Annu. Rev. Microbiol. 2002. 56:457–87
Copyright c ? 2002 by Annual Reviews. All rights reserved
First published online as a Review in Advance on May 10, 2002
WHAT ARE BACTERIAL SPECIES?
Frederick M. Cohan
Department of Biology, Wesleyan University, Middletown, Connecticut 06459-0170;
speciation, ecotype, species concept, genetic exchange, systematics,
fundamental unit of biological diversity, the species. The past half-century of bacte-
rial systematics has been characterized by improvements in methods for demarcating
by a theory-based concept of species. Eukaryote systematists have developed a uni-
versal concept of species: A species is a group of organisms whose divergence is
capped by a force of cohesion; divergence between different species is irreversible;
and different species are ecologically distinct. In the case of bacteria, these universal
properties are held not by the named species of systematics but by ecotypes. These are
populations of organisms occupying the same ecological niche, whose divergence is
purged recurrently by natural selection. These ecotypes can be discovered by several
universal sequence-based approaches. These molecular methods suggest that a typical
named species contains many ecotypes, each with the universal attributes of species.
A named bacterial species is thus more like a genus than a species.
Bacterial systematics has not yet reached a consensus for defining the
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
BACTERIAL SPECIES AS PHENOTYPIC
AND GENETIC CLUSTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
THE UNIVERSAL DYNAMIC PROPERTIES OF SPECIES . . . . . . . . . . . . . . . . . . . 460
THE PECULIAR SEXUAL HABITS OF BACTERIA . . . . . . . . . . . . . . . . . . . . . . . . 464
WHY THE BIOLOGICAL SPECIES CONCEPT IS
INAPPROPRIATE FOR BACTERIOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465
BACTERIA FORM SPECIES LIKE EVERYONE ELSE . . . . . . . . . . . . . . . . . . . . . . 466
BACTERIAL SPECIATION AS AN EVERYDAY PROCESS . . . . . . . . . . . . . . . . . . 467
SEQUENCE-BASED APPROACHES TO
IDENTIFYING BACTERIAL SPECIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471
Discovery of Ecotypes as Sequence Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471
Discovery of Ecotypes as Star Clades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474
Discovery of Ecotypes Through Multilocus Sequence Typing . . . . . . . . . . . . . . . . . 478
RECOMMENDATIONS FOR BACTERIAL SYSTEMATICS . . . . . . . . . . . . . . . . . . 480
These are formative times for systematics. Owing to recent technological devel-
opments, the study of biological diversity has matured into a powerful science.
Systematists have invented robust methods for deriving evolutionary trees (35)
as well as powerful computer algorithms for implementing these methods (91).
Also, the technology for DNA sequencing is readily accessible, so that system-
atic research today is typically based on sequence variation and sequence-derived
phylogenies. Together, the new methods of systematics and a wealth of sequence
data are allowing systematists to reconstruct the history of life and the origins of
biodiversity with confidence.
For microbiologists, the new systematics has been particularly fruitful.
Microbial systematists have built a universal tree of life, such that any newly
discovered organism can be placed near its closest relatives (18,104). Sequence
surveys have fostered discovery of new bacterial taxa at all levels. Sequence data
have frequently turned up organisms with no known close relatives; these or-
ganisms represent new divisions within the bacterial world (18). At the other
extreme, sequence surveys have fostered the discovery of new species. For ex-
ample, sequence data revealed the Lyme disease spirochete Borrelia burgdor-
feri (sensu lato) to consist of several species, each with its own etiology (7,8).
Sequence-based approaches have allowed systematists to characterize the
species diversity even among uncultured bacteria, and species names have now
been given to many uncultured organisms, pending further characterization
Despite these remarkable successes, however, bacterial systematics has not
yet reached a consensus for defining the fundamental unit of biological diver-
sity, the species. Bacterial species exist—on this much bacteriologists can agree.
Bacteriologists widely recognize that bacterial diversity is organized into discrete
phenotypic and genetic clusters, which are separated by large phenotypic and
genetic gaps, and these clusters are recognized as species (85). Beyond agree-
ing on the existence of species, however, bacteriologists differ on operational
procedures for identifying species most appropriately (32). Moreover, we fail to
agree on whether a bacterial species should be conceived simply as a cluster of
phenotypically and genetically similar organisms, or whether we should also ex-
pect a species to have special genetic, ecological, evolutionary, or phylogenetic
I argue that there are bacterial taxa called “ecotypes” (15), which share the
quintessential set of dynamic properties held by all eukaryotic species. I demon-
strate that, alas, the species generally recognized in bacterial systematics do not
have these universal properties: Each named “species” appears to contain many
sequence-based approaches for discovering ecotypes and recommend a means of
incorporating ecotypes into bacterial taxonomy.
BACTERIAL SPECIES AS PHENOTYPIC
AND GENETIC CLUSTERS
Bacterial systematics began in much the same way as the systematics of animals
organisms identified species simply as phenotypic clusters (87). While macrobiol-
ogists generally surveyed morphological characters and microbiologists generally
investigated metabolic characters, systematists of all major groups were success-
ful in carving biological diversity into the phenotypic clusters they identified as
Macrobial and microbial systematics split profoundly with Mayr’s publication
of the Biological Species Concept in 1944 (57), in which evolutionary theory was
incorporated into systematics. The Biological Species Concept and several later
concepts of species changed zoologists’ and botanists’ views of what a species
should represent. A species was no longer merely a cluster of similar organisms;
a species was now viewed as a fundamental unit of ecology and evolution, with
certain dynamic properties. In the case of Mayr’s Biological Species Concept,
a species was viewed as a group of organisms whose divergence is opposed by
systematics has not incorporated theory-based concepts of species.
improvements in methods for demarcating species as clusters. Technological ad-
fatty acid content (81) frequently provide important diagnostic phenotypic char-
acters (67). Automated equipment for assaying metabolism, such as the microtiter
plate reader, allows many more strains to be assessed for many more metabolic
Bacterial systematists have also developed improved statistical methods for
demarcating phenotypic clusters. Numerical taxonomy, developed by Sneath &
Sokal (86), was designed as an objective, mathematical approach for demarcating
clusters. In this method, large numbers of strains are assayed for many phenotypic
traits, including degradation or metabolism of certain chemicals, the ability to
produce and survive various antibiotics, the ability to grow on various carbon or
nitrogen sources, staining reactions, and morphology (32). The multidimensional
space of phenotypic diversity can then be collapsed onto two or three axes, for
example, by principal component analysis. Practitioners of numerical taxonomy
have clearly illustrated the discrete nature of phenotypic clusters within many
bacterial genera (9). Note that phenotype-based numerical taxonomy is ultimately
much more than a method for delimiting species; it is also a venture into the
natural history of a bacterial group. By studying the phenotypic variation within a
taxon, we learn about the ecological diversity that gives meaning to the taxonomic
Over the past three decades, bacterial systematists have added molecular tech-
niques to their arsenal for demarcating clusters. In the early 1970s bacteriologists
adopted a genomic approach for discovering clusters. With whole-genome DNA-
as measured by the fraction of their genomes that are homologous. Johnson (44)
nearly always shared 70% or more of their genomes and that strains from different
species nearly always shared less than 70%. A 70% level of homology over the
genome was thus adopted as a gold standard for determining whether two strains
should be considered different species (100).
Systematists have more recently utilized DNA sequence divergence data, par-
& Goebel (88) found that strains that are more than 3% divergent in 16S rRNA
are nearly always members of different species, as determined by DNA-DNA
hybridization, whereas strains that are less than 3% divergent may or may not be
generally members of different species. A cutoff of 3% divergence was therefore
recommended as a conservative criterion for demarcating species.
Molecular approaches have made bacterial systematics much more accessible.
The existence of multiple species within a group can now be inferred by general
differences between the clusters. In the case of uncultivated bacteria, molecular
methods are all that is available for identifying the diversity of species.
terial systematics. This is because the molecular cutoffs for demarcating species
have been calibrated to yield the species groupings already determined by pheno-
typic clustering. The 70% cutoff for DNA-DNA hybridization was calibrated to
yield the phenotypic clusters previously recognized as separate species, and the
3% cutoff for 16S rRNA divergence was calibrated to yield the species previously
determined by DNA-DNA hybridization and phenotypic clustering. Because bac-
terial systematics is lacking a theory-based concept of species, all we can do is
by phenotypic criteria. I demonstrate that, if we adopt an ecological and evolu-
tionary theory of species, molecular approaches will give us much more than the
phenotypic clusters of yore; we will be able to identify nearly every ecologically
distinct population of bacteria.
THE UNIVERSAL DYNAMIC PROPERTIES OF SPECIES
Evolutionary biologists and systematists of animals and plants have widely be-
lieved that species are more than just clusters of closely related and similar organ-
by one or more forces of cohesion.
The principal insight of Mayr’s (57) Biological Species Concept is that there is
a reason why organisms form the tight clusters discovered by systematists. Every
and (at least for highly sexual animal and plant species) this cohesive force is
genetic exchange. So long as organisms can successfully interbreed, argued Mayr,
they will remain phenotypically and genetically similar, but when they lose the
species as a reproductive community, a group of organisms with the potential to
interbreed and produce viable and fertile offspring.
ulation genetic theory predicts that recurrent interbreeding between populations
that even low levels of genetic exchange between populations should limit their
genetic divergence [although probably not in the way Mayr envisioned; see (30)].
Second, Mayr pointed out a striking empirical correspondence between the phe-
notypic clusters previously recognized as species and groups of organisms with
the potential to interbreed.
Systematists have pointed out that the Biological Species Concept does not fe-
licitously accommodate hybridization between species of plants (89) and species
of bacteria (75,76). That is, there are many pairs of species (as defined by phe-
notypic clustering) that occasionally exchange genes yet retain their integrity as
distinct phenotypic clusters. In these cases, genetic exchange is clearly not acting
effectively as a force of cohesion.
Concept, a species is a group of organisms whose divergence is capped by one or
more forces of cohesion. In the case of sexual species, the predominant cohesive
into account that genetic exchange between two groups is not always sufficient to
are considered separate species, according to the Cohesion Species Concept. With
regard to highly sexual species, the Cohesion Species Concept may be understood
isms whose divergence is constrained by genetic exchange. We see that the Cohe-
sion Species Concept is especially useful in accommodating bacterial groups that
form separate phenotypic clusters despite recurrent recombination between them.
Another limitation of the Biological Species Concept is that it does not accom-
enough to take into account species that fall outside the box built by the Biological
Species Concept. Templeton (92) argues that asexual species are subject to their
own powerful force of cohesion. This force is natural selection, which can purge
all genetic diversity from an asexual population. Consider the fate of an adaptive
members of the population. For example, Ferea et al. (29) found that in asexual
an oxygenated environment involved many mutations, some suppressing anaero-
bic metabolism and others augmenting oxidative phosphorylation. Each of these
mutations was brought to fixation (i.e., to a frequency of 100%) by natural selec-
tion. In the absence of recombination, the entire genome of the successful mutant
is brought to fixation, and so all the genetic diversity within the population (at all
loci) is purged to zero. An asexual species may then be understood as a group
of organisms whose divergence is constrained and recurrently reset to zero by
intermittent bouts of natural selection. [This diversity-purging process is called
“periodic selection” (5).]
when an asexual lineage evolves into a new ecological niche (i.e., uses a different
by adaptive mutants from its former population (Figure 1). For example, within
between organisms. The asterisk represents an adaptive mutation in one individual of
Species 1. Because of the absence (or rarity) of recombination, the adaptive mutant
and its clonal descendants replace the rest of the genetic diversity within the species.
its demise with the next periodic selection event. Because asexual (or rarely sexual)
species differ in the resources they use, the adaptive mutant from one species (e.g.,
Species 1) does not out-compete the organisms of other species, and the genetic diver-
sity within these other species remains untouched. Once populations of organisms are
divergent enough to escape one another’s periodic selection events, these populations
are free to diverge permanently and have reached the status of distinct species. Used
with permission from the American Society of Microbiology (14a).
The transience of diversity within an asexual (or rarely sexual) species and
an experimental population of Escherichia coli inoculated with a single clone, a
mutant population arose that fed on a waste metabolite from the original (95).
While the original population and the cross-feeding population continued to co-
exist, each population endured its own periodic selection events. That is, adaptive
tions. Owing to the differences in ecology between populations, however, periodic
selection from one population failed to affect the diversity within the other. When
two asexual populations reach the point that they can survive each other’s periodic
selection events, a force of cohesion no longer caps their divergence, and so they
may be considered separate species.
In summary, any species, whether highly sexual (such as animals and plants)
or completely asexual (such as experimental microbial populations engineered
to lack sex), is subject to forces of cohesion. Within highly sexual populations,
genetic exchange is a powerful force of cohesion, although genetic exchange be-
tween groups that recombine only rarely (as in hybridizing species) does not
constrain divergence. Within asexual species, periodic selection is a powerful
force of cohesion that recurrently resets the genetic diversity within the species to
While cohesion is the fundamental attribute claimed for all species (13,15,61,
with distinct evolutionary fates [the Evolutionary Species Concept by Simpson
(83) and Wiley (103)].
Another corollary is that species occupy different ecological niches either by
utilizing different kinds of resources or by utilizing the same kinds of resources at
different times or within different microhabitats [the Ecological Species Concept
by van Valen (98)]. This is most clearly true for asexual species, as asexual or-
ganisms must diverge ecologically before they can escape one another’s periodic
without constraint from one another. Early twentieth-century ecologists demon-
strated that two species cannot coexist unless they differ at least somewhat in the
resources they consume [the competitive exclusion principle (31)]. Thus, in the
highly sexual world of animals and plants, speciation requires both reproductive
divergence and ecological divergence (25,31,54,59).
Although the various species concepts I have discussed emphasize different at-
tial property: that species are evolutionary lineages that are irreversibly separate,
each with its own evolutionary tendencies and historical fate (19).
Systematists have thus delineated general attributes of species that apply well
for organisms at the extremes of sexuality, including species that exchange genes
every generation as well as species that never exchange genes. As we shall see,
bacteria fall into neither extreme, but nevertheless they fall into species that fit all
the universal attributes of species I have delineated.
THE PECULIAR SEXUAL HABITS OF BACTERIA
Genetic exchange in bacteria differs profoundly from that in the highly sexual eu-
karyotes, the animals and plants, for which species concepts were invented. First,
recombination in bacteria is extremely rare in nature. Several laboratories have
taken a retrospective approach to determining the historical rate of recombina-
tion in nature (27,28,79). Based on surveys of diversity in allozymes, restriction
recognition sites, and DNA sequences, recombination rates have been estimated
from the degree of association between genes or parts of genes [(37,39,40); for
limitations of this approach, see (16,56)]. Survey-based approaches have shown
that in most cases a given gene segment is involved in recombination at about the
same rate or less, as mutation (27,28,56,79,82,102). Less commonly, as in the
cases of Helicobacter pylori and Neisseria gonorrhoeae, recombination occurs
at least one order of magnitude more frequently than mutation, although survey
methods do not allow us to determine by how much (66,90).
Also in contrast to the case for animals and plants, recombination in bacteria
is promiscuous. Whereas animal groups typically lose the ability to exchange
genes entirely by the time their mitochondrial DNA sequences are 3% divergent
(6), bacteria can undergo homologous recombination with organisms at least as
divergent as 25% in DNA sequence (21,52,99).
There are, nevertheless, some important constraints on bacterial genetic ex-
change. Ecological differentiation between populations may prevent them from
for Streptococcus pyogenes populations adapted for throat versus skin infection at
duction or conjugation, is limited by the host ranges of the respective phage and
Finally, homologous recombination is limited by the resistance to integration of
divergent DNA sequences because mismatch repair tends to reverse integration
of a mismatched donor-recipient heteroduplex (77,99) and because integration
requires a 20–30-bp stretch of nearly perfectly matched DNA (51,52,74).
In addition, recombination events in bacteria are localized to a small fraction of
the genome. Segments transduced or transformed in the laboratory are frequently
that recombination in nature is likewise highly localized within the chromosome
Finally, recombination in bacteria is not limited to the transfer of homologous
recombination, whereby a heterologous gene from a donor is integrated along
with flanking homologous DNA (34,52). Alternatively, heterologous genes may
be integrated along with a transposable element brought into the recipient on a
plasmid or phage (97). Genomic analyses have recently shown that a sizeable
fraction of bacterial species’ genomes (frequently 5%–15%) has typically been
acquired from other species (65).
WHY THE BIOLOGICAL SPECIES CONCEPT IS
INAPPROPRIATE FOR BACTERIOLOGY
Systematists of animals and plants are indebted to Mayr’s (57) Biological Species
Concept for the infusion of evolutionary theory into systematics. Even though
animal and plant species are usually discovered as phenotypic clusters (87), sys-
tematists have understood that there is a force of cohesion that holds each cluster
together and that organisms in different clusters are no longer bound by a force of
Why has bacterial systematics not been similarly transformed by evolutionary
theory? It is not for lack of wrestling with the concept. For example, in the early
1960s, Ravin (75,76) attempted to apply the Biological Species Concept to bac-
teria. Recognizing that bacteria are sexual and that bacteria can exchange genes
even with distant relatives, Ravin (76) defined “genospecies” as groups of bacteria
that could exchange genes and “taxospecies” as the phenotypic clusters of main-
stream bacterial systematics. In contrast to the case for animals and plants, the
genospecies and taxospecies so defined did not correspond well: Many clusters
retained their phenotypic distinctness despite their inclusion within a genospecies
(76). The lack of correspondence between genospecies and taxospecies suggested
that the ability to exchange genes had little effect on the evolution of phenotypic
divergence in bacteria. Perhaps discouraged, bacteriologists did not attempt to ap-
ply the Biological Species Concept (or any other theory-based concept of species)
for another three decades.
Dykhuizen & Green (23) proposed in 1991 to classify bacteria into species
of strains that recombine with one another but not with strains from other such
identify groups that have and have not been exchanging genes.
cuity of genetic exchange: Bacteria do exchange genes both within and between
the clusters we recognize as named species (48,55). Nevertheless, in practice it is
within named bacterial species than between them.
two closely related animal or plant populations cannot be permanent until the rate
of recombination between the populations is severely reduced compared to the
rate of recombination within populations. Owing to the high rate of recombination
recombination. The genetic basis of ecological divergence among ecotypes is assumed
be due to acquisition of different gene loci. Recombination occurs between ecotypes
at rate cb. The fitness penalty for recombination at any of the genes responsible for
ecological divergence is s, such that the fitness of each nonrecombinant genotype
(e.g., ABCDE) is 1, and the fitness of a single-locus recombinant (e.g., ABCDe) is
1−s. A mathematical model shows the equilibrium frequency of maladaptive foreign
alleles in each ecotype to be cb/s, which, given the low rate of recombination in
bacteria, is a negligible frequency (14). Used with permission from American Society
for Microbiology (14a).
Ecological divergence between ecotypes is stable with respect to recurrent
eliminate interpopulation divergence if it were to proceed at the same rate as
recombination within populations.
In contrast, because recombination in bacteria is so rare, recurrent recombi-
nation between bacterial species cannot hinder their divergence (14). Even if re-
combination between species were to occur at the same rate as recombination
within them, natural selection against interspecies recombinants could easily limit
the frequency of recombinant genotypes to negligible levels (Figure 2). While the
evolution of sexual isolation is an important milestone in the origin of animal
and plant species, it is irrelevant to the evolution of permanent divergence in the
bacterial world. The Biological Species Concept is thus a red herring for bacterial
Nevertheless, bacteriologists need not envy the macrobial world for its tidy
application of the Biological Species Concept. It turns out that there is an ap-
propriate species concept for bacteria. Moreover, bacteria and eukaryotes both fit
comfortably within a universal concept of species.
BACTERIA FORM SPECIES LIKE EVERYONE ELSE
mutant (14,15): An ecotype is a set of strains using the same or similar ecological
resources, such that an adaptive mutant from within the ecotype out-competes
to extinction all other strains of the same ecotype; an adaptive mutant does not,
however, drive to extinction strains from other ecotypes (Figure 1). For example,
an adaptive mutant from an ecotype of Streptococcus pyogenes that is genetically
adapted to infecting our throats would out-compete to extinction other members
of its own ecotype but would not out-compete closely related ecotypes genetically
adapted to infecting our skin.
If they were entirely asexual, bacterial ecotypes defined in this way would have
the universal properties of species. I earlier discussed how asexual populations
separate (free to diverge from one another indefinitely), and the ecotypes would
be ecologically distinct.
Let us now add the reality of rare but promiscuous genetic exchange to these
ecotypes. Would they still retain the universal qualities of species? We might
imagine that periodic selection would not be an effective force of cohesion within
place the adaptive mutation into many genetic backgrounds within the ecotype,
such that selection would fail to purge sequence diversity at all loci. However,
this is not the case. Under rates of recombination typical of bacteria, selection
will purge each locus, on average, of 99.9% of its sequence diversity (13). Thus,
In contrast, there is no effective force of cohesion binding different ecotypes.
Ecotypes are defined to be free to diverge without the constraint of one another’s
periodic selection events; moreover, as we have seen, the rare recombination oc-
curring in bacteria is unable to prevent adaptive divergence between ecotypes.
In summary, the bacterial ecotypes defined here share the fundamental proper-
ecotypes have diverged to the point of escaping one another’s periodic selection
events, there is no force that can prevent their divergence; and bacterial ecotypes
are ecologically distinct. Bacterial ecotypes are therefore evolutionary lineages
that are irreversibly separate, each with its own evolutionary tendencies and his-
torical fate (19,83,103). A species in the bacterial world may be understood as an
evolutionary lineage bound together by ecotype-specific periodic selection.
BACTERIAL SPECIATION AS AN EVERYDAY PROCESS
How frequently do bacterial populations split irreversibly into lineages with sep-
arate evolutionary fates? By applying the principles of population genetics and
much greater than that in the highly sexual world of animals and plants.
ecological (25,31) divergence, but speciation in bacteria requires only ecological
Second, speciation in highly sexual eukaryotes requires allopatry (i.e., that the
incipient species inhabit different geographical regions) (58), or at least microal-
lopatry (i.e., that they inhabit different microhabitats) (11,101). This is because
highly sexual populations cannot diverge as long as they are exchanging genes at a
high rate; allopatry (or microallopatry) provides the only mechanism for reducing
genetic exchange between populations in early stages of speciation. In contrast, as
I have shown, genetic exchange is too rare to hinder divergence between bacterial
populations, and so the need for allopatry in bacterial speciation is greatly reduced
(but not necessarily eliminated, as we shall see).
Third, the extremely large population sizes of bacteria make rare mutation and
recombination events much more accessible to a bacterial population than is the
case for macroorganisms.
Fourth, whereas each animal and plant species is genetically closed to all other
(65,106). So, while animal and plant species must evolve all their adaptations
on their own, bacteria can take up existing adaptations from a great diversity of
other species. Homologous recombination can substitute an adaptive allele from
another species into an existing gene in the recipient (55); recombination can also
introduce entirely novel genes and operons from other species (3,33,46,47,65).
By granting an entirely new metabolic function, heterologous gene transfer has
the potential to endow a strain with a new resource base, such that the strain and
its descendants are instantaneously a new species—beyond the reach of periodic
selection within the strain’s former population. Since 5%–15% of the genes in a
typical bacterial genome have been acquired from other species (65), it is possible
that many speciation events in the past have been driven by the acquisition of new
The transfer of adaptations across species is facilitated by the peculiar charac-
teristics of bacterial genetic exchange. Incorporation of highly divergent DNA is
by the localized nature of bacterial recombination, whereby only a small fraction
of the donor’s genome is integrated. This allows for the transfer of a generally
useful adaptation (i.e., useful in the genetic backgrounds and the ecological niches
case for most eukaryotes, where the processes of meiosis and fertilization yield
hybrids that are a 1:1 mix of both parents’ genomes.
Finally, genetic exchange between ecotypes (48) may enhance speciation by
preventing a nascent ecotype from being extinguished by an adaptive mutant from
the parental ecotype (15) (Figure 3). This can occur if ecological divergence be-
tween incipient ecotypes involves several mutational steps. In the early stages of
such divergence, nearly every periodic selection event may be limited to purging
the diversity within its own ecotype. Occasionally, however, an extraordinarily
fit adaptive mutant from the parental ecotype might out-compete all strains from
each newly divergent ecotype in the figure has already undergone several private periodic
selection events. However, in the figure we suppose that an extraordinarily competitive adap-
tive mutant (with asterisk) has appeared in the ancestral ecotype, such that this mutant would
out-compete the membership of the nascent ecotype as well as its own ecotype membership.
(A) When there is no recombination between the newly divergent ecotypes, the adaptive
mutant could extinguish the membership of the other ecotype, and the speciation process
would be terminated. (B) When the adaptive mutation can be transferred from one ecotype
to the other, periodic selection is less likely to cause extinction of one ecotype by another.
The transfer of the adaptive mutation would cause a private periodic selection event within
the nascent ecotype. Because the two ecotypes would then share the adaptive mutation, one
ecotype would not be able to extinguish the other. Used with permission from the Society for
Systematic Biology (15).
Facilitation of speciation by recombination among ecotypes. It is assumed that
the nascent ecotype (as well as all the other strains from its own ecotype). In this
case, the speciation process would be quashed by a periodic selection event be-
fore the two incipient ecotypes had diverged sufficiently to be completely free of
one another (Figure 3A). However, recombination between two incipient species
mutation could be transferred from the parental ecotype to a recipient in the other
ecotype, and the new ecotype would lose its disadvantage.
Although recurrent recombination is not sufficient to prevent ecological di-
vergence between ecotypes (13), recombination should be sufficient to allow an
adaptive mutation to pass between ecotypes and enable the recipient ecotype to
become fixed for the adaptation by natural selection. Preventing divergence be-
tween ecotypes requires recurrent recombination at a high rate, but initiating a
natural selection event in a recipient population requires only a single recombina-
tional transfer into the recipient ecotype. Given the enormous population sizes of
bacterial populations, such a transfer event is not unlikely.
In summary, population genetic principles suggest that the rare but promiscu-
ous nature of bacterial genetic exchange, as well as the large population sizes of
bacteria, should foster a much higher rate of speciation in bacteria than is possible
in plants and animals. Nevertheless, important questions remain unresolved.
It is not clear, for example, whether bacterial speciation can proceed without
allopatry. As I have discussed, allopatry is unnecessary for evolution of sexual
isolation between incipient ecotypes because sexual isolation is not a necessary
step in the origin of bacterial ecotypes. However, allopatry may be necessary to
give a nascent ecotype a chance to gradually build up its ecological distinctness
from the parental ecotype before being exposed to periodic selection from the
actually occurs in nature.
Fortunately, these issues can now be addressed by model experimental sys-
tems developed for studying the origins of ecological diversity in the bacterial
world. In these model systems, a clone and its descendants are cultured in liquid
in the laboratory and are allowed to evolve on their own. In one system, using
E. coli, bacteria are cultured in a chemostat (95); in another (also using E. coli),
the bacteria are maintained in serial batch culture (43,80). In yet another sys-
tem (using Pseudomonas fluorescens), the culture medium is neither replenished
nor stirred (72). In all these systems, no extrinsic source of DNA is provided, so
novel genes cannot be introduced by horizontal transfer. Moreover, all vectors of
recombination have been eliminated from the E. coli systems.
evolution of a new ecotype, which utilized acetate secreted by the original clone.
In experiments in a nonstirred environment, as performed by Rainey & Travisano
(72), ecotypes have replicably arisen that are specifically adapted to different parts
of the structured environment (i.e., the surface, the bottom, and the water column).
In other experiments, molecular markers have demonstrated the existence of a di-
in only a subset of the population, indicating that multiple ecotypes are present. In
some cases, the putative ecotypes have coexisted over years of evolution (80,95).
The rate at which new ecotypes can be formed is striking. In the case of the
in the course of several days. In the unstructured environments of the chemostat
and serial batch culture, ecotypes originated within several weeks.
gradually build up their ecological distinctness. Because some incipient ecotypes
have coexisted for years (80,95), it appears that nascent ecotypes have evolved to
for speciation even in the simplest of environments: The chemostat environment
does not have daily or seasonal fluctuations; the stirring eliminated the possibility
of adaptation to different microhabitats; no extrinsic DNA was present; and only
one carbon source was introduced into the system. The bacterial metabolism itself
created a diversity of resources (by secreting acetate), and this was all that was
needed to foster speciation. Thus, the evolution of new ecotypes would appear to
be an ineluctable process in the bacterial world.
SEQUENCE-BASED APPROACHES TO
IDENTIFYING BACTERIAL SPECIES
Discovery of Ecotypes as Sequence Clusters
The theory of evolutionary genetics provides a compelling rationale for using
sequence data to characterize bacterial diversity (69). Given enough time, each
bacterial ecotype is expected to be identifiable as a sequence cluster, where the
average sequence divergence between ecotypes is much greater than the average
based on DNA sequence data (Figure 4).
The rationale can be outlined from a phylogenetic perspective. Suppose a new
original mutant (Figure 4A)]. However, this ecotype is not yet a sequence cluster;
one would not conclude from the sequence-based phylogeny that two populations
exist within this group. After periodic selection, however, the diversity within the
new ecotype is purged (Figure 4B). Likewise, periodic selection events within the
ancestral ecotype will purge diversity within that ecotype as well (Figures 4C,D).
Note that owing to the diversity-purging effect of periodic selection within each
ecotype, the ecotypes eventually appear as separate sequence clusters and each is
a monophyletic group (Figure 4E). Although this result is seen most clearly in the
case of no recombination, Palys et al. (69) showed that under the extremely low
rates of recombination occurring in bacteria different ecotypes are nevertheless
expected to fall eventually into different sequence clusters for any gene shared
to periodic selection (Figure 5). Each adaptive mutant within the ecotype would
two ecotypes will become distinct sequence clusters. (A) The derived ecotype con-
sists of the descendants of a mutant (X) capable of utilizing a new ecological niche.
The adaptive mutant in the derived ecotype (*) is capable of out-competing all other
members of the derived ecotype. (B) The adaptive mutant (*) has driven all the other
lineages within the derived ecotype to extinction. (C) With time, the derived ecotype
a mutation that allows it to out-compete other members of its ecotype. (D) The adap-
tive mutant (**) has out-competed other members of the ancestral ecotype. (E) The
ancestral ecotype is becoming more genetically diverse. At this point, each ecotype
is a distinct sequence cluster as well as a monophyletic group. Used with permission
from the Society for Systematic Biology (15).
A phylogenetic perspective on periodic selection. As demonstrated here,
sequence clusters. (A) The ecotype initially contains two distinct sequence clusters.
Then an adaptive mutation occurs in lineage X. (B) Because the adaptive mutant can
purge diversity from the entire ecotype, only one cluster survives periodic selection.
ulation now forms a single sequence cluster. Used with permission from the American
Society for Microbiology (69).
Sympatric members of a single ecotype cannot be split among multiple
drive to extinction cells from all the clusters within the ecotype, and the cluster
then the clusters must belong to different ecotypes.
There is one exception to this conclusion: Geographically isolated populations
able to compete with subpopulations from other regions, so sequence divergence
between the geographically isolated subpopulations could proceed indefinitely.
Divergence among geographically isolated members of the same ecotype would
be especially likely for bacteria with low mobility (perhaps pathogens of nonmo-
bile hosts) but would not be possible for highly mobile organisms like Bacillus,
where intercontinental migration of spores occurs extremely frequently (79). In
geographic region (i.e., within migration range) must represent different ecotypes.
sequence clusters and groups known to be ecologically distinct (69,96).
The correspondence between ecologically distinct populations and sequence
clusters has proven useful for bacterial systematics in several ways (68). First,
previously characterized sequence differences between taxa can be used diagnos-
tically to identify unknown isolates.
Second, the correspondence between ecotypes and sequence clusters has en-
abled us to discover ecological diversity among uncultured bacteria. Increasingly
for 16S rRNA (20,41,62). For example, David Ward and coworkers have found
that sequence clusters of uncultured Synechococcus strains from Yellowstone hot
springs correspond to populations inhabiting distinct microenvironments defined
by temperature, photic zone, and stage of ecological succession [(73); D. Ward,
of sequence diversity of 16S rRNA in the genus Frankia uncovered previously
unknown taxa with unique host specificities (64).
Discovery of Ecotypes as Star Clades
While sequence clusters provide a useful criterion for discovering ecotypes, a
serious problem remains. A sequence-based phylogeny from almost any named
bacterial species reveals a hierarchy of clusters, subclusters, and sub-subclusters.
This raises the possibility that a typical named bacterial species may contain many
cryptic and uncharacterized ecotypes, each corresponding to some small subclus-
ter. The challenge is to determine which level of subcluster, if any, corresponds to
ecotypes. Fortunately, the peculiar population dynamics of bacteria allows us to
identify the clusters that correspond to ecotypes.
Jason Libsch and I have developed a model for identifying the clusters corre-
sponding to ecotypes [(15); J. Libsch & F.M. Cohan, unpublished results]. Our
“star clade” approach assumes that the sequence diversity within an ecotype is
constrained largely by periodic selection and much less by genetic drift (random
fluctuation in gene frequencies within a population, most notably within popula-
tions of small size). This assumption is correct if the population size of a bacterial
ecotype is 1010or greater. [If sequence diversity in populations of this size were
limited only by genetic drift, sequence diversity would be far greater than the
0.5%–1.0% generally seen within sequence clusters (14)].
Consider next the consequences of periodic selection on the phylogeny of an
ecotype. Nearly all strains randomly sampled from an ecotype should trace their
ancestries directly back to the adaptive mutant that caused and survived the last
periodic selection event. Thus, the phylogeny of an ecotype should be consistent
are equally closely related to one another (Figure 6). In contrast, a population
whose sequence diversity is limited by genetic drift will have a phylogeny with
In an asexual ecotype, a sequence-based phylogeny would yield a perfect star
clade, with only minor exceptions due to homoplasy (i.e., convergent nucleotide
by periodic selection versus genetic drift. (a) In a population of small size, genetic
drift causes coalescence of many pairs of lineages. Moreover, if recombination is
frequent, there is no opportunity for genome-wide purging of diversity. Consequently,
the phylogeny has many nodes. (b) In a bacterial population, characterized by large
population size and rare recombination, the population’s phylogeny is expected to
resemble a star. Following periodic selection, each strain traces its ancestry directly
back to the adaptive mutant that precipitated the periodic selection event. In addition,
population sizes are too large for genetic drift to create coalescences between pairs of
strains with appreciable frequency.
The phylogenetic signatures of populations whose diversity is controlled
state). However, in an ecotype subject to high rates of recombination, particularly
with other ecotypes, the sequence-based phylogeny can deviate significantly from
a perfect star clade. For example, suppose that a large segment from a divergent
ecotype is recombined into one recipient within the ecotype and that this recipient
later donates this foreign segment to another member of the ecotype (Figure 7).
a deviation from a perfect star clade.
We have used a computer simulation to determine how closely an ecotype’s
sequence-based phylogeny should resemble a perfect star clade (15). In general,
within groups recombining only rarely (e.g., Staphylococcus aureus) (E. Feil,
personal communication), the phylogeny of an ecotype is expected to closely
star. Here a member of the ecotype has received a divergent donor’s sequence in a gene
on which the phylogeny is based, and the divergent sequence is in turn transferred to
another member of the ecotype. Each of these strains then appears to be the other’s
closest relative, and a node is added to the phylogeny. The star clade computer simu-
lation determines how many nodes an ecotype’s phylogeny is expected to have, taking
into account the recombination and mutation parameters of the taxon.
Recombination causes a bacterial ecotype’s phylogeny to deviate from a
resemble a perfect star clade; within a taxon with more frequent recombination
[e.g., Neisseria meningitidis (27)], the phylogeny of an ecotype is expected to
deviate to a greater extent from the star form. Our approach is to determine,
for a given taxon, how closely an ecotype’s sequence-based phylogeny should
resemble a star clade and then to identify the largest groups of strains that are each
consistent with what is expected for an ecotype. Here the phylogenies are based
on a concatenation of several gene loci, usually seven.
The number of nodes within a tree quantifies the degree of resemblance of an
coalescence of two or more lineages yields an additional node (Figure 8). In the
case of S. aureus, where individual alleles are subject to mutation three times
more frequently than recombination per gene (E. Feil, personal communication),
The Star computer program simulated sequence evolution within an ecotype, at the
seven loci used by MLST. The program took into account the recombination and mu-
tation parameters for N. meningitidis and S. aureus, as estimated from sequence data.
Sequence evolution was simulated over many replicate runs. From each replicate run,
a 95% bootstrap-supported phylogeny of the ecotype was determined using PAUP∗’s
heuristic parsimony algorithm (91), based on a concatenation of the seven loci. The
figure indicates for each taxon the fraction of times a particular number of nodes
was found in the ecotype’s phylogeny, over all replicate runs. Owing to the low fre-
quency of recombination in S. aureus, an ecotype in this taxon usually has just one
significant node; therefore, any collection of strains from S. aureus that has more than
one node likely contains more than one ecotype. In the more frequently recombining
N. meningitidis, an ecotype can have up to two significant nodes.
The number of nodes predicted to occur within an ecotype’s phylogeny.
our simulations have shown that an ecotype is only rarely, by chance, expected to
have more than one significant node in its phylogeny (F.M. Cohan, unpublished
results) (Figure 8). Ecotypes may be identified, then, as the largest groups of
strains whose phylogeny contains one significant node. On the other hand, the
greater recombination rates within N. meningitidis [where recombination occurs
from a perfect star clade (F.M. Cohan, unpublished results) (Figure 8). An ecotype
within N. meningitidis is expected to contain at most one or two significant nodes.
Accordingly, we may tentatively identify ecotypes within N. meningitidis as the
largest clusters whose phylogenies contain at most two significant nodes. As we
within most named species of bacteria.
Although the star clade approach produces a theory-based criterion for testing
whether a set of strains are members of the same ecotype (i.e., the maximum
number of nodes expected within an ecotype’s phylogeny), this approach does not
As we shall see, the MLST approach developed by B. Spratt and coworkers (50)
produces accurate hypotheses for demarcating strains into ecotypes.
Discovery of Ecotypes Through Multilocus Sequence Typing
In MLST, strains of a named species are surveyed for partial sequences (usually
∼450 bp) of seven gene loci that produce “housekeeping proteins” (proteins that
are not involved in niche-specific adaptations and are presumably interchangeable
quantified in MLST as the number of loci that are different. Two strains are scored
as different for a locus whether they differ by one nucleotide substitution or by
sified into “clonal complexes”: All strains that are identical with a particular strain
vides the “Burst” computer algorithm developed by E. Feil for assigning strains
into clonal complexes according to criteria set by the user.
ically distinct clusters. The various hypervirulent lineages within N. meningitidis
(1,12,50,109) and within Streptococcus pneumoniae (36,108) have been distin-
guished by MLST as separate clonal complexes. For example, one clonal complex
of serogroup A in N. meningitidis causes pandemic and epidemic meningitis, par-
associated with disease. One clonal complex of serogroup C causes localized out-
breaks in primary schools, university dormitories, and prisons, where conditions
are crowded; and one clonal complex of serogroup B causes disease more spo-
radically (1,12,50,109). It is especially impressive that recombination within N.
of the ten clonal complexes identified within N. meningitidis (27). In the case of
N. meningitidis, the Burst computer algorithm by E. Feil has identified groups of
strains that are identical to a “central” strain at five or more loci; in other species,
a criterion of identity at six or more loci has been used (28). The numbers indicate
multilocus sequence types, as listed at the MLST website. The numbers in parentheses
indicate the number of strains with a given sequence type. Within the inside circle is
the “central” sequence type that is identical to the rest of the clonal complex at five
or more loci. In the next circle are sequence types identical to the central strain at
exactly six loci. In the outside circle are sequence types identical to the central strain at
five loci. The straight line indicates identity between peripheral strains at six loci. The
the star clade approach shows almost every clonal complex to have the phylogenetic
properties of an ecotype.
Demarcation of clonal complexes by MLST. The figure indicates three
MLST from correctly identifying ecologically distinct groups (50). It will be in-
teresting to see how well MLST holds up in analysis of the most frequently re-
combining bacteria, including H. pylori (90) and N. gonorrhoeae (66).
Why does MLST work so well? I have previously hypothesized that the clonal
ecotypes are expected to accumulate little sequence diversity. It was the intuitive
periodic selection events for a given strain to accumulate divergence at one or two
loci out of seven, whether by mutation or recombination—this yields the 6/7 and
5/7 criteria used in MLST. Because MLST’s 6/7 and 5/7 criteria are intuitively
based, we should test analytically whether MLST’s clonal complexes are indeed
Fortunately, the star clade approach can test whether the clonal complexes
the recombination and mutation parameters estimated for the particular taxon. To
this end, I have tested the strains of each of the ten clonal complexes within
N. meningitidis for inclusion within a single ecotype. As shown in Figure 8, using
is expected to have at most two significant nodes in its phylogeny. As it turns out,
the phylogenies of all the MLST clonal complexes within N. meningitidis contain
one or two nodes, with the exception of the ET-37 complex, which has three.
Because an N. meningitidis ecotype is so unlikely to contain three nodes, these
results suggest that the ET-37 complex contains two ecotypes.
be subsumed within a single ecotype. In all but one case, the pool of strains from
different N. meningitidis clonal complexes contained too many nodes to fit within
a single ecotype.
The same pattern has emerged from analysis of ecotypical diversity within
S. aureus: With few exceptions the clonal complexes demarcated by MLST are
each consistent with the phylogeny of one ecotype, and each pair of complexes
represents more than one ecotype.
complexes should change with the rate of recombination. Intuitively, one might
undergo changes in more than two out of seven loci between periodic selection
events. It will be interesting to simulate evolution within an ecotype (using the
simulation developed for the star clade algorithm) to determine how the optimal
criterion for demarcating MLST complexes changes with recombination rate.
complexes yielded by MLST have phylogenies consistent with ecotypes, at least
within S. aureus and N. meningitidis. The clonal complexes produced by MLST
thus produce reliable hypotheses about the membership of ecotypes, and these
also be tested directly by an ecological investigation of the putative ecotypes.
bacterial species may actually be more like a genus than a species.
RECOMMENDATIONS FOR BACTERIAL SYSTEMATICS
A principal aim of systematics is to discover, describe, and classify the diversity
of living organisms. Systematists have concluded that the basic unit of biological
diversity is the species, with these quintessential properties: Species are groups of
organisms whose divergence is hindered by one or more forces of cohesion; they
are ecologically distinct from one another; and they are irreversibly separate. In
species of bacterial systematics but by ecotypes. These are populations of organ-
by natural selection. Named species appear to contain many such ecotypes.
It should not come as a surprise that named species contain this magnitude
of ecological diversity. For decades, systematists have known that there is con-
siderable variation in metabolic traits within named species (49); also, DNA-
DNA hybridization experiments have demonstrated a great diversity in genomic
content within named species (63). More recently the sequencing of multiple
genomes within species has shown considerable variation in the genes contained
We have before us an urgent but accessible goal: to characterize the ecotypical
diversity within our most familiar and important named species. A comprehensive
study of ecological diversity would demand no less. From a clinical point of view,
identifying a pathogen with its ecotype, and thus its distinct virulence properties,
will be invaluable; indeed, this was a primary motivation in developing MLST
(50). Finally, from an evolutionary genetic point of view, statistical techniques
for estimating dynamic properties [such as recombination (37) and migration (84)
rates] from sequence survey data require that the strains sampled come from a
clonal complexes yielded by MLST give a good first approximation for ecotype
demarcation. The star clade method can then test whether the clonal complexes
obtained are consistent with ecotypes. Apparently, in most cases they will be.
are indeed ecologically distinct, and the ecological differences can be character-
ized. Subtractive hybridization is a promising method for discovering the sets of
genes not shared by two ecotypes and may suggest the nature of their ecological
differences (2,45). Also, ecological differences between ecotypes can be charac-
terized by differences in the expression levels of all the genes they share by using
microarray technology (29).
Sequence-based approaches are particularly important for discovering the eco-
logical diversity among uncultured bacteria, which we now know to constitute
the great majority of bacterial diversity (18). The most discerning sequence-based
methods must be utilized, or we will likely underestimate the ecological diversity
among uncultured bacteria. For example, limiting sequence surveys of uncultured
bacteria to 16S rRNA data would likely miss many closely related ecotypes be-
cause multilocus sequence typing has revealed multiple ecotypes within named
species of cultivated bacteria, and these ecotypes are typically identical or nearly
identical in their 16S rRNA sequences.
We are left, then, with the practical, taxonomic matter of classifying the eco-
typical diversity within named species. I recommend that when putative ecotypes
demarcated by MLST and/or the star clade approach are confirmed to be ecologi-
genus, species, and ecotype name. For example, the virulent serogroup A clonal
complex and ecotype of N. meningitidis might be named N. meningitidis ecoty-
pus africana for its role in epidemics and pandemics in sub-Saharan Africa. The
clusters we identify in this fashion are the fundamental units of ecology and evo-
lution. They deserve our attention and they deserve a name.
I am grateful to E. Feil for sharing unpublished data about S. aureus and for
many enlightening discussions about MLST. I am grateful to M. Dehn for help in
and research grants from Wesleyan University.
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