Multitype $\Lambda$-coalescents

TL;DR

This paper characterizes multitype $ ext{Lambda}$-coalescent processes with colour-changing blocks, providing conditions for their structure, and generalizes classical coalescent classification to multitype settings with various applications.

Contribution

It introduces a comprehensive framework for multitype $ ext{Lambda}$-coalescents, extending Pitman's classification to processes with colour changes and cross-type mergers.

Findings

Characterization of multitype $ ext{Lambda}$-coalescents with permutation invariance

Conditions for processes to come down from infinity

Unification of models like seed-bank, island, and branching coalescents

Abstract

Consider a multitype coalescent process in which each block has a colour in $\{1,\ldots,d\}$. Individual blocks may change colour, and some number of blocks of various colours may merge to form a new block of some colour. We show that if the law of a multitype coalescent process is invariant under permutations of blocks of the same colour, has consistent Markovian projections, and has asychronous mergers, then it is a multitype $\Lambda$-coalescent: a process in which single blocks may change colour, two blocks of like colour may merge to form a single block of that colour, or large mergers across various colours happen at rates governed by a $d$-tuple of measures on the unit cube $[0,1]^d$. We go on to identify when such processes come down from infinity. Our framework generalises Pitman's celebrated classification theorem for singletype coalescent processes, and provides a unifying setting for numerous examples that have appeared in the literature including the seed-bank model, the island model and the coalescent structure of continuous-state branching processes.