations

Overproduction and limited resources

Gene and allele differences in organisms

Natural selection and descent with modification

Members of a single species that may, or may not, share a common space

Members of the same community

Interbreeding members of the same species that share a common space

Multiple species interacting together in an ecosystem

By targeting their host cell for cell death

By digesting the host cell they infected

By taking the nutrients of the host cell they infected

By using the cellular machinery of the host cell they infected