Conclusions
Darwin described the complexity of nature as a tangled bank, where organisms depend on each other in “so complex a manner” (Darwin 1859). Burgeoning understanding of the largely cryptic role microbes play in host ecology and evolution further tangles an already tangled bank, challenging our classic views on adaptation, and potentially calling into question many of our long-standing assumptions about the causes of natural selection and traits underlying adaptation. Although microbial effects on adaptation complicate our ability to disentangle genotype vs. environment effects on phenotypes, it is an important field of study because microbial communities may also provide novel avenues of adaptation. As a result, understanding the role microbes play in adaptive responses has the potential to provide new approaches for increasing population resilience to environmental change in both natural and managed systems (Mueller et al. 2019). Ultimately, to harness the potential adaptive power of microbes requires carefully delineating microbe-mediated adaptation, testing it adequately, and eventually identifying the conditions favoring it. We hope this article stimulates discussion around these topics and provides a framework to accomplish these goals.