The relative frequency between two continuous-state branching processes with immigration and their genealogy

TL;DR

This paper introduces a new stochastic process called the $ ext{Lambda}$-asymmetric frequency process ($ ext{Lambda}$-AFP) to study the relative frequency of two continuous-state branching processes with immigration, analyzing its properties, limits, and dualities.

Contribution

It defines the $ ext{Lambda}$-AFP, proves its Feller property, derives large population limits, and explores duality conditions, including connections to $ ext{Lambda}$-coalescents.

Findings

The $ ext{Lambda}$-AFP is a Feller process.

Large population limits of the process are characterized.

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

Abstract

When two (possibly different in distribution) continuous-state branching processes with immigration are present, we study the relative frequency of one of them when the total mass is forced to be constant at a dense set of times. This leads to a SDE whose unique strong solution will be the definition of a $\Lambda$-asymmetric frequency process ($\Lambda$-AFP). We prove that it is a Feller process and we calculate a large population limit when the total mass tends to infinity. This allows us to study the fluctuations of the process around its deterministic limit. Furthermore, we find conditions for the $\Lambda$-AFP to have a moment dual. The dual can be interpreted in terms of selection, (coordinated) mutation, pairwise branching (efficiency), coalescence, and a novel component that comes from the asymmetry between the reproduction mechanisms. In the particular case of a pair of equally distributed continuous-state branching processes, the associated $\Lambda$-AFP will be the dual of a $\Lambda$-coalescent. The map that sends each continuous-state branching process to its associated $\Lambda$-coalescent (according to the former procedure) is a homeomorphism between metric spaces.