Through the microbial looking glass: our shifting understanding of the holobiont and microbes as mediators of organismal biology

Our understanding and appreciation for the role of microbial symbioses in the evolution and speciation of macro-organisms has fluctuated over time. Even in contemporary times, our collective understanding of the microbiome has shifted from an adaptationist focus on beneficial interactions, to better incorporate ecological, stochastic, and neutral interactions. From the 1900s to today, the holobiont concept has repeatedly been proposed as a guiding hypothesis and principle. However, it has also received continuous dispute and critique, often regarding the universality of processes, fidelity of partnerships, and lack of explicit delineation of holobionts from individual units. Modern interpretation of the holobiont is meant to be an extension of multilevel selection theory, and natural selection need not always act at this collective level. The solution may be for scientists to remove conceptual mind blocks and consider a hazy, flexible, and dynamic definition of the holobiont. Such a frame of mind will be similar to the lack of clarity generally surrounding the biological species concept-a muddled yet unifying concept. Viewing the world through the "microbial looking glass" of the holobiont will offer a more accurate understanding of biological systems and will facilitate research progress.