Non-vanishing sharp-fronted travelling wave solutions of the Fisher-Kolmogorov model

Abstract

The Fisher-Kolmogorov-Petrovsky-Piskunov (KPP) model, and generalizations thereof, involves simple reaction-diffusion equations for biological invasion that assume individuals in the population undergo linear diffusion with diffusivity D, and logistic proliferation with rate λ. For the Fisher-KPP model, biologically relevant initial conditions lead to long-time travelling wave solutions that move with speed c=2√λD. Despite these attractive features, there are several biological limitations of travelling wave solutions of the Fisher-KPP model. First, these travelling wave solutions do not predict a well-defined invasion front. Second, biologically relevant initial conditions lead to travelling waves that move with speed c=2√λD > 0. This means that, for biologically relevant initial data, the Fisher-KPP model cannot be used to study invasion with c ≠ 2√λD, or retreating travelling waves with c < 0. Here, we reformulate the Fisher-KPP model as a moving boundary problem and show that this reformulated model alleviates the key limitations of the Fisher-KPP model. Travelling wave solutions of the moving boundary problem predict a well-defined front that can propagate with any wave speed, -∞ < c < ∞. Here, we establish these results using a combination of high-accuracy numerical simulations of the time-dependent partial differential equation, phase plane analysis and perturbation methods. All software required to replicate this work is available on GitHub.