Processes and patterns of interaction as units of selection: An introduction to ITSNTS thinking - PubMed (original) (raw)

Processes and patterns of interaction as units of selection: An introduction to ITSNTS thinking

W Ford Doolittle et al. Proc Natl Acad Sci U S A. 2018.

Abstract

Many practicing biologists accept that nothing in their discipline makes sense except in the light of evolution, and that natural selection is evolution's principal sense-maker. But what natural selection actually is (a force or a statistical outcome, for example) and the levels of the biological hierarchy (genes, organisms, species, or even ecosystems) at which it operates directly are still actively disputed among philosophers and theoretical biologists. Most formulations of evolution by natural selection emphasize the differential reproduction of entities at one or the other of these levels. Some also recognize differential persistence, but in either case the focus is on lineages of material things: even species can be thought of as spatiotemporally restricted, if dispersed, physical beings. Few consider-as "units of selection" in their own right-the processes implemented by genes, cells, species, or communities. "It's the song not the singer" (ITSNTS) theory does that, also claiming that evolution by natural selection of processes is more easily understood and explained as differential persistence than as differential reproduction. ITSNTS was formulated as a response to the observation that the collective functions of microbial communities (the songs) are more stably conserved and ecologically relevant than are the taxa that implement them (the singers). It aims to serve as a useful corrective to claims that "holobionts" (microbes and their animal or plant hosts) are aggregate "units of selection," claims that often conflate meanings of that latter term. But ITSNS also seems broadly applicable, for example, to the evolution of global biogeochemical cycles and the definition of ecosystem function.

Keywords: evolution; microbiome; natural selection; persistence; process.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

A cartoon illustrating ITSNTS. Hand-drawn arrows are meant to represent the steps or components of a cyclical process with intermediate metabolites or stages A–D. Shaded symbols (rectangles, ellipses, and so forth) are microbial taxa comprising guilds that implement each step. The process or interaction, by existing, encourages the evolution of entities (taxa) that implement its steps, and these in turn reconstruct the process or interaction as a whole. The process thus persists, and changes in the mode of implementation that promote persistence are selected for. (As discussed briefly in the text, a similar scheme could be developed for developmental or regulatory interactions and the genes that carry them out.)

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