Stochastic mechanisms forming large clones during colonisation of new areas

TL;DR

This study demonstrates that stochastic processes during species-range expansions can lead to large clonal populations, with initial asexual dominance replaced by sexual recruitment over time, supported by empirical data from seaweed species.

Contribution

The paper introduces a stochastic model showing how large clones form during colonization, contrasting with selection-based explanations, and validates it with empirical data.

Findings

Large clones dominate early colonization stages.

Long-distance dispersal shortens clone dominance periods.

Empirical data supports stochastic expansion mechanisms.

Abstract

In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises from natural selection favouring asexual recruitment in young populations. Alternatively, as we show here, this pattern may result from stochastic processes during species-range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that eventually favour sexual recruitment, colonisation starts with an asexual wave. Long after colonisation is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long-distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our stochastic model makes detailed predictions different from a selection model, and comparing these with empirical data from a postglacially established seaweed species (Fucus radicans) shows that in this case a stochastic mechanism is strongly supported.