Together microbes are the greatest source of biomass on Earth. Collectively their metabolic power is great. Even though a limited number of microbes are capable of causing disease, microbes are totally indispensable for mankind.
Genetic variation mechanisms cause genetic change in the microbial cell. The selection pressure from changing environmental factors saves those microbes that adapt. Microbes live in the microenvironment where they may interact with other microbes in a consortium. The small size of microbes has impact on fitness as it affects metabolism, generation time, colonization ability and growth rate.