Origins of Bacterial Species--Biology 537
February 6, 2003
Week 2: Alternative views of bacterial diversity
Required reading
(Lawrence 2002)
Lawrence suggests that periodic selection is not an important force in bacterial
evolution, and he emphasizes sexual isolation as the quintessential step in
speciation. He proposes a model in which two ecologically distinct populations
gradually become divergent in sequence, and thereby sexually isolated,
throughout the chromosome. His model is based on the idea that recombination
events in a region containing a niche-specific gene will be unsuccessful, and thus
a certain region flanking a niche-specific gene will be protected from
recombination. As each population evolves further into its respective niche,
more regions along the chromosome will contain niche-specific genes, and so
theses regions, too, will become protected from recombination. Eventually, all
parts of the chromosome become divergent and thus protected from
recombination.
(Vulic et al. 1999)
Vulic et al. note that the adaptive changes allowing a niche invasion can be
promoted by modulation of the mismatch repair system. A debilitated mismatch
repair system more readily allows uptake of potentially adaptive genes from
other species. While an incipient species is lacking a fully functional mismatch
repair system, it rapidly acquires sequence divergence from its parental
population. This acquired sequence divergence contributes to sexual isolation
when mismatch repair is reinstated.
(Gogarten et al. 2002)
Another possibility is that there are no taxa with the dynamic attributes of
species. Gogarten et al. suggest that there is no force of cohesion within a
bacterial population, that periodic selection is ineffective at purging diversity.
Further, Gogarten et al. predict a great deal of flux between ecologically distinct
genotypes, owing to the high rate of gene acquisition by horizontal transfer. Any
one lineage would spend only a short time in a particular ecological niche before
it acquires new genes through horizontal transfer and moves to another niche.
This view does not predict that ecologically distinct groups within a named
species would have any significant longevity as separate clades.
Optional reading
(Dykhuizen and Green 1991)
Dykhuizen and Green propose a way to bring bacterial within the fold of the
Biological Species Concept, by proposing a phylogenetic approach toward
identifying groups of strains that undergo recombination, and those that do not.
References
Dykhuizen, D. E., and L. Green. 1991. Recombination in Escherichia coli and the
definition of biological species. J Bacteriol 173:7257-7268.
Gogarten, J. P., W. F. Doolittle, and J. G. Lawrence. 2002. Prokaryotic evolution in
light of gene transfer. Mol Biol Evol 19:2226-2238.
Lawrence, J. G. 2002. Gene transfer in bacteria: speciation without species? Theor
Popul Biol 61:449-460.
Vulic, M., R. E. Lenski, and M. Radman. 1999. Mutation, recombination, and
incipient speciation of bacteria in the laboratory. Proc Natl Acad Sci U S A
96:7348-7351.