The frequency process in a non-neutral two-type continuous-state branching process with competition and its genealogy

TL;DR

This paper analyzes a two-type continuous-state branching process with competition, focusing on the relative frequency dynamics, genealogy, and large population limits, providing new insights into coexistence conditions.

Contribution

It introduces the culled frequency process, generalizes previous models, and establishes its duality and genealogy, along with deriving a large population limit as an ODE.

Findings

The culled frequency process can be described by a branching-coalescing Markov chain.

Conditions for the process to have a moment dual are identified.

Large population limits lead to a deterministic ODE, with coexistence conditions analyzed.

Abstract

We consider a population growth model given by a two-type continuous-state branching process with immigration and competition, introduced by Ma. We study the relative frequency of one of the types in the population when the total mass is forced to be constant at a dense set of times. The resulting process is described as the solution to an SDE, which we call the culled frequency process, generalizing the $\Lambda$-asymmetric frequency process introduced by Caballero et al. We obtain conditions for the culled frequency process to have a moment dual and show that it is given by a branching-coalescing continuous-time Markov chain that describes the genealogy of the two-type CBI with competition. Finally, we obtain a large population limit of the culled frequency process, resulting in a deterministic ordinary differential equation (ODE). Two particular cases of the limiting ODE are studied to determine if general two-type branching mechanisms and general Malthusians can lead to the coexistence of the two types in the population.