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A Model For The Evolutionary Dynamics Of Cross Feeding Polymorphisms In Microorganisms

2136 words - 9 pages

Abstract Understanding mechanisms of evolutionary
diversification is central to evolutionary biology. Microbes
constitute promising model systems for observing processes
of diversification directly in the laboratory. One of the main
existing paradigms for microbial diversification is the evolution
of cross-feeding polymorphisms, in which a strain specializing
on a primary resource coexists with a cross-feeding
strain that specializes on a waste product resulting from
consumption of the primary resource. Here I propose a
theoretical model for the evolutionary dynamics through
which cross-feeding polymorphisms can gradually emerge
from a single ancestral strain. The model is based on the
framework of adaptive dynamics, which has proved to be
very useful for studying adaptive processes of divergence
under sympatric conditions. In particular, the phenomenon
of evolutionary branching serves as a general paradigm for
diversification. I show that evolutionary branching naturally
occurs in evolutionary models of cross-feeding if (1) there is
a trade-off between uptake efficiencies on the primary and
secondary resources, and (2) this trade-off has positive curvature.
The model also suggests that the evolution of crossfeeding
should be more likely in chemostat cultures than in
serial batch cultures, which conforms with empirical observations.
Overall, the model provides a theoretical metaphor
for the evolution of cross-feeding polymorphisms.
Key words Crossfeeding · Polymorphism · Adaptive dynamics
· Evolutionary branching · Tradeoff · Frequency
dependence
M. Doebeli
Departments of Zoology and Mathematics, University of British
Columbia, 6270 University Blvd., Vancouver, BC, Canada V6T 1Z4
Tel. 1-604-822-3326; Fax 1-604-822-2416
e-mail: doebeli@zoology.ubc.ca
Introduction
Understanding the origin and evolution of diversity is one
of the central problems in population biology. In particular,
understanding the processes of speciation is of fundamental
importance. Although traditional discussions of speciation
are based on geographic patterns of species distributions
(Mayr 1963; Turelli et al. 2001), recent theoretical developments
have studied the ecological mechanisms that can
drive adaptive divergence between different lineages
(Geritz et al. 1998; Dieckmann and Doebeli 1999; Doebeli
and Dieckmann 2000). In this context, the phenomenon of
evolutionary branching is of particular importance. During
this evolutionary process, frequency-dependent selection
drives an evolving lineage to a point in phenotype space
where selection turns disruptive, after which the lineage
splits into two diverging phenotypic branches. Evolutionary
branching is a paradigm for evolutionary diversification
emerging from the theoretical framework of adaptive dynamics
(Metz et al. 1996; Dieckmann and Law 1996; Geritz
et al. 1998). The main conceptual idea underlying this
theory for evolutionary dynamics is that the phenotypic
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