"Constraints on communities and characters"
In the late 1990’s, Sydney Brenner lamented the fact that the molecular biology revolution of the 60’s had diminished the dual role of theory and experiment in answering big questions in biology. Can we predict evolutionary dynamics? How do ecology and evolution interact to give rise to the complexity we observe in Nature? In this talk, motivated by past successes in physics, I argue that understanding constraints on biological systems provides a path forward to reconnecting theory and experiment in biology. I present two vignettes which illustrate the power of elucidating constraints at two levels of biological organization. First, I present a study of abundance dynamics in a complex three-species microbial community consisting a photosynthetic alga, and bacterium and a microbial predator. Understanding how interacting communities of microbes respond to changes in their environment is important for disciplines from climate change to human health but due to strong, and often highly non-linear, interactions between organisms community response to perturbations is challenging to predict. Through a series of experiments we reveal two constraints on abundance dynamics in this system: (1) the community exhibits surprisingly deterministic dynamics on very long timescales, and (2) the community responds coherently along "ecomodes" when subjected to changing light levels [Phys. Rev. X, 2015]. In second study we ask how constraints on phenotypic variation limit the capacity of organisms to adapt to multiple simultaneous selection pressures. We select Escherichia coli for faster migration through a porous environment, a process which depends on both motility and growth. We find that a trade-off between swimming speed and growth rate constrains the evolution of faster migration. Evolving faster migration in rich medium results in slow growth and fast swimming, while evolution in minimal medium results in fast growth and slow swimming. A model of the evolutionary process shows that the genetic capacity of an organism to vary traits can qualitatively depend on its environment, which in turn alters its evolutionary trajectory [eLife, 2017]. We explore the possibility that phenotypic constraints and genetic architecture can provide a route to predicting evolutionary dynamics.