If organisms and genomes were completely separated from one another for all of their lives, then evolution would be relatively simple, as would be the discussion of evolution. However, almost all organisms live either in close proximity, attached or enveloped by other cells and organisms. Most could not survive without the presence of the other organisms. These interactions cause a variety of effects on evolution and genomes. The simplest interactions are those that involve an organism that becomes dependent on another organism for some biological process or particular molecules. For example, humans are dependent on gut microorganisms for much of their digestion of food. But, microbes also have analogous associations (Figure 16.1). Some microbes become dependent on other microbes for some of their nutrition. Some cells are in contact with cells of another species for much (or all) of their life cycles. These associations can range from coincidental to pathogenic. However, pathogens that do not kill their hosts ensure that a ready supply of host organisms will be available for infection by some of their progeny. Therefore, this evolutionary relationship is relatively stable over extended periods of time, although genetic changes are inevitable (Figure 16.1). Other symbiotic relationships can develop as well. Commensal relationships are common. In this form of symbiosis, one of the species gains from the other, but has no effect on the other organism. However, often, it becomes dependent on its host and begins to lose genes as they become unnecessary (Figure 16.1).