64
Basic biogeography: estimating biodiversity and mapping nature
Science from 1990 to 2010, with 47 per cent of those
records falling between 2006 and 2010, suggesting an
ongoing acceleration in the use of ESUs.
ESUs represent something of a conundrum for those
projects that seek to fully enumerate more traditional
and intuitive conservation units (i.e. species) as a baseline unit of biodiversity (e.g. the Catalogue of Life,
Species 2000, Encyclopedia of Life; Table 4.1 ), in the
sense that they do not coincide perfectly with either a
species or subspecies as a formal taxonomic category
(e.g. Zink, 2004 ) and therefore are not fully accounted
for in taxonomic lists. Furthermore, discrepancies
between species and ESUs could lead to very different
assessments of local and regional biodiversity, which
might lead to different inferences in biogeographical
and ecological analyses (see Riddle & Hafner, 1999 ;
Kelt & Brown, 2000 ).
Under the Moritz (1994) defi nition, the discrepancy
between ESU diversity and species diversity can be estimated and appears to be considerable, but not overwhelming. In a survey of vertebrate taxonomic species
(Avise & Walker, 1999 ), 56 per cent contained at least
two, but less than seven, lineages that could be considered as separate ESUs; Riddle and Hafner (1999) estimated an average of 2.7 ESUs per species in desert
rodents from western North America; and Zink (2004)
estimated 1.9 ESUs per avian species.
Some have argued that ESUs delineated using the
Moritz defi nition should qualify for recognition as distinct species under the PSC (Vogler & DeSalle, 1994 ).
Most often, however, practitioners are properly hesitant to name new species formally based on a single -
gene or - genome (most commonly mtDNA in animals
and cpDNA in plants) assay of geographic, phylogenetic, and population genetic variation because a
single gene tree will often not be entirely congruent
with the species tree (Avise, 2005 ; Edwards, 2009 ). In
other words, a single mtDNA gene tree, even if resulting in several reciprocally monophyletic lineages
(i.e. ESUs), is not always going to refl ect the same
phylogeny for different genes drawn from the nuclear
genome.
At the same time, biogeographers and conservation
biologists do recognize the great operational utility of
mtDNA or cpDNA assays as a powerful ‘ fi rst approximation ’ of evolutionary and biogeographical pattern
and processes (Zink & Barrowclough, 2008 ). They
have indeed been used extensively to assay patterns of
biodiversity, to postulate associated historical processes
and to develop conservation prescriptions.
further analysis across a broader range of taxa and
geographical regions, the practical offshoots of a PSC
perspective have taken root in several tangible forms,
which we will discuss next.
4.3.2 Evolutionarily Signifi cant Units ( ESU s )
The most frequently cited molecular - based units that
sidestep the task of formally naming new species are
called Evolutionarily Signifi cant Units, or ESUs. The
concept of an ESU originated with Ryder (1986) , but
has since been developed variously by Waples (1991) ,
Moritz (1994) , and Crandall et al . (2000) . The standard way to identify ESUs is through analysis of mitochondrial DNA (mtDNA) in animals or chloroplast
DNA in plants (cpDNA). This is because these
two organelle molecules, being maternally inherited,
evolve very rapidly and therefore are more likely than
nuclear genes to refl ect the history of long - term population divergence through isolation.
Moritz (1994) offered the defi nition that an ESU
should be reciprocally monophyletic for mtDNA alleles
and show signifi cant divergence of allele frequencies at
nuclear loci. This is an attempt to delineate the ‘ major
historical units ’ that arise from long - term geographical
isolation and evolutionary divergence (Avise, 2005 ).
Such ‘ natural groups ’ may be morphologically cryptic
but hold within them not only the imprint of their past
history, but the possibility of differing responses to
future environmental change. Whether or not they
satisfy the criteria for the application of the biological
species concept, or merely represent sub - specifi c variation, these ESUs may warrant individual conservation
attention.
Alternative defi nitions have attempted to align ESUs
with the ‘ Distinct Population Segments ’ (DPS) amendment to the 1973 Endangered Species Act in the USA
(Waples, 1991 ), with incorporation of a broader recognition of evolutionary processes (Crandall et al .,
2000 ) or with the desirable goal of achieving genealogical concordance across multiple genes (Avise,
2005 ).
Regardless of defi nition, the underlying power of the
ESU concept – with its reliance on molecular data and
the ability to ‘ do ’ biogeography and conservation
without the necessity of formally naming new species
– appears to remain intact. 420 hits were returned
from a query using ‘ Evolutionar * Signifi cant Unit * ’ in
a recent (23 October 2010) topic search of the Web of
Basic biogeography: estimating biodiversity and mapping nature
Science from 1990 to 2010, with 47 per cent of those
records falling between 2006 and 2010, suggesting an
ongoing acceleration in the use of ESUs.
ESUs represent something of a conundrum for those
projects that seek to fully enumerate more traditional
and intuitive conservation units (i.e. species) as a baseline unit of biodiversity (e.g. the Catalogue of Life,
Species 2000, Encyclopedia of Life; Table 4.1 ), in the
sense that they do not coincide perfectly with either a
species or subspecies as a formal taxonomic category
(e.g. Zink, 2004 ) and therefore are not fully accounted
for in taxonomic lists. Furthermore, discrepancies
between species and ESUs could lead to very different
assessments of local and regional biodiversity, which
might lead to different inferences in biogeographical
and ecological analyses (see Riddle & Hafner, 1999 ;
Kelt & Brown, 2000 ).
Under the Moritz (1994) defi nition, the discrepancy
between ESU diversity and species diversity can be estimated and appears to be considerable, but not overwhelming. In a survey of vertebrate taxonomic species
(Avise & Walker, 1999 ), 56 per cent contained at least
two, but less than seven, lineages that could be considered as separate ESUs; Riddle and Hafner (1999) estimated an average of 2.7 ESUs per species in desert
rodents from western North America; and Zink (2004)
estimated 1.9 ESUs per avian species.
Some have argued that ESUs delineated using the
Moritz defi nition should qualify for recognition as distinct species under the PSC (Vogler & DeSalle, 1994 ).
Most often, however, practitioners are properly hesitant to name new species formally based on a single -
gene or - genome (most commonly mtDNA in animals
and cpDNA in plants) assay of geographic, phylogenetic, and population genetic variation because a
single gene tree will often not be entirely congruent
with the species tree (Avise, 2005 ; Edwards, 2009 ). In
other words, a single mtDNA gene tree, even if resulting in several reciprocally monophyletic lineages
(i.e. ESUs), is not always going to refl ect the same
phylogeny for different genes drawn from the nuclear
genome.
At the same time, biogeographers and conservation
biologists do recognize the great operational utility of
mtDNA or cpDNA assays as a powerful ‘ fi rst approximation ’ of evolutionary and biogeographical pattern
and processes (Zink & Barrowclough, 2008 ). They
have indeed been used extensively to assay patterns of
biodiversity, to postulate associated historical processes
and to develop conservation prescriptions.
further analysis across a broader range of taxa and
geographical regions, the practical offshoots of a PSC
perspective have taken root in several tangible forms,
which we will discuss next.
4.3.2 Evolutionarily Signifi cant Units ( ESU s )
The most frequently cited molecular - based units that
sidestep the task of formally naming new species are
called Evolutionarily Signifi cant Units, or ESUs. The
concept of an ESU originated with Ryder (1986) , but
has since been developed variously by Waples (1991) ,
Moritz (1994) , and Crandall et al . (2000) . The standard way to identify ESUs is through analysis of mitochondrial DNA (mtDNA) in animals or chloroplast
DNA in plants (cpDNA). This is because these
two organelle molecules, being maternally inherited,
evolve very rapidly and therefore are more likely than
nuclear genes to refl ect the history of long - term population divergence through isolation.
Moritz (1994) offered the defi nition that an ESU
should be reciprocally monophyletic for mtDNA alleles
and show signifi cant divergence of allele frequencies at
nuclear loci. This is an attempt to delineate the ‘ major
historical units ’ that arise from long - term geographical
isolation and evolutionary divergence (Avise, 2005 ).
Such ‘ natural groups ’ may be morphologically cryptic
but hold within them not only the imprint of their past
history, but the possibility of differing responses to
future environmental change. Whether or not they
satisfy the criteria for the application of the biological
species concept, or merely represent sub - specifi c variation, these ESUs may warrant individual conservation
attention.
Alternative defi nitions have attempted to align ESUs
with the ‘ Distinct Population Segments ’ (DPS) amendment to the 1973 Endangered Species Act in the USA
(Waples, 1991 ), with incorporation of a broader recognition of evolutionary processes (Crandall et al .,
2000 ) or with the desirable goal of achieving genealogical concordance across multiple genes (Avise,
2005 ).
Regardless of defi nition, the underlying power of the
ESU concept – with its reliance on molecular data and
the ability to ‘ do ’ biogeography and conservation
without the necessity of formally naming new species
– appears to remain intact. 420 hits were returned
from a query using ‘ Evolutionar * Signifi cant Unit * ’ in
a recent (23 October 2010) topic search of the Web of