Dynamics of lineages in adaptation to a gradual environmental change

TL;DR

This paper models how populations adapt to gradual environmental changes, analyzing both macroscopic PDE behavior and microscopic lineage histories, revealing how ancestral traits influence current populations with a trait lag.

Contribution

It introduces a stochastic individual-based model combined with PDE analysis to study lineage dynamics under environmental change, highlighting ancestral trait influence.

Findings

Population persists with a trait lag under environmental change.

Lineage paths approximate Ornstein-Uhlenbeck processes in large populations.

Ancestral traits shape current trait distributions with a measurable lag.

Abstract

We investigate a simple quantitative genetics model subjet to a gradual environmental change from the viewpoint of the phylogenies of the living individuals. We aim to understand better how the past traits of their ancestors are shaped by the adaptation to the varying environment. The individuals are characterized by a one-dimensional trait. The dynamics -- births and deaths -- depend on a time-changing mortality rate that shifts the optimal trait to the right at constant speed. The population size is regulated by a nonlinear non-local logistic competition term. The macroscopic behaviour can be described by a PDE that admits a unique positive stationary solution. In the stationary regime, the population can persist, but with a lag in the trait distribution due to the environmental change. For the microscopic (individual-based) stochastic process, the evolution of the lineages can be traced back using the historical process, that is, a measure-valued process on the set of continuous real functions of time. Assuming stationarity of the trait distribution, we describe the limiting distribution, in large populations, of the path of an individual drawn at random at a given time $T$. Freezing the non-linearity due to competition allows the use of a many-to-one identity together with Feynman-Kac's formula. This path, in reversed time, remains close to a simple Ornstein-Uhlenbeck process. It shows how the lagged bulk of the present population stems from ancestors once optimal in trait but still in the tail of the trait distribution in which they lived.