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.