UC Santa Barbara

The controversy around questions like "Is the holobiont (host and all its microbes) a unit of selection?" betrays our field's confusion. Terms like "holobiont" and "unit of selection" are vague and vary between researchers. We bring clarity by building a mathematical theory of holobiont evolution and multilevel selection. This theory is expressive enough to precisely ask questions like the one we posed and strong enough to calculate an answer. Through it we prove that a general model of holobiont evolution is equivalent to the more traditional replicator-mutator equations. This demonstrates that individual and multilevel perspectives of evolution are mathematically equivalent.

These investigations also raise an unexpected side-question: how should a parent optimally distribute limited resources among its offspring? This is as relevant to a mother bird splitting food between her chicks as it is to a bacterium dividing high-quality organelles between its daughters. These are the theories of optimal offspring size and bacterial aging respectively, and we unify the two through a resource distribution model. From this we uncover conditions which select for birthing a "runt of the litter."