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Proc. R. Soc. B (2007) 274, 1709–1714
doi:10.1098/rspb.2007.0224
Published online 15 May 2007
Reproductive isolation of sympatric morphs
in a population of Darwin’s finches
Sarah K. Huber1,*, Luis Fernando De León2,3, Andrew P. Hendry2,
Eldredge Bermingham3 and Jeffrey Podos1,4
1
Graduate Program in Organismic and Evolutionary Biology, and 4Department of Biology, University of Massachusetts,
Amherst, MA 01003, USA
2
Redpath Museum and Department of Biology, McGill University, 859 Sherbrooke Street West,
Montreal, Quebec, Canada H3A 2K6
3
Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Panama
Recent research on speciation has identified a central role for ecological divergence, which can initiate
speciation when (i) subsets of a species or population evolve to specialize on different ecological resources
and (ii) the resulting phenotypic modes become reproductively isolated. Empirical evidence for these two
processes working in conjunction, particularly during the early stages of divergence, has been limited. We
recently described a population of the medium ground finch, Geospiza fortis, that features large and small
beak morphs with relatively few intermediates. As in other Darwin’s finches of the Galápagos Islands, these
morphs presumably diverged in response to variation in local food availability and inter- or intraspecific
competition. We here demonstrate that the two morphs show strong positive assortative pairing, a pattern
that holds over three breeding seasons and during both dry and wet conditions. We also document
restrictions on gene flow between the morphs, as revealed by genetic variation at 10 microsatellite loci. Our
results provide strong support for the central role of ecology during the early stages of adaptive radiation.
Keywords: ecological speciation; adaptive radiation; assortative mating; adaptive divergence;
genetic divergence
1. INTRODUCTION
Bimodal populations, although rare, provide outstanding
opportunities to study the early stages of adaptive
diversification (Smith 1993; Smith & Skulason 1996;
Orr & Smith 1998; Gislason et al. 1999; Rundle & Nosil
2005). We have recently described a bimodal population
of the medium ground finch (Geospiza fortis) at El
Garrapatero on Santa Cruz Island, Galápagos, Ecuador
(figure 1). This population features birds that fall mainly
into large and small beak size morphs, with relatively few
intermediates, a pattern that has been confirmed statistically (Hendry et al. 2006; Huber & Podos 2006). If other
G. fortis populations are any guide (Price 1987; Grant
1999; Keller et al. 2001; Grant & Grant 2006), this
variation has a strong additive genetic basis and reflects
selection imposed by variation in the size and hardness of
seeds. The bimodality has almost certainly arisen owing to
specialization by the two morphs on different food types,
perhaps coupled with intra- or interspecific competition,
and reflecting processes thought to have driven the
adaptive radiation as a whole (Lack 1947; Boag & Grant
1981; Schluter 2000; Grant & Grant 2002; Herrel et al.
2005). Moreover, the structure of vocal mating signals
(songs) of males at El Garrapatero differs between the
morphs in acoustic parameters that correspond to
differences in beak size and vocal performance (Huber &
Podos 2006). The presence of ecologically driven
bimodality in beak size, coupled with divergence in mating
* Author for correspondence (shuber@bio.umass.edu).
Received 16 February 2007
Accepted 23 April 2007
signals, suggests that this population might be in an early
stage of speciation, a possibility that we investigate here.
We examined three factors that may influence incipient
ecological speciation in El Garrapatero G. fortis: the
strength of assortative pairing; the persistence of assortative pairing over time and across variable ecological
conditions; and levels of gene flow between the morphs.
Our study focused on breeding pairs of G. fortis during
2004–2006. Climatic conditions varied widely during
these years, which allowed us to test for the strength and
stability of assortative pairing under variable ecological
conditions. Virtually no rain fell in 2004 and in the first
two months of 2005, making this the most extreme
drought in the 40-year period of record (Grant & Grant
2006). Some breeding occurred during this period but at a
low rate. Heavy rains fell in March 2005, and the number
of breeding pairs increased considerably. More typical
rainfall prevailed in 2006.
2. MATERIAL AND METHODS
We studied pairs of G. fortis during the breeding season in
January–April 2004, January–May 2005 and January–March
2006 at El Garrapatero, Santa Cruz Island, Galápagos,
Ecuador (GPS coordinates: 00840 0 20 00 –41 0 20 00 S; 90813 0 10 00 –
14 0 40 00 W). Birds were captured in mist nets and banded with
unique combinations of one metal and three colour bands.
We took the following measurements on each bird (Grant
et al. 1985): beak length; beak depth; and beak width. We
then collected a small volume of blood from the ulnar vein of
1709
This journal is q 2007 The Royal Society
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1710 S. K. Huber et al.
Reproductive isolation of Darwin’s finches
(a)
20
frequency
15
10
5
0
–2
–1
0
1
beak PC1
2
3
(b)
(c)
(d )
Figure 1. (a) The bimodal distribution of beak sizes for
G. fortis at El Garrapatero in 2004 (white bars, females; black
bars, males). Bimodality has been inferred for this population
by statistical comparison of fits with unimodal and bimodal
distributions (Hendry et al. 2006). (b) Representative small
morph (left) and large morph (right) birds; both are mature
males caught at the same time in the same mist net. (c) A large
ground finch (G. magnirostris) and the same large morph
G. fortis shown in (b). (d ) A small ground finch (G. fuliginosa).
Scale barZ5 mm. Photo credits Andrew Hendry.
each bird, using a 27-gauge needle and filter paper treated
with EDTA.
Focal observations of individuals were used to determine
pairing status. Repeat observations of pairs were made every
3–4 days throughout the breeding season or until nestlings
fledged. The occurrence of two or more of the following
behaviours was used to identify mated pairs: nest building
by both the male and female; copulation; mate guarding;
feeding of the female by the male during incubation or
courtship; feeding of nestlings; back-and-forth calling
between the male and the female. This study was restricted
to pairs that bred (i.e. females that laid eggs). Nests of the two
morphs are fully interspersed at our study site (S.K. Huber
2005, unpublished data), and patterns of assortative pairing
could thus be attributed to assortative mate choice rather
than spatial segregation.
To test for assortative pairing, we first calculated a
composite measure of beak size, using a principal components analysis that included beak length, depth and width
Proc. R. Soc. B (2007)
(as in Grant 1999). Across all birds at El Garrapatero banded
between 2004 and 2006, PC1 explained 88.3% of the
variation in beak measurements (eigenvalueZ2.65). Assortative pairing was then tested by plotting PC1 for males against
PC1 of the females with which they were paired. Nonparametric Spearman’s rank correlations were used to
determine the degree of assortative pairing. These correlations were calculated based on pairs formed under dry
conditions (2004 to early 2005, nZ21 pairs), under very wet
conditions (late 2005, nZ33 pairs) and under moderately
wet conditions (2006, nZ26 pairs). The 2004 to early 2005
dataset did not contain any duplicate individuals. Some
individuals were included in more than one of the three
datasets. However, no individuals paired with multiple mates
within a given year, and all birds that bred in multiple years
changed mates from one year to the next.
The consequences of assortative pairing depend largely on
the extent of extra-pair fertilizations (EPFs) and whether
EPFs occur within or between the morphs. EPFs have been
documented in G. fortis of another, small island population
( Keller et al. 2001). Even low rates of EPFs would
presumably eliminate any genetic differences between the
morphs that accrue through assortative mating. To assess
levels of intermorph gene flow, we divided the birds into small
and large beak size classes, between which we examined
patterns of genetic variation across 10 microsatellite loci. If
genetic differences are present, then EPFs between the
morphs are either absent or do not contribute substantially
to genetic exchange between the morphs.
Total DNA was extracted from blood samples collected
in 2004 and 2005 using a modified proteinase K phenol–
chloroform protocol (Sambrook et al. 1989). Fragments were
amplified by polymerase chain reaction ( PCR) for 10
unlinked dinucleotide microsatellites (Petren 1998). PCR
products were analysed using a multi-capillary sequencer ABI
3100. Genetic work was carried out at Naos Molecular
Laboratories at the Smithsonian Tropical Research Institute
in Panama.
Genetic comparisons were made between small and large
beak size classes within each year. Birds were assigned to these
classes by performing a principal components analysis of beak
length, depth and width for all banded birds in a given year.
We then placed individuals into two groups (small or large)
based on a cluster analysis of PC1 (SPSS v. 12.0, 2003).
Individuals that were within 0.5 s.d. of the small/large ‘cutoff’ were considered intermediate and removed from the
analysis (nZ47). These intermediate birds were encompassed by both the upper tail of the small beak morph
distribution and the lower tail of the large beak morph
distribution, and thus could not be assigned reliably to either
class. If gene flow is not influenced by beak size (the null
expectation), then removing birds with intermediate beak
sizes should have no effect on our results.
Measures of genetic variation, including allelic diversity,
frequency and heterozygosity, were calculated using
GENEPOP v. 3.4 ( Raymond & Rousset 1995), FSTAT
v. 2.9.3.2 and ARLEQUIN v. 3.0 (table 1). We tested for
Hardy–Weinberg equilibrium using the Markov chain
method as implemented in GENEPOP v. 3.4 (dememorizationZ10 000, batchesZ100, iterations per batchZ5000;
table 1). We tested for linkage disequilibrium between pairs
of loci using GENEPOP v. 3.4 (table 2).
Genetic differences between beak size classes were
analysed in several ways. First, we tested for significant
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S. K. Huber et al.
Reproductive isolation of Darwin’s finches
1711
Table 1. Overall genetic diversity for large and small morph G. fortis sampled at El Garrapatero (small morph, nZ197; large
morph, nZ59). (Shown are the number of birds analysed (N ), number of alleles (NA ), observed heterozygosities (HO),
expected heterozygosities (HE), FIS and p values from a Hardy–Weinberg test for heterozygote deficits across all birds. Italicized
p values indicate those that remained significant after a sequential Bonferroni correction.)
locus
N
NA
HO
HE
FIS
p
Gf03
Gf04
Gf05
Gf07
Gf08
Gf09
Gf11
Gf12
Gf13
Gf16
254
254
254
245
253
256
243
252
255
255
14
5
10
19
24
15
29
16
13
11
0.818
0.472
0.677
0.861
0.885
0.578
0.844
0.885
0.878
0.808
0.854
0.474
0.681
0.870
0.925
0.601
0.937
0.901
0.870
0.789
0.041
K0.002
0.006
0.010
0.043
0.039
0.100
0.017
K0.010
K0.025
0.017
0.502
0.255
0.007
0.004
0.266
!0.001
0.204
0.751
0.673
(a)
(b)
(c) 4
(d )
(e)
male beak PC1
3
2
1
0
–1
–2
–2
–1
0
1
2
female beak PC1
3
–2
–1
0
1
2
female beak PC1
3
–2
–1
0
1
2
female beak PC1
3
Figure 2. Assortative pairing by beak size in pairs of G. fortis at El Garrapatero. (a) A breeding pair of small morph individuals
(photo credit Eric Hilton) and (b) a breeding pair of large morph individuals (photo credit Sarah Huber) photos not to scale. (c)
Assortative pairing under dry conditions (2004, early 2005). (d ) The pattern under very wet conditions ( late 2005) and (e) the
pattern under moderately wet conditions (2006). Male and female ‘beak PC1’ values are scores along the first principal
component based on beak length, depth and width.
differences in allele frequencies by using the ‘genic differentiation’ option in GENEPOP v. 3.4. Second, we estimated
genetic distances using F-statistics (FST) and R-statistics
(R ST; Weir & Cockerham 1984; Slatkin 1995). These
analyses used 10 000 randomizations. For FST across all loci
(table 3; FSTZ0.017), the 95% confidence interval was
computed to be 0.011–0.024 using FSTAT.
Multilocus genotypes were also used to assess population
structuring. First, we used factorial correspondence analysis
in GENETIX v. 4.05 (Belkhir et al. 2004) to determine the
similarity of allelic states between the morphological classes.
We used a t-test for differences between beak morphs in scores
for factors 1 and 2. Second, we used the Bayesian approach
implemented in STRUCTURE v. 2.1 (Pritchard et al. 2000).
We ran five simulations for each putative number of clusters
(KZ1–5). In each case, we used the admixture model with
burn-in of 100 000 and Monte Carlo Markov chain iteration
value of 500 000. The most probable number of clusters
was always KZ1, but visual inspection suggested some
Proc. R. Soc. B (2007)
differences between the clusters. We therefore tested for
significant differences in cluster placement when KZ2.
Specifically, the results of a STRUCTURE analysis give a
probability between one and zero that an individual belongs
to cluster 1 or cluster 2 (the sum of probabilities for both
clusters is one). Probabilities were arcsine square root
transformed, and a t-test was used to compare values between
the two putative clusters for each morphs in all five iterations.
This analysis revealed whether morphs were being randomly
placed into the two clusters.
3. RESULTS
We found strong positive assortative pairing by beak size
(figure 1a,b) for dry conditions in 2004 and early 2005
(rZ0.742, pZ0.001; figure 2c), very wet conditions in late
2005 (rZ0.390, pZ0.025; figure 2d ) and moderately wet
conditions in 2006 (rZ0.705, p!0.001; figure 2e).
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1712 S. K. Huber et al.
Reproductive isolation of Darwin’s finches
Table 2. Pairs of loci that showed significant ( p!0.05)
linkage disequilibrium across all loci. (Italicized p values
indicate those that remain significant after sequential
Bonferroni corrections.)
loci
a2
d.f.
p
Gf05 & Gf11
Gf04 & Gf13
Gf03 & Gf05
Gf05 & Gf08
Gf04 & Gf08
Gf08 & Gf11
Gf07 & Gf08
infinity
19.44
8.24
7.04
6.95
6.67
6.37
2
2
2
2
2
2
2
!0.0001
!0.0001
0.016
0.030
0.031
0.036
0.041
Table 3. Genetic differences between small and large beak size
classes. (Genic differentiation p values were obtained using
GENEPOP. Italicized p values indicate those that remained
significant after a sequential Bonferroni correction.)
locus
FST
R ST
genic differentiation ( p)
Gf03
Gf04
Gf05
Gf07
Gf08
Gf09
Gf11
Gf12
Gf13
Gf16
overall
0.026
0.030
0.012
0.038
0.015
0.005
0.017
0.007
0.006
0.012
0.017
0.032
0.016
0.038
0.001
0.051
K0.000
0.050
0.041
0.049
0.012
0.040
0.005
0.006
!0.001
!0.0001
0.0001
0.009
!0.0001
0.064
0.006
0.006
!0.0001
Multiple lines of evidence, consistent across loci,
indicate restricted gene flow between the large and small
morphs. First, we found several signatures of population
admixture (Hardy–Weinberg deficits and linkage disequilibrium) when pooling all of the birds (tables 1 and 2).
Second, allele frequencies differed significantly between
beak size classes at nine of the ten loci (table 3). Third,
genetic divergence measures between the large and
small beak size classes were non-trivial (FSTZ0.017;
R STZ0.040) and differed significantly from zero (table 3).
Some population structure was also evident based on
multilocus genotypes. In particular, GENETIX revealed
significant differences between the large and small
beak size classes on each of the first two factors (factor
1: tZK2.30, pZ0.02; factor 2: tZ2.56, pZ0.01).
STRUCTURE found few differences between the morphs
(with a single cluster always being most likely; mean
ln( p)ZK10318.92), but this is expected when groups are
only moderately differentiated (Pritchard et al. 2000). Yet,
the assignment of individuals to clusters when KZ2 was
not random with respect to beak size for large morph
individuals in two of the five iterations (iteration 1:
tZK3.24, pZ0.001; iteration 2: tZ2.47, pZ0.014) and
for small morph individuals in one of the five iterations
(tZ5.65, p!0.001).
4. DISCUSSION
Our genetic data reveal that the large and small beak
morphs at El Garrapatero represent two partially distinct
gene pools. Our behavioural data suggest that this genetic
Proc. R. Soc. B (2007)
divergence can be attributed, at least in part, to females’
choice of males with similar beak sizes. With this evidence,
we are in a position to consider factors that might promote
and maintain the observed bimodality.
One possible factor promoting bimodality is disruptive
selection in sympatry (Rueffler et al. 2006), in this case
against birds with intermediate beak sizes. Indeed, we have
found that intermediate birds survive at lower rates
between years in comparison with large and small morphs
(A. P. Hendry & J. Podos 2006, unpublished data). Such
disruptive selection could, in principle, lead to a purely
sympatric origin of reproductive isolation (Higashi et al.
1999; Kondrashov & Kondrashov 1999; Ryan et al. 2007).
This process is especially probable under two conditions.
The first condition is that traits under divergent selection
(here beak size) are the same as, or are genetically or
phenotypically linked to, traits that influence mate choice
(here beak size, a visual cue and song, a vocal mating
signal; see Ratcliffe & Grant 1983; Podos 2001). The
second condition is that mating is assortative with respect
to those traits (Grant et al. 2000), as shown here.
Another factor that may promote bimodality is initial
divergence during a period of allopatry. The El Garrapatero G. fortis morphs may have originated at different
places on the same island, or on different islands, under
distinct ecological conditions and divergent selection
regimes. Following secondary contact, these differences
could have led to assortative mating and reduced gene flow
through the sympatric processes described above. Indeed,
this scenario of initial allopatric divergence followed by
further sympatric divergence mirrors a widely accepted
model of speciation in many taxa, including Darwin’s
finches (Grant 1999; Schluter 2000).
Yet another factor potentially influencing bimodality is
introgression with other Darwin’s finch species. At our
study site, G. fortis is sympatric with a smaller ground finch
species, Geospiza fuliginosa, and a larger ground finch
species, Geospiza magnirostris. Perhaps large G. fortis
historically hybridized with G. magnirostris or small
G. fortis hybridized with G. fuliginosa. We have identified
at least one instance of hybridization in our population, in
which a large morph G. fortis female mated with a
G. magnirostris male. This sort of interspecies mating
could increase phenotypic and genetic variation in
G. fortis, which might then facilitate the emergence of
bimodality (Seehausen 2004). Indeed, the Galápagos
ground finches may be a promising system for determining
how hybridization facilitates speciation (Mallet 2007)
rather than just hampering it (Grant & Grant 2002).
In conjunction with previous work on Darwin’s finches,
our results support the role of ecologically mediated
phenotypic divergence as an important driving force in the
early stages of adaptive radiation. Divergence is initiated
when variation in food types, food availability or
competition imposes divergent selection (in allopatry) or
disruptive selection (in sympatry) on beak morphology
(Boag & Grant 1981; Grant 1999). Resulting adaptive
divergence then imposes secondary consequences on the
evolution of mating signals (Ratcliffe & Grant 1983; Podos
2001; Podos & Nowicki 2004; Podos et al. 2004; Huber &
Podos 2006). This divergence in mating signals may then
cause assortative mating and thus help maintain reproductive isolation in sympatry. Beak morphology in
Darwin’s finches may therefore be regarded as one of the
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Reproductive isolation of Darwin’s finches
elusive ‘magic traits’ of speciation (Gavrilets 2004), given
that it is both a target of divergent selection and a
component in mating signals that drive assortative mating
(Grant & Grant 1997; Podos & Hendry 2006). We have
identified a population that seems to be in the early stages
of this process.
It is uncertain whether or not the two morphs at El
Garrapatero will ultimately diverge to the level of welldefined species. For instance, immigration from a nearby
unimodal population (Hendry et al. 2006), which might
itself have considerable gene flow between birds with large
and small beaks, could hamper further divergence between
the morphs at El Garrapatero. Additionally, environmental
conditions may eventually change to the extent that ‘hybrid’
offspring no longer have reduced fitness, as has been the case
for established species of Darwin’s finches (Grant & Grant
1996). Regardless, our data support the hypothesis that the
early stages of assortative mating and reproductive isolation
are driven by ecological divergence.
The collection of data in this study was done in concordance
with Animal use Protocols approved by the University of
Massachusetts Amherst.
We thank the Galápagos National Park and the Charles
Darwin Research Station for providing support to this
research. Anthony Herrel, Ana Gabela, Eric Hilton, Haldre
Rogers and Steve Johnson provided assistance in the field.
Funding was provided by National Science Foundation grant
IBN 0347291 to J.P., and National Science Foundation grant
DDIG 0508730 to J.P. and S.K.H. Additional funding for
S.K.H. was provided by an American Ornithologists’ Union
Student Research grant, an Explorer’s Club grant, a Sigma Xi
Grant-in-Aid of Research, an Animal Behaviour Society
Student Research grant and a University of Massachusetts
Woods Hole Fellowship. Support for L.F.D. was provided by
Instituto para la Formación y Aprovechamiento de Recursos
Humanos and Secretaria Nacional de Ciencia, Tecnologı́a e
Innovación.
REFERENCES
Belkhir, K., Chikhi, L., Raufaste, N. & Bonhomme, F. 2004
GENETIX logiciel sous WindowsTM pour la génétique des
populations, 4.05. Montpellier, France: Laboratoire Génome,
Populations, Interactions CNRS UMR 5000, Université de
Montpellier II.
Boag, P. T. & Grant, P. R. 1981 Intense natural selection in a
population of Darwin’s finches (Geospizinae) in the
Galápagos. Science 214, 82–85. (doi:10.1126/science.
214.4516.82)
Gavrilets, S. 2004 Fitness landscapes and the origin of species.
Princeton, NJ: Princeton University Press.
Gislason, D., Ferguson, M. M., Skulason, S. & Snorrason,
S. S. 1999 Rapid and coupled phenotypic and genetic
divergence in Icelandic Arctic char (Salvelinus alpinus).
Can. J. Fish. Aquat. Sci. 56, 2229–2234. (doi:10.1139/
cjfas-56-12-2229)
Grant, P. R. 1999 Ecology and evolution of Darwin’s finches.
Princeton, NJ: Princeton University Press.
Grant, B. R. & Grant, P. R. 1996 High survival of Darwin’s
finch hybrids: effects of beak morphology and diets.
Ecology 77, 500–509. (doi:10.2307/2265625)
Grant, P. R. & Grant, B. R. 1997 Mating patterns of Darwin’s
finch hybrids determined by song and morphology. Biol.
J. Linn. Soc. 60, 317–343. (doi:10.1006/bijl.1996.0103)
Grant, P. R. & Grant, B. R. 2002 Unpredictable evolution in a
30-year study of Darwin’s finches. Science 296, 707–711.
(doi:10.1126/science.1070315)
Proc. R. Soc. B (2007)
S. K. Huber et al.
1713
Grant, P. R. & Grant, B. R. 2006 Evolution of character
displacement in Darwin’s finches. Science 313, 224–226.
(doi:10.1126/science.1128374)
Grant, P. R., Abbot, I., Schluter, D., Curry, R. L. & Abbott,
L. K. 1985 Variation in the size and shape of Darwin’s
finches. Biol. J. Linn. Soc. 25, 1–39.
Grant, P. R., Grant, B. R. & Petren, K. 2000 The allopatric
phase of speciation: the sharpbeaked ground finch
(Geospiza difficilis) on the Galápagos islands. Biol.
J. Linn. Soc. 69, 287–317. (doi:10.1006/bijl.1999.0382)
Hendry, A. P., Grant, P. R., Grant, B. R., Ford, H. A., Brewer,
M. J. & Podos, J. 2006 Possible human impacts on adaptive
radiation: beak size bimodality in Darwin’s finches. Proc. R.
Soc. B 273, 1887–1894. (doi:10.1098/rspb.2006.3534)
Herrel, A., Podos, J., Huber, S. K. & Hendry, A. P. 2005 Bite
performance and morphology in a population of Darwin’s
finches: implications for the evolution of beak shape. Funct.
Ecol. 19, 43–48. (doi:10.1111/j.0269-8463.2005.00923.x)
Higashi, M., Takimoto, G. & Yamamura, N. 1999 Sympatric
speciation by sexual selection. Nature 402, 523–526.
(doi:10.1038/990087)
Huber, S. K. & Podos, J. 2006 Beak morphology and song
features covary in a population of Darwin’s finches
(Geospiza fortis). Biol. J. Linn. Soc. 88, 489–498. (doi:10.
1111/j.1095-8312.2006.00638.x)
Keller, L. F., Grant, P. R., Grant, B. R. & Petren, K. 2001
Heritability of morphological traits in Darwin’s finches:
misidentified paternity and maternal effects. Heredity 87,
325–336. (doi:10.1046/j.1365-2540.2001.00900.x)
Kondrashov, A. S. & Kondrashov, F. A. 1999 Interactions
among quantitative traits in the course of sympatric
speciation. Nature 400, 351–354. (doi:10.1038/22514)
Lack, D. 1947 Darwin’s finches. Cambridge, UK: Cambridge
University Press.
Mallet, J. 2007 Hybrid speciation. Nature 446, 279–283.
(doi:10.1038/nature05706)
Orr, M. R. & Smith, T. B. 1998 Ecology and speciation.
Trends Ecol. Evol. 13, 502–506. (doi:10.1016/S01695347(98)01511-0)
Petren, K. 1998 Microsatellite primers from Geospiza fortis
and cross-species amplification in Darwin’s finches. Mol.
Ecol. 7, 1782–1784.
Podos, J. 2001 Correlated evolution of morphology and vocal
signal structure in Darwin’s finches. Nature 409, 185–188.
(doi:10.1038/35051570)
Podos, J. & Hendry, A. P. 2006 The biomechanics of
ecological speciation. In Ecology and biomechanics: a
mechanical approach to the ecology of animals and plants
(eds A. Herrel, T. Speck & N. Rowe), pp. 301–321. Boca
Raton, FL: CRC Press.
Podos, J. & Nowicki, S. 2004 Beaks, adaptation, and
vocal evolution in Darwin’s finches. Bioscience
54, 501–510. (doi:10.1641/0006-3568(2004)054[0501:
BAAVEI]2.0.CO;2)
Podos, J., Southall, J. A. & Rossi-Santos, M. 2004 Vocal
mechanics in Darwin’s finches: correlation of beak gape
and song frequency. J. Exp. Biol. 207, 607–619. (doi:10.
1242/jeb.00770)
Price, T. 1987 Diet variation in a population of Darwin’s
finches. Ecology 68, 1015–1028. (doi:10.2307/1938373)
Pritchard, J. K., Stephens, M. & Donnelly, P. 2000 Inference
of population structure using multilocus genotype data.
Genetics 155, 945–959.
Ratcliffe, L. M. & Grant, P. R. 1983 Species recognition in
Darwin’s finches (Geospiza, Gould). I. Discrimination by
morphological cues. Anim. Behav. 31, 1139–1153.
(doi:10.1016/S0003-3472(83)80021-9)
Raymond, M. & Rousset, F. 1995 GENEPOP version 1.2:
population genetics software for exact tests and ecumenicism. J. Hered. 86, 248–249.
Downloaded from http://rspb.royalsocietypublishing.org/ on February 10, 2017
1714 S. K. Huber et al.
Reproductive isolation of Darwin’s finches
Rueffler, C., Van Dooren, T. J. M., Leimar, O. & Abrams,
P. A. 2006 Disruptive selection and then what? Trends Ecol.
Evol. 21, 238–245. (doi:10.1016/j.tree.2006.03.003)
Rundle, H. D. & Nosil, P. 2005 Ecological speciation. Ecol. Lett.
8, 336–352. (doi:10.1111/j.1461-0248.2004.00715.x)
Ryan, P. G., Bloomer, P., Moloney, C. L., Grant, T. J. &
Delport, W. 2007 Ecological speciation in South Atlantic
island finches. Science 315, 1420–1423. (doi:10.1126/
science.1138829)
Sambrook, J., Fritsch, E. F. & Maniatis, T. 1989 Molecular
cloning: a laboratory manual, 2nd edn. New York, NY: Cold
Spring Harbor Laboratory Press.
Schluter, D. 2000 The ecology of adaptive radiation. Oxford,
UK: Oxford University Press.
Proc. R. Soc. B (2007)
Seehausen, O. 2004 Hybridization and adaptive radiation. Trends
Ecol. Evol. 19, 198–207. (doi:10.1016/j.tree.2004.01.003)
Slatkin, M. 1995 A measure of population subdivision based
on microsatellite allele frequencies. Genetics 139, 457–462.
Smith, T. B. 1993 Disruptive selection and the genetic basis
of bill size polymorphism in the African finch Pyrenestes.
Nature 363, 618–620. (doi:10.1038/363618a0)
Smith, T. B. & Skulason, S. 1996 Evolutionary significance of
resource polymorphisms in fishes, amphibians, and birds.
Annu. Rev. Ecol. Syst. 27, 111–133. (doi:10.1146/annurev.
ecolsys.27.1.111)
Weir, B. S. & Cockerham, C. C. 1984 Estimating F-statistics
for the analysis of population structure. Evolution 38,
1358–1370. (doi:10.2307/2408641)