Fig. 2.

Host–microbiome interactions can affect both health and fitness. Dysbiosis is associated with a number of negative health outcomes, including obesity, asthma, and certain cancers. Negative health outcomes are not sufficient evidence for coevolution of the microbiome and host, however. Not all of these diseases result in negative fitness consequences by limiting reproductive success. Microbiomes potentially impact host fitness at multiple stages of life by affecting survival through reproductive years or reducing fertility. In infancy, microbes extract energy from non-digestible components of milk, increasing nutrient acquisition at this vulnerable age. During childhood, a stable microbiome prevents invasion of deadly pathogens. In adulthood, the microbiome potentially influences fertility, either by altering nutrition or causing disease. Finally, the microbiome may be important for lifespan. Although lifespan after menopause will not result in more children, the grandmother hypothesis predicts that care of extended kin results indirectly in higher fitness [139]. IBD inflammatory bowel disease