Lloyd A. Demetrius is an American mathematician and theoretical biologist at the Department of Organismic and Evolutionary Biology, Harvard University.

Since 1990, he has been with the Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, first as a visiting professor (1990–1992), and then as an associate in population genetics.

He has also pioneered the application of the methodology of quantum mechanics to the study of allometric relations between metabolic rate and generation time in cells.

Gibbs, and directionality theory is the natural extension of statistical mechanics, the study of the collective behaviour of inorganic matter.

Evolutionary entropy describes the multiplicity of trajectories induced by interparticle forces and defined in terms of temporal progression of instantaneous microstates.

Statistical mechanics, one of the pillars of modern physics, is concerned with deducing the thermodynamic properties of aggregates of inanimate matter from its microstructure.

[2][5] This relation, Demetrius has shown, provides a conceptual framework for understanding the origin of life: the transition from an abiotic system, defined by inorganic matter – solids, liquids and gases – to the emergence of organized chemical assemblies capable of Darwinian evolution.