A quantitative definition of organismality and its application to lichen

TL;DR

This paper proposes a formal, functional definition of what constitutes an organism, emphasizing its role as a coarse-grained, predictive model of biological systems, and applies this to the complex case of lichen symbiosis.

Contribution

It introduces a new, formal framework for defining organisms based on their predictive utility and applies it to a complex symbiotic system, revealing insights into organismality.

Findings

Complex mixtures of entities can serve as optimal organisms in certain contexts.

Ecological factors influence the best coarse-graining choices for defining organisms.

Traditional organism concepts may not always align with functional, predictive definitions.

Abstract

The organism is a fundamental concept in biology. However there is no universally accepted, formal, and yet broadly applicable definition of what an organism is. Here we introduce a candidate definition. We adopt the view that the "organism" is a functional concept, used by scientists to address particular questions concerning the future state of a biological system, rather than something wholly defined by that system. In this approach organisms are a coarse-graining of a fine-grained dynamical model of a biological system. Crucially, the coarse-graining of the system into organisms is chosen so that their dynamics can be used by scientists to make accurate predictions of those features of the biological system that interests them, and do so with minimal computational burden. To illustrate our framework we apply it to a dynamic model of lichen symbiosis---a system where either the lichen or its constituent fungi and algae could reasonably be considered "organisms." We find that the best choice for what organisms are in this scenario are complex mixtures of many entities that do not resemble standard notions of organisms. When we restrict our allowed coarse-grainings to more traditional types of organisms, we find that ecological conditions, such as niche competition and predation pressure, play a significant role in determining the best choice for organisms.