Genetic Differentiation, Niche Divergence, and the Origin and Maintenance of the Disjunct Distribution in the Blossomcrown Anthocephala floriceps (Trochilidae)

Abstract

Studies of the origin and maintenance of disjunct distributions are of special interest in biogeography. Disjunct distributions can arise following extinction of intermediate populations of a formerly continuous range and later maintained by climatic specialization. We tested hypotheses about how the currently disjunct distribution of the Blossomcrown (Anthocephala floriceps), a hummingbird species endemic to Colombia, arose and how is it maintained. By combining molecular data and models of potential historical distributions we evaluated: (1) the timing of separation between the two populations of the species, (2) whether the disjunct distribution could have arisen as a result of fragmentation of a formerly widespread range due to climatic changes, and (3) if the disjunct distribution might be currently maintained by specialization of each population to different climatic conditions. We found that the two populations are reciprocally monophyletic for mitochondrial and nuclear loci, and that their divergence occurred ca. 1.4 million years before present (95% credibility interval 0.7-2.1 mybp). Distribution models based on environmental data show that climate has likely not been suitable for a fully continuous range over the past 130,000 years, but the potential distribution 6,000 ybp was considerably larger than at present. Tests of climatic divergence suggest that significant niche divergence between populations is a likely explanation for the maintenance of their disjunct ranges. However, based on climate the current range of A. floriceps could potentially be much larger than it currently is, suggesting other ecological or historical factors have influenced it. Our results showing that the distribution of A. floriceps has been discontinous for a long period of time and that populations exhibit different climatic niches have taxonomic and conservation implications.

Full text

Genetic Differentiation, Niche Divergence, and the
Origin and Maintenance of the Disjunct Distribution in
the Blossomcrown
Anthocephala floriceps
(Trochilidae)
Marı
´
a Lozano-Jaramillo
1
, Alejandro Rico-Guevara
2
, Carlos Dani el Cadena
1
*
1 Laboratorio de Biologı
´
a Evolutiva de Vertebrados, Departamento de Ciencias Biolo
´
gicas, Unive rsidad de los Andes, Bogota
´
, Colombia, 2 Department of Ecology &
Evolutionary Biology, University of Connecticut, Storrs, Connecticut, United States of America
Abstract
Studies of the origin and maintenance of disjunct distributions are of special interest in biogeography. Disjunct distributions
can arise following extinction of intermediate populations of a formerly continuous range and later maintained by climatic
specialization. We tested hypotheses about how the currently disjunct distribution of the Blossomcrown (Anthocephala
floriceps), a hummingbird species endemic to Colombia, arose and how is it maintained. By combining molecular data and
models of potential historical distributions we evaluated: (1) the timing of separation between the two populations of the
species, (2) whether the disjunct distribution could have arisen as a result of fragmentation of a formerly widespread range
due to climatic changes, and (3) if the disjunct distribution might be currently maintained by specialization of each
population to different climatic conditions. We found that the two populations are reciprocally monophyletic for
mitochondrial and nuclear loci, and that their divergence occurred ca. 1.4 million years before present (95% credibility
interval 0.7–2.1 mybp). Distribution models based on environmental data show that climate has likely not been suitable for
a fully continuous range over the past 130,000 years, but the potential distribution 6,000 ybp was considerably larger than
at present. Tests of climatic divergence suggest that significant niche divergence between populations is a likely
explanation for the maintenance of their disjunct ranges. However, based on climate the current range of A. floriceps could
potentially be much larger than it currently is, suggesting other ecological or historical factors have influenced it. Our results
showing that the distribution of A. floriceps has been discontinous for a long period of time and that populations exhibit
different climatic niches have taxonomic and conservation implications.
Citation: Lozano-Jaramillo M, Rico-Guevara A, Cadena CD (2014) Genetic Differentiation, Niche Divergence, and the Origin and Maintenance of the Disjunct
Distribution in the Blossomcrown Anthocephala floriceps (Trochilidae). PLoS ONE 9(9): e108345. doi:10.1371/journal.pone.0108345
Editor: William J. Etges, University of Arkansas, United States of America
Received January 17, 2014; Accepted August 26, 2014; Published September 24, 2014
Copyright: ß 2014 Lozano-Jaramillo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The study was financed by the Facultad de Ciencias at the Universidad de los Andes, Bogota
´
, Colombia. The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* Email: ccadena@uniandes.edu.co
Introduction
The limits of the geographic ranges of populations and species
reflect the interplay of a variety of ecological and evolutionary
forces such as migration, extinction and speciation [1–3].
Understanding how such forces underlie the origin and mainte-
nance of disjunct distributions, in which closely related taxa or
members of the same species occur in widely separate areas, is of
central interest in biogeography [4,5]. Hypotheses that may
account for the disjunct distributions of species or close relatives
include long-distance dispersal or the extinction of intermediate
populations of a formerly continuous range, possibly as a result of
geographic or climatic events, or human intervention. After
disjunct distributions arise, the question becomes how are they
maintained. Likely explanations for the maintenance of disjunct
distributions are (1) environmental unsuitability of intervening
areas and (2) adaptation to different environmental conditions in
geographically separate areas [1,6–11].
When historical distributions cannot be studied directly (i.e.,
using the fossil record), testing hypotheses about the origin of
disjunct distributions can be accomplished using molecular
phylogenetic estimates of divergence times between populations,
which can be correlated with historical events [12–17]. This
approach has provided insights into pervasive biogeographic
patterns, such as the disjunct distribution of many organisms
occurring in separate continents. For instance, based on the
estimated time of lineage divergence, disjunct distributions of
organisms occurring in America and Africa has been attributed to
the split of Gondwana [17–20], transoceanic dispersal [21–23],
human-mediated introductions [13], or various combinations of
these processes [24].
Inferences about historical ranges and whether disjunct
distributions might be the result of extinction of intermediate
populations can also be made using ecological niche-modeling
tools [25,26] to generate historical estimates of potential species
distributions based on climatic data [27–30]. For example, such
models have indicated that some species with currently disjunct
distributions may have been widely distributed in the past [29,31].
If currently disjunct populations are relicts of more widespread
lineages and one can construct models of the potential distribu-
tions at different times in the past, then one would expect to find a
reduction in the connectivity between populations through time,
with population separation matching the divergence dates
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estimated using molecular data. In addition, climatic data and
statistical analyses based on null models can be used to evaluate
the hypothesis that disjunct distributions are maintained at present
time as a result of differentiation in climatic preferences between
populations found in disjunct areas. Specifically, this hypothesis
predicts that disjunct populations occur under different climatic
environments as a result of niche divergence and that intervening
areas are unsuitable for their occurrence [30].
The Blossomcrown (Anthocephala floriceps Gould, 1854), the
single representative of a monotypic genus of hummingbird
(Trochilidae) endemic to Colombia, is a good model in which to
study disjunct distributions: two sedentary subspecies recognized
based on plumage variation live in regions separated by more than
900 km (Fig. 1). Anthocephala floriceps floriceps is restricted to the
foothills and mid elevations of the Sierra Nevada de Santa Marta
in northern Colombia (500–1700 m), whereas A. f. berlepschi is
found in the Andes (1200–2300 m) in Tolima and Huila
departments [32–34]. In this study, we used DNA sequence data
and niche modeling tools to (1) determine the timing of divergence
between the two populations of A. floriceps, (2) assess whether the
disjunct distribution of the species could have arisen as a result of
fragmentation of a formerly widespread range owing to climate
change over the Pleistocene, and (3) evaluate whether its disjunct
distribution might be maintained by unsuitable intervening areas
or specialization of each isolated population to different climatic
conditions (niche divergence).
Materials and Methods
Molecular analyses
We used nuclear and mitochondrial DNA sequence data to
examine genetic differentiation and to estimate the timing of
divergence between populations of A. floriceps. These data
allowed us to gain insight about factors potentially involved with
the origin of their disjunct ranges. We extracted DNA from tissue
samples of three museum specimens of A. f. floriceps and two of A.
f. berlepschi (Table 1) using a phenol/chlorophorm protocol [35].
We then amplified and sequenced two mitochondrial (ND2 and
ND4) and two nuclear genes (Bfib7 and ODC introns 6 and 7) for
all individuals using published primers and protocols [36,37]. We
did not estimate gametic phase for the nuclear loci; apparent
heterozygosities were coded as ambiguities using IUPAC codes.
We combined our data (GenBank accession numbers KJ826445–
KJ826464) with sequences of the same genes from three
individuals of A. f. berlepschi obtained from GenBank
(GU167208.1, GU166876, GU167098.1, GU166955.1; Table 1;
[38]). As outgroups, we used four of the closest living relatives of
Anthocephala identified by phylogenetic analyses of the Trochili-
dae [37,39]. We obtained sequences for the ND2 and ND4 genes
of the following outgroups from GenBank: Campylopterus
hemileucurus (EU042534.1, EU042214.1), Klais guimeti
(AY830495.1, EU042317.1), Orthorhyncus cristatus
(AY830508.1, EU042328.1), and Stephanoxis lalandi
(GU167250.1, GU166919.1).
To estimate the divergence time between the two populations of
A. floriceps, we constructed a chronogram in BEAST 1.5.2 [40]
based on a concatenated matrix including sequences of both
mitochondrial genes for the two populations and outgroups. We
conducted this analysis using the HKY+G substitution model,
which was selected as the best fit to the data according to the
Akaike Information Criterion (AIC) in ModelTest 3.7 [41]. To
calibrate our tree based on analyses including ND4 data, we used
the ND2 substitution rate of 2.5% divergence per million years
[42], and related the corrected distances for ND2 with the
distances obtained combining ND2 and ND4 data using a linear
regression. Because the slope of the regression was 1.11 (r
2
= 0.99),
we multiplyed the ND2 per-lineage rate of 0.0125 by 1.11, and
fixed the product (0.0139) as the mean rate for calibration. We
fitted a relaxed molecular clock with lognormal rate-variation, and
ran 50 million generations sampling every 1000 steps and
discarding the first 10,000 as burn-in. We used TRACER v1.5
to check that effective sample sizes of parameter estimates were
greater than 200. As an additional way to examine relationships
among mtDNA and nucDNA sequences, we also constructed
haplotype networks (for concatenated mitochondrial data and
separately for each nuclear locus) using the median-joining
algorithm in the software Network 4.5.1.6 [43].
Ecological niche modeling
We first used ecological niche modeling tools to (1) determine
whether areas located in between the two disjunct distribution
ranges of A. floriceps are unsuitable for its occurrence, and (2) to
assess whether the distribution of A. floriceps could have been
more widespread in the past (i.e., at different periods in the
Pleistocene). For these analyses, we used 43 localities obtained
from museum specimens ([44], Global Biodiversity Information
Facility (GBIF: http://www.gbif.org)), field observations (N.
Gutie´rrez, pers. comm.), and published data [34]. We character-
ized each locality with 19 climatic variables at 1 km x 1 km
resolution obtained from WorldClim [45]; these variables are
commonly used in ecological niche modeling and indicate annual
trends, seasonality, and extreme values in temperature and
precipitation. We considered all of Colombia and western
Venezuela and generated a model of the potential distribution of
A. floriceps in this area at present using the maximum enthropy
algorithm implemented in Maxent 3.3.2 [27]. We used default
settings to obtain a logistic model output with continuous values
ranging from 0 to 100, with higher values indicating greater
probabilities of occurrence. Following model-validation using the
area under the receiver-operating-characteristic (ROC) curve and
a binomial test of omission [27], we projected the model onto
climate layers for 6,000 years before present (ybp), the Last Glacial
Maximum (LGM; aprox. 21,000 ybp), and 130,000 ybp [45,46].
To distinguish climatically suitable from unsuitable sites, we
applied the ‘‘fixed cumulative value 10’’ threshold rule in Maxent
[47]. We visually assessed the extent of potential distributions at
these different time periods.
We also used ecological niche modeling to evaluate whether the
currently disjunct distribution of A. floriceps might be maintained
by specialization of each population to different climatic condi-
tions. To accomplish this, we first modeled the potential
distribution at present of each population separately using the 19
climatic variables. We then projected models generated for each
population onto the geographic region where the other population
occurs to assess whether each model would classify the localities
where the other population has been recorded as climatically
suitable (i.e., model interprediction). Low model interprediction
would support the hypothesis of climatic specialization maintain-
ing disjunct ranges. However, because the two populations occur
in geographically distinct areas where climate may differ
considerably irrespective of the presence or absence of the study
species, lack of interprediction of distribution models does not
necessarily reflect intrinsic niche divergence between populations;
populations may have equivalent fundamental niches yet occupy
different environments (i.e., different realized niches) due solely to
geographic differences in climate [10,48,49]. Thus, we sought to
determine whether the environments where populations occurred
were more or less similar that expected by chance based on
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differences in the climatic conditions of the regions within which
the ranges of each population are embedded. To do so, we
examined climatic divergence between populations relative to a
null divergence model using the climatic background of the range
of each population, an approach that allows for explicit testing of
niche divergence vs. niche conservatism [10]. For this analysis we
used the 19 WorldClim variables and also elevation; we reduced
these 20 variables using a principal component analysis (PCA) and
then employed the first four principal components (accounting for
c. 97% of the variance, see below) as observed niche values. To
establish background variation in climate, we extended polygons
depicting the known distribution range of each population of A.
floriceps [50] 20 km in all directions and randomly placed 1000
points within each expanded polygon. Values for elevation and the
19 climatic variables were extracted for all of these points. Niche
divergence and conservatism were assessed by comparing the
observed difference in mean niche values to the difference in mean
background (i.e., null) values for each of the four principal
components. Niche divergence, i.e., specialization to different
climates, as a potential factor accounting for the maintenance of
disjunct distributions would be supported if population niches were
more divergent than expected based on background divergence
[10]. Tests were conducted in R version 2.12.2 [51].
Figure 1. Current distribution of the Blossomcrown (
Anthocephala floricep
s). The blue area corresponds to A. f. floriceps from the Sierra
Nevada de Santa Marta and the red to A. f. berlepschi from the Andes. The locations of different montane regions mentioned in the text are indicated.
doi:10.1371/journal.pone.0108345.g001
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Results
Molecular analyses
Genealogies showed the same pattern for all genes: each
subspecies of A. floriceps formed a monophyletic group comprising
distinct haplotypes (Fig. 2). Although our sample sizes were low,
this pattern suggested the two populations have been isolated for a
considerable time span, long enough to have achieved reciprocal
monophyly in both mitochondrial and nuclear loci. Furthermore,
the chronogram based on mtDNA sequences indicated that the
two subspecies were reciprocally monophyletic groups whose
divergence dates to c. 1.4 million years before present (mybp; 95%
credibility interval 0.7–2.1 mybp; Fig. 3).
Ecological niche modeling
The area under the ROC curve for the model predicting the
potential distribution of A. floriceps at present was close to one
(0.983), indicating it performed substantially better than chance.
Additionally, the binomial test of omission was significant (p,
0.001), suggesting that the species’ distribution was adequately
predicted based on climate. This model suggested that environ-
mental conditions suitable for the occurrence of A. floriceps existed
well beyond the boundaries of its current range in the Andes
(Fig. 4a). This indicates that, based on the climatic variables
studied, at least part of the range disjunction cannot be attributed
to climatic unsuitability of intervening areas.
Because the model based on climatic data adequately predicted
the present-day distribution (i.e., point localities) of A. floriceps,
assuming niche conservatism one can use such models to examine
the potential distribution of the species in the past based on
historical climate. None of the historical distribution ranges
estimated by the model were sufficiently large suggesting there
was potential for the species to be continuously distributed in the
past (Fig. 4). However, the potential distribution for 6,000 ybp was
considerably larger and more continuous than the potential
distribution at present (Fig. 4b); at this time, the Sierra Nevada de
Santa Marta appears to have been connected to the northern end
of the Cordillera Oriental of the Andes (i.e., Serranı
´
a de Perija´) by
areas suitable for the presence of A. floriceps across the intervening
lowlands. Moreover, environments potentially suitable for the
species appear to have been more extensively distributed in the
northern sector of the Cordillera Central and in the Serranı
´
ade
San Lucas and surrounding lowlands 6,000 ybp relative to the
present. In contrast, much of the area now occupied by A. floriceps
(including all of the range of A. f. floriceps in the Sierra Nevada de
Santa Marta) appear to have been unsuitable for the species
21,000 ybp (Fig. 4c). Finally, for 130,000 ybp, the model identi-
fied continuous areas of potentially high climatic suitability along
the eastern slope of the Cordillera Oriental and extending into
lowland areas east of the Andes, but revealed no potential
connections between the currently disjunct populations (Fig. 4d).
Potential distribution models constructed separately for each
population based on present-day climate also had area under
ROC curves close to one (A. f. floriceps: 0.981, A. f. berlepschi:
0.944). However, the distribution model constructed for each
population did not predict the current distribution of the other
(Fig. 5), implying that each population inhabits environments with
different climatic conditions. This result was supported by tests of
niche divergence and conservatism (Table 2). The axis explaining
most of the variation (PC1; 40%) was largely associated with
elevation and temperature and was the only one revealing
significant niche conservatism. The other three axes (jointly
accounting for c. 57% of environmental variation) revealed
significant niche divergence between populations associated with
precipitation and seasonality (Table 2; Table S1). The Andean
Table 1. Specimens of A. floriceps included in molecular phylogenetic analyses.
Taxon Tissue number Locality
A. f. floriceps ICN 36492 Santa Marta, Cuchilla de San Lorenzo
A. f. floriceps ICN 36491 Santa Marta, Cuchilla de San Lorenzo
A. f. floriceps ICN 36467 Santa Marta, Cuchilla de San Lorenzo
A. f. berlepschi ANDES-BT 1311 Huila, Algeciras, Vereda Las Brisas, Finca Be
´
lgica
A. f. berlepschi ANDES-BT 1315 Huila, Algeciras, Vereda Las Brisas, Finca Be
´
lgica
A. f. berlepschi IAvH 1253 Huila, Palestina, Parque Nacional Natural Cueva de los Gua
´
charos
A. f. berlepschi IAvH 1269 Huila, Palestina, Parque Nacional Natural Cueva de los Gua
´
charos
A. f. berlepschi IAvH 1255 Huila, Palestina, Parque Nacional Natural Cueva de los Gua
´
charos
ICN: Instituto de Ciencias Naturales, Universidad Nacional de Colombia; ANDES-BT: Banco de Tejidos, Museo de Historia Natural de la Universidad de los Andes; IAvH:
Instituto Alexander von Humboldt.
doi:10.1371/journal.pone.0108345.t001
Figure 2. Haplotype networks showing that no alleles are
shared between populations of
A. floriceps
in any of the genes
analyzed. Blue corresponds to A. f. floriceps and red to A. f. berlepschi.
Circle size is proportional to the number of individuals with each
haplotype; hatches indicate mutational steps. (a) ND2, (b) ND4, (c) Bfib7
and (d) ODC.
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population lives in less humid and less seasonal environments than
the population from the Sierra Nevada de Santa Marta (Fig. S1).
Discussion
The origin and maintenance of the disjunct distribution
in A. floriceps
Our estimates of potential distributions based on climatic data
indicated that in four time periods over the last 130,000 ybp,
including the present, climatic conditions have likely not been
suitable for A. floriceps to have had a fully continuous distribution.
The only possible exception to this pattern is the inferred
connection between the Sierra Nevada de Santa Marta and the
northern stretches of the Cordillera Oriental (Serranı
´
a de Perija´)
suggested by the predicted potential distribution for 6,000 ybp
(Fig. 4b). Also at 6,000 ypb, the species appears to have had a
more extensive potential distribution along the Cordillera Central,
which may have allowed for connectivity between this mountain
range, the Serranı
´
a de Perija´ and the Sierra Nevada de Santa
Marta via the Serranı
´
a de San Lucas and surrounding areas, a
region in which climatically suitable areas appeared to have been
considerably more extensive than at present (Fig. 4). If either
scenario is correct, then the species must have gone extinct not
only from the lowland environments separating the Sierra Nevada
de Santa Marta from the Perija´, but also from the full extent of the
Perija´, the Serranı
´
a de San Lucas and the Cordillera Oriental,
mountain systems where it does not presently exist.
We note, however, that estimates of potential historical
distributions based on ecological niche modeling must be
considered cautiously because the realized conditions under which
species exist at present (i.e., those used to build ecological niche
models) may not fully represent their fundamental niches and
could lead to potentially misleading reconstructions of their
geographic ranges at other times. Especially in scenarios where
combinations of climatic conditions that existed in the past are not
equivalent to those existing in the present, i.e., non-analogous
climates, models based only on present-day conditions may not
accurately estimate historical distributions [52,53]. We suspect this
likely applies to our estimate of potential distribution for A.
floriceps at 21,000 ybp, when its potential range appeared to have
been substantially reduced, with no suitable environments in the
Sierra Nevada de Santa Marta, the region where one of its
present-day populations is endemic (Fig. 4c). Based on patterns of
genetic variation indicating marked distinctiveness of the Santa
Marta population (see below), that the species was absent from this
mountain range at this time and colonized it subsequently seems
unlikely.
Because GIS layers depicting estimates of historical climate in
our study region are unavailable for dates earlier than those we
examined, we cannot address the possibility that the range of A.
floriceps became disjunct at an earlier moment in history using
ecological niche modeling. Can molecular data provide insights
about the origin of its disjunct distribution? Our molecular-clock
analysis suggests that the divergence between mtDNA clades dates
to c 1.4 million mybp (credibility interval 0.7–2.1 mybp),
suggesting that divergence occurred prior to the period for which
historical climate data are available. However, we note that the
inferred timing of divergence reflects gene divergence, which may
be considerably older than population/taxon divergence [54,55].
This is a likely possibility considering that Neotropical montane
birds often show strong population genetic differentiation even
along continuous ranges [56,57].
Our results are consistent with the hypothesis that the currently
disjunct distribution of A. floriceps may persist due to specializa-
tion of each isolated population to different climatic conditions.
Ecological niche models suggest that populations of A. floriceps are
divergent in their climatic niches beyond what one would expect
given the climatic background where they exist, implying that a
Figure 3. Divergence-time estimates (mya) between populations of
A. floriceps
and outgroups, based on two mitochondrial genes
using a Bayesian relaxed molecular-clock analysis. Node bars indicate 95% credibility intervals on node ages; scale bar shows time in million
years. Values on each clade indicate posterior probabilities when greater than 0.7. Symbols indicate individuals having identical sequences in A. f.
floriceps (*) and A. f. berlepschi (").
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plausible explanation for the maintenance of their disjunct ranges
is climatic niche divergence. It makes sense that both populations
exhibit a conserved niche axis related to elevation and temper-
ature because their elevational ranges overlap broadly [34].
However, our analyses revealed significant niche divergence in
relation to precipitation and seasonality, with the Andean
population occupying less humid and less seasonal environments.
If this reflects that each population is adapted to specific climatic
conditions and not simply that realized climatic conditions differ
between regions but fundamental climatic niches do not, then
climatic restrictions likely do not allow the species’ geographic
distribution to become fully continuous [10,48,49,58–60].
Although our models failed to reveal continuous potential
distributions in the past and at present and populations showed
significant climatic divergence, climatic unsuitability of intervening
areas and niche divergence between populations are not sufficient
explanations for the c. 900-km discontinuity in the present-day
range of A. floriceps. The modeled potential distribution at present
(Fig. 4a) indicates that environmental conditions suitable for its
occurrence exist through much of the Cordillera Central of the
Figure 4. Potential distributions for
A. floriceps
predicted using climatic data in Maxent. Models are shown for climatic conditions of (a)
the present, (b) 6,000 ybp, (c) 21,000 ybp and (d) 130,000 ybp. Dots on the present distribution map indicate localities used to build the models.
Darker colors denote areas of greater climatic suitability; areas in white are below the minimum suitability threshold and are therefore considered to
be unsuitable.
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Table 2. Divergence on niche axes between populations of A. floriceps.
Niche axes
PC1 PC2 PC3 PC4
Pairwise comparison
A. f. floriceps vs A. f. berlepshi 0.78C 1.23 D 0.70 D 1.37 D
(0.58, 1.26) (0.30, 1.55) (0.63, 0.89) (1.13, 2.31)
Variance explained (% ) 40% 24% 22% 11%
Top four variable loadings elevation* bio16 bio17* bio3*
bio6 bio13 bio14* bio14
bio11 bio12 bio12* bio15
bio10 bio18 bio18 bio17*
Instances of significant niche divergence (D) or conservatism (C) are shown in bold (t-test; p,0.05). Values in parentheses represent the 95% confidence intervals of the
null distributions based on background divergence between the geographic ranges of each population. For each niche axis, the top four environmental variables
loading on it are shown (asterisks indicate opposite sign). bio3 = isothermality, bio6 = minimum temperature of coldest month, bio10 = mean temperature of warmest
quarter, bio 11 = mean temperature of coldest quarter, bio12 = annual precipitation, bio13 = precipitation of wettest month, bio14 = precipitation of driest month,
bio15 = precipitation seasonality, bio16 = precipitation of wettest quarter, bio17 = precipitation of driest quarter, bio18 = precipitation of warmest quarter. For full results
of principal components analysis see Fig. S1.
doi:10.1371/journal.pone.0108345.t002
Figure 5. Model of potential distribution constructed based on localities of
A. f. berlepschi
projected onto the region where
A. f.
floriceps
occurs (indicated by a blue shape; (a)). Model of potential distribution constructed based on localities of A. f. floriceps projected onto
the region where A. f. berlepschi occurs (indicated by a red shape; (b)). Red and blue dots indicate localities used to build the models for A. f. berlepschi
and A. f. floriceps, respectively. Darker colors denote areas of greater climatic suitability in a continuous scale (i.e., no cutoff threshold was established
in Maxent). Note that localities of each population have low suitability according to the model constructed with data from the other population,
indicating niche divergence.
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Colombian Andes, a region lacking obvious environmental
discontinuities [2]. Also, suitable conditions exist along the western
slope of the Cordillera Oriental albeit with some notable
environmental breaks (Fig. 4a; [2]). Thus, based on climatic
conditions, the distribution range of A. floriceps could potentially
be larger than it currently is, especially in the Cordillera Central. A
similar result was obtained in a recent study examining disjunct
populations of Painted Buntings (Passerina ciris) in North
America, where areas not occupied by the species were found to
be potentially suitable for its occurrence [30]. The restricted
distribution range of the Andean form A. f. berlepschi likely reflects
ecological factors not accounted for by climatic variation (e.g.,
biotic interactions) or historical factors limiting range expansion.
The influence of historical factors is likely, considering A. f.
berlepschi is one of several members of a distinctive assemblage of
codistributed taxa restricted to an area of endemism in the
departments of Tolima and Huila [8,61,62].
Taxonomic and conservation implications
Our divergence time estimates between populations of A.
floriceps (1.4 mybp) suggest an older date than the reported
divergence times for phylogroups within some Neotropical
hummingbird species [60,63–65] and even between several
lineages recognized as different species of hummingbirds [66].
Our analyses further showed that subspecies do not share
haplotypes in four different genes including nuclear loci, with
their four-fold higher coalescence times relative to mtDNA,
indicating long-term isolation without gene flow. We realize our
sample sizes are not large enough to provide a robust test of
reciprocal monophyly, but given the strong divergence and
geographic isolation, we suspect our conclusions would be robust
to analyses with larger sample sizes.
In conclusion, our data suggest that the current distribution of
A. floriceps has been disjunct for a relatively long time.
Furthermore, each population occurs under distinct climatic
conditions, which likely reflects evolved differences in their
climatic niche. Our results revealing strong genetic and climatic
divergence between populations of A. floriceps, together with
morphological differences that led to their recognition as different
subspecies, arguably have taxonomic implications. The evidence
for marked divergence and reciprocal monophyly in mitochon-
drial and nuclear loci, in addition to climatic differentiation and
morphological diagnosability, implies that each population could
be considered a full species under several species concepts [67–71].
Applying the criterion of reproductive isolation central to the
biological species concept is impossible owing to the allopatric
distributions of the two populations, but divergence in several
respects between them, relative to divergence between ‘‘good’’
species of hummingbirds [72], may suffice to consider them to be
reproductively isolated [73]. In any event, the likelihood that the
two forms may eventually come into contact appears extremely
unlikely, so their status as independently evolving units will most
likely be maintained and should probably prevail in terms of
establishing their taxonomic status [74]. At the very least, our work
shows that these populations are divergent lineages meeting the
criteria for recognition as evolutionarily significant units worthy of
attention from a conservation standpoint and requiring indepen-
dent management [75,76]. Their distinctiveness has likely been
overlooked as a consequence of traditional taxonomy treating
them as conspecific, a situation that may apply to several other
populations of Neotropical birds with disjunct ranges [77].
Supporting Information
Figure S1 Bivariate plots showing climatic differences
between localities occupied by Anthocephala floriceps
floriceps in the Sierra Nevada de Santa Marta (blue) and
A. f. berlepschi in the Andes (red). Note that A. f. berlepschi
occurs in drier areas with more stable temperature and less
seasonal precipitation than A. f. floriceps.
(TIF)
Table S1 Variables used to characterize the ecological
niches of populations of Anthocephala floriceps and their
loadings on the first four axes obtained following
principal components analyses. These four axes accounted
for 97% of the variation. The variables with the four highest
loadings on each principal component are shown in bold.
(DOCX)
Acknowledgments
We thank the Facultad de Ciencias at Universidad de Los Andes for
funding and the Instituto de Gene´tica at Universidad de Los Andes for
allowing access to their facilities. We thank J. L. Parra for providing DNA
sequences and F. G. Stiles for authorizing the use of tissue samples from the
Instituto de Ciencias Naturales at Universidad Nacional de Colombia. We
thank members of the Laboratorio de Biologı
´
a Evolutiva de Vertebrados at
Universidad de Los Andes, especially A. Morales, P. Pulgarı
´
n, N.
Gutie´rrez, and S. Gonza´lez, for sharing ideas and providing assistance
throughout the study. E. Tenorio provided valuable help with analyses and
figures. The manuscript was improved thanks to helpful comments by K.
Hurme and two anonymous reviewers.
Author Contributions
Conceived and designed the experiments: MLJ CDC. Performed the
experiments: MLJ CDC. Analyzed the data: MLJ ARG CDC. Contributed
reagents/materials/analysis tools: MLJ CDC. Wrote the paper: MLJ ARG
CDC.
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