Hybridization of glaucous gull (Larus hyperboreus) and herring gull (Larus argentatus) in Iceland: mitochondrial and microsatellite data.
ABSTRACT Large white-headed gulls provide an interesting group of birds for studies of hybridization. The group is composed of 20 species of recent origin, often with weak reproductive barriers. Here we report the results from a study on the glaucous gull Larus hyperboreus, an Arctic species which has been breeding in Iceland for centuries, and the herring gull Larus argentatus which has a wide distribution in Europe but colonized Iceland in 1920s. Previous studies, based on morphological variation indicated hybridization between the two species in Iceland, have been questioned as it may just reflect variation within the species. Here we evaluate whether hybridization has occurred between the two species in Iceland by studying variation in microsatellites and mtDNA. The analysis is based on feathers taken from wings sampled in Iceland over a period of 40 years. The results are compared with samples obtained from East Greenland and published sequences of samples obtained throughout Europe. The genetic analysis reveals a distinctive grouping of the two species, although they present a shallow genealogy and an extensive sharing of the genetic variants between the two species. Several individuals show admixture for molecular markers, which may result from an incomplete lineage sorting although geographical patterns of both mtDNA haplotypes and microsatellites strongly indicate a recent hybridization in Iceland.
- SourceAvailable from: J.-M. Pons[Show abstract] [Hide abstract]
ABSTRACT: Recent genetic studies have shown that introgression rates among loci may greatly vary according to their location in the genome. In particular, several cases of mito-nuclear discordances have been reported for a wide range of organisms. In the present study, we examine the causes of discordance between mitochondrial (mtDNA) and nuclear DNA introgression detected in North American populations of the Great Black-backed Gull (Larus marinus), a Holarctic species, from the Nearctic North American Herring Gull (Larus smithsonianus). Our results show that extensive unidirectional mtDNA introgression from Larus smithsonianus into Larus marinus in North America cannot be explained by ancestral polymorphism but most likely results from ancient hybridization events occurring when Larus marinus invaded the North America. Conversely, our nuclear DNA results based on 12 microsatellites detected very little introgression from Larus smithsonianus into North American Larus marinus. We discuss these results in the framework of demographic and selective mechanisms that have been postulated to explain mito-nuclear discrepancies. We were unable to demonstrate selection as the main cause of mito-nuclear introgression discordance but cannot dismiss the possible role of selection in the observed pattern. Among demographic explanations, only drift in small populations and bias in mate choice in an invasive context may explain our results. As it is often difficult to demonstrate that selection may be the main factor driving the introgression of mitochondrial DNA in natural populations, we advocate that evaluating alternative demographic neutral hypotheses may help to indirectly support or reject hypotheses invoking selective processes.Heredity advance online publication, 9 October 2013; doi:10.1038/hdy.2013.98.Heredity 10/2013; 112(3). · 3.80 Impact Factor
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ABSTRACT: We studied the influence of glacial oscillations on the genetic structure of seven species of white-headed gull that breed at high latitudes (Larus argentatus, L. canus, L. glaucescens, L. glaucoides, L. hyperboreus, L. schistisagus, and L. thayeri). We evaluated localities hypothesized as ice-free areas or glacial refugia in other Arctic vertebrates using molecular data from 11 microsatellite loci, mitochondrial DNA (mtDNA) control region, and six nuclear introns for 32 populations across the Holarctic. Moderate levels of genetic structure were observed for microsatellites (F(ST)= 0.129), introns (Φ(ST)= 0.185), and mtDNA control region (Φ(ST)= 0.461), with among-group variation maximized when populations were grouped based on subspecific classification. Two haplotype and at least two allele groups were observed across all loci. However, no haplotype/allele group was composed solely of individuals of a single species, a pattern consistent with recent divergence. Furthermore, northernmost populations were not well differentiated and among-group variation was maximized when L. argentatus and L. hyberboreus populations were grouped by locality rather than species, indicating recent hybridization. Four populations are located in putative Pleistocene glacial refugia and had larger τ estimates than the other 28 populations. However, we were unable to substantiate these putative refugia using coalescent theory, as all populations had genetic signatures of stability based on mtDNA. The extent of haplotype and allele sharing among Arctic white-headed gull species is noteworthy. Studies of other Arctic taxa have generally revealed species-specific clusters as well as genetic structure within species, usually correlated with geography. Aspects of white-headed gull behavioral biology, such as colonization ability and propensity to hybridize, as well as their recent evolutionary history, have likely played a large role in the limited genetic structure observed.Ecology and Evolution 06/2012; 2(6):1278-95. · 1.66 Impact Factor
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ABSTRACT: Following range expansion and colonization, hybridization between Herring and Caspian Gulls, Larus argentatus and L. cachinnans, takes place in central and eastern Europe. To examine how hybrid zone is affected by the abundance dynamics of these species and their reproductive performance, we studied a mixed colony at Włocławek Reservoir, central Poland, for over 7 years, from 2002 to 2009, and included data from the species monitoring from 1990 to 2001. To evaluate the species abundance dynamics and possible mechanisms of reproductive isolation, breeders (n = 226 individual birds) were trapped on nests and colour-ringed; breeding performance was studied in detail for 202 breeding pairs with both mates known. Between 2002 and 2009 the proportion of Caspian Gulls among breeders had strongly increased (from 14% to 42%), whereas the proportion of Herring Gulls had declined (from 70% to 35%). The frequency of hybrids varied a little with no clear trend (mean 20%, range 15–28%). The colony size during that time was approximately stable, with 125–135 breeding pairs. 32 individuals originating from outside the zone, ringed as nestlings in the core range of either species, were recorded as breeders at the study site, documenting dispersal of parental species into the zone. The immigration of the two parental species showed contrasting temporal patterns in the two compared decades, 1990–1999 vs. 2000–2009. The immigration of Herring Gulls as measured by the reencounter probability declined nearly three times, while approximately twofold increase was seen in Caspian Gulls. Birds tended to choose phenotypically similar mates, so that there were fewer heterospecific pairs than expected under random mating. Numbers of homospecific, heterospecific and mixed pairs were similar during 7 years. On average, males of Caspian Gulls were significantly heavier than males of Herring Gulls. Caspian Gull pairs bred on average 7 days earlier than pairs of Herring Gulls. No differences in clutch size, clutch volume or hatching success among pairs of different composition were found, indicating weak postzygotic isolation. Current abundance of species in the hybrid zone is changing dynamically and is primarily driven by the strength of immigration from outside the zone.Acta Ornithologica 12/2012; · 1.68 Impact Factor
Hybridization of glaucous gull (Larus hyperboreus)
and herring gull (Larus argentatus) in Iceland:
mitochondrial and microsatellite data
Freydı ´s Vigfu ´sdo ´ttir, Snæbjo ¨rn Pa ´lsson*and Agnar Ingo ´lfsson
Department of Biology, University of Iceland, Sturlugata 7, 101 Reykjavı ´k, Iceland
Large white-headed gullsprovide an interesting group of birds for studies of hybridization.The group
is composed of 20 species of recent origin, often with weak reproductive barriers. Here we report the
results from a study on the glaucous gull Larus hyperboreus, an Arctic species which has been breeding
in Iceland for centuries, and the herring gull Larus argentatus which has a wide distribution in Europe
but colonized Iceland in 1920s. Previous studies, based on morphological variation indicated
hybridization between the two species in Iceland, have been questioned as it may just reflect variation
within the species. Here we evaluate whether hybridization has occurred between the two species in
Iceland by studying variation in microsatellites and mtDNA. The analysis is based on feathers taken
from wings sampled in Iceland over a period of 40 years. The results are compared with samples
obtained from East Greenland and published sequences of samples obtained throughout Europe.
The genetic analysis reveals a distinctive grouping of the two species, although they present a shallow
genealogy and an extensive sharing of the genetic variants between the two species. Several
individuals show admixture for molecular markers, which may result from an incomplete lineage
sorting although geographical patterns of both mtDNA haplotypes and microsatellites strongly
indicate a recent hybridization in Iceland.
Keywords: gulls; introgression; Arctic; expansion; cytochrome b; phylogeography
Species distributions and genetic population structure
have been found to reflect climatic oscillations during
the Quaternary glacial periods (Hewitt 2000; Newton
2003). Ancestral populations split up during glacial
periods and evolved in allopatry in distinct refugia
(Hewitt 1996). Following the retreat of the glaciers,
populations of several species have expanded and
formed secondary contact zones (Barton & Hewitt
1985; Hewitt 2004), often involving various forms of
hybridization (Grant & Grant 1992).
Avian hybrids are widespread and more than 9% of
all bird species are known to have hybridized in nature,
although this is more common in some taxa than others
(Grant & Grant 1992; Randler 2002). Hybridization
among large white-headed gull species has been
reported from many areas (McCarthy 2006), e.g.
Larus smithsonianus!Larus hyperboreus in the Mack-
enzie Delta, Canada (Spear 1987), Larus glaucescens!
Larus occidentalis on the Pacific coast of North America
(Bell 1996), Larus argentatus!Larus fuscus in western
France (Yesou 1991), L. argentatus!Larus cachinnans
in Russia (Panov & Monzikov 1999) and Poland (Gay
et al. 2007), and L. argentatus!L. hyperboreus in
Iceland (Ingo ´lfsson 1970). The widespread instances
of incomplete reproductive isolation in this group
provide an excellent system to study hybridization and
introgression between species of recent origin. The large
white-headed gull species originated ca 100–600 kyr
ago, possibly in two main glacial refugia, the Atlantic
and the Aralo-Caspian refugium (Crochet et al. 2003;
Liebers et al. 2004).
The glaucous gull (L. hyperboreus) is an Arctic
circumpolar species that has undoubtedly been a
breeding bird in Iceland for a long time (Ingo ´lfsson
1970). The breeding distribution of the European
herring gull (L. argentatus) is confined to northern
Europe, ranging from Kola Peninsula to France and
recently to Iceland (Olsen & Larsson 2004). The
distribution of L. argentatus increased markedly during
the twentieth century, which led to its colonization in
Iceland ca 1925, where it came into contact with
L. hyperboreus (Ingo ´lfsson 1970). In Iceland, both
species breed in colonies, often large, the former
usually on grassy slopes and cliffs by the sea
(Guðmundsson 1955) and the latter more often on
relatively level ground near the sea (Ingo ´lfsson 1982,
personal observations). Despite colonial breeding,
dispersal is known to be substantial in large gulls
(Cramps & Simmons 1983; Coulson 1991) and as is
reflected by the recent colonization of Iceland.
Ingo ´lfsson (1970, 1987) described hybridization
among the two species based on morphological
variation and observation of mated pairs. Apparently
pure glaucous gulls predominated in western Iceland
while apparently pure herring gulls and hybrids were
common in southern and eastern Iceland. A colony
that apparently consisted exclusively of glaucous gulls
was known in southwestern Iceland in the early 1900,
Phil. Trans. R. Soc. B (2008) 363, 2851–2860
Published online 2 June 2008
One contribution of 16 to a Theme Issue ‘Hybridization in animals:
extent, processes and evolutionary impact’.
*Author for correspondence (firstname.lastname@example.org).
This journal is q 2008 The Royal Society
but by 1970 it consisted mostly of apparently pure
L. argentatus and possible hybrids (Ingo ´lfsson 1970).
In another instance, a colony consisting mostly of
apparently pure glaucous gulls in southeastern Iceland
in 1963 had changed to a colony of mostly herring gull-
like birds and putative hybrids by 1973 (Ingo ´lfsson
1987). Studying the allozyme and morphological
variation, Snell (1991a,b) argued against hybridization,
claiming that the variation found in gulls in Iceland
simply reflected the natural variation within the species
(but see rebuttal by Ingo ´lfsson 1993).
A previous study of the genetics of the herring gull
Iceland (Snell 1991a). Studies by Bell (1996) and
Crochet (2000) indicated that the situation in gulls was
more complex than simple surveys of plumage pheno-
types or allozyme variation had shown. Similarly, more
have shown discrepancies between gene trees and the
taxon phylogeny, resulting from incomplete lineage
sorting and/or hybridization (Crochet et al. 2002,
2003; Liebers et al. 2004; Gay et al. 2007). Genetic
work by Liebers et al. (2004) on L. hyperboreus from
Novaja Semlija, Svalbard and Baffinland indicated a
phylogeographical split between Europe and America,
and a similar split was described between the European
L. argentatus and the American L. smithsonianus.
In order to solve the dispute by Ingo ´lfsson and Snell,
on whether hybridization occurs among glaucous gulls
(L. hyperboreus) and herring gulls (L. argentatus) in
Iceland, a better understanding of various aspects of
the biology of these two species is needed. Here we
study variation at five microsatellite loci and a sequence
variation in mtDNA (cytochrome b) in samples
obtained in Iceland over a period of 40 years, and
from Greenland. The mtDNA sequences are in
addition compared with sequence data from samples
obtained elsewhere in Europe and from Canada
(Liebers et al. 2004). To look for signs of incomplete
lineage sorting and hybridization, we examine firstly
whether the mtDNA genealogy confines separate
groups for the two species. Secondly, we look for
signs of temporal and geographical patterns. As
incomplete lineage sorting should be independent of
geographical locations, a local sharing of haplotypes in
a recent contact zone would point to hybridization.
2. MATERIAL AND METHODS
Samples were obtained from various breeding colonies in
seven regions of Iceland (figure 1) and from Kulusuk in East
Greenland (table 1). According to Ingo ´lfsson (1970), western
Iceland was dominated by L. hyperboreus-like birds, while
eastern and southern Iceland was dominated by L. argentatus-
like birds and apparent hybrids. In one colony in southeastern
Iceland (location number 6 in table 1), both the types as well
as their hybrids were common. Samples originated from three
time periods. Samples from period 1 (collected by Ingo ´lfsson
1964–1973) and period 2 (by Snell 1985–1986) were obtained
from specimens at the collection of The Icelandic Institute of
Natural History. Sampling from period 3 (2005–2006) was
carried out in the field, by shooting or Cannon netting, near
breeding colonies. For the purpose of the analysis, gulls were
assigned to two groups on the basis of primary pattern (i.e. the
coloration of wing primary feathers), those scoring 0.0–1.0
in hybrid index, HI (see Ingo ´lfsson 1970) being termed
L. hyperboreus, and those scoring between 1.1 and 5.0 being
termed L. argentatus. The index reflects different degrees of
black patterns, from no trace (HIZ0) to black subterminal
bands with sharp edges (HIZ5). This grouping of genotypes
based on a HI is supported by Bayesian clustering (BAPS;
Marttinen et al. 2006). For comparison, mtDNA sequences
from 137 individuals from Liebers et al. (2004) were included
in this study.
(b) Molecular analysis
DNA was extracted from feathers in 300 ml of 6% Chelex 100
(Biorad) containing 3 ml proteinase K (0.5 mg mlK1), in-
cubated at 658C for 2 hours and at 958C for 5 min. A 971 bp
fragment of the mtDNA cytochrome b gene was amplified
using primers L14967 and H15938, as in Crochet et al.
(2003). For the museum specimens, two smaller overlapping
segments of the cytochrome b gene were amplified separately,
using primers L15440 (50-GCCAAACCCTCGTAGAAT-
GA-30) with H15938, and H15619 (50-GTAGGGGTGGA-
ATGGGATTT-30) with L15008 (Crochet et al. 2003).
Polymerase chain reaction (PCR) amplifications were carried
out in 10 ml volume containing 1!amplification buffer,
0.09 U Taq DNA polymerase, 1.5 mM MgCl2, 1 mM of
each dNTP and 10 mM of each primer. Cycling conditions
were 948C for 40 s, 568C for 30 s and 728C for 60 s for 30
cycles for primers L14967-H15938. For primers used on
museum specimens, the cycling conditions were 948C for
60 s, 538C for 30 s and 728C for 60 s, for 38 cycles.
Prior to sequencing, excess primers and nucleotides were
enzymatically removed from PCR amplification products
using a mixture of exonuclease I and Antarctic phosphatase
(New England BioLabs). Cycle sequencing was carried out
using the BIGDYE TERMINATORv. 1.1 Cycle Sequencing Kit on
ABI PRISM 3100 Genetic Analyzer (Applied Biosystems).
Five microsatellite loci (HG14, HG16, HG18, HG25 and
HG27), developed for the North American herring gull
L. smithsonianus, were amplified following the procedures in
Crochet et al. (2003). PCR was performed in a 15 ml reaction
containing the same concentrations as listed above; the
products were sent for genotyping to GATCBiotech AG,
Germany and scored with the GENE MARKER v. 5.1 software
package (SoftGenetics LLC 2004).
Figure 1. Sampling locations of gulls in Iceland. Numbers
refer to locations listed in table 1. Samples included in
population comparisons per location are 1 (a3w), 2 (h1w, h2
andh3), 3 (h1w), 4(a1, a2 and a3e) and5 and 6 (a1and h1e).
The main nesting areas of L. hyperboreus are in northwestern
Iceland (2 and 3), whereas L. argentatus is mainly nesting in
eastern Iceland (4–6).
2852 F. Vigfu ´sdo ´ttir et al.Hybridization of glaucous and herring gulls
Phil. Trans. R. Soc. B (2008)
(c) Statistical analysis
Basic statistics of mtDNA diversity, including nucleotide and
haplotype diversity for each sample, were calculated with
ARLEQUIN v. 3 (Excoffier et al. 2005). Deviations from
equilibrium population dynamics, resulting from population
differences among all sequences(Rogers &Harpending 1992).
The introgression ratio, IG, was calculated according to
Belahbib et al. (2001), with modifications. The original IG
reflects the amount of locally shared haplotypes between two
species and is expected to be one when there is no difference
between the species and zero when they are totally different.
allsamplescombinedofspeciesy, andvice versa.The standard
errors and CIs of the IG values were estimated by non-
parametric bootstrapping, with the program boot in the
R package (available at http://www.r-project.org/). Bootstrap
samples were obtained by resampling 1000 individuals within
the subpopulations of each species. To detect signatures of
population structure, hierarchical analyses of molecular
variance (AMOVA) were calculated among species and
populations (Excoffier et al. 1992), and for all pairwise
comparisons between samples. The analyses were based on
haplotype frequencies (Wright’s FST) and also on variance in
divergence between sequences (FST). The overall relationship
among the haplotypes is presented with a median-joining
network (Bandelt et al. 1999) constructed with NETWORK
v. 4.201 (www.fluxus-engineering.com). Equal weights were
assumed for each variable position.
Genetic diversity parameters of the microsatellite variation,
including number of alleles (na) and unbiased expected
heterozygosity (He), were summarized with GENETIX v. 4.05
(Belkhir et al. 2004). The effective number of alleles was
calculated as neZ1/(1KHe) (Weir 1990). Departures from
Hardy–Weinberg equilibrium (HWE) and linkage disequili-
brium were tested with random permutations as implemented
with GENETIX. The hierarchical partition of variation among
species and populations were conducted using ARLEQUIN v. 3
(Excoffier et al. 2005). Pairwise FST values (Weir &
Cockerham 1984) and the proportion of variation based on
variance in allele size (RST) were calculated (Slatkin 1995).
The pairwise differences were summarized with a multi-
dimensional scale plot, also known as principal coordinate
analysis (Venables & Ripley 1994). Bonferroni-adjusted
probabilities were used (Sokal & Rohlf 1995) in cases of
multiple and non-orthogonal tests.
The genetic structure, including both information from
microsatellites and the mtDNA haplotypes together, was
analysed by the Bayesian clustering method and the
admixture analyses implemented in BAPS v. 4.14 (Marttinen
et al. 2006). The analysis was based on samples that had been
scored for three or more markers. In BAPS, the a priori
information of the nine samples listed in table 2 was used for
clustering. Evidence for admixture was considered significant
for individuals with a Bayesian p!0.05, as recommended by
Marttinen et al. (2006).
(a) Analysis of the mitochondrial cytochrome b
Sequences of a length of 850 bp of the cytochrome b
were obtained from 214 gulls, with 41 polymorphic
sites, defining 20 haplotypes (table 2). Sequences were
EU526315–EU526334). Haplotypes differ from each
other by a maximum of 10 mutations (figure 2). Five
haplotypes are identical to the haplotypes reported by
Liebers et al. (2004). The haplotypes form three main
clades: two European clades (I and II) and a North
American clade (III; figure 2). Only one European
L. hyperboreus, from west Iceland (h3), has an
American haplotype (H1). Four L. hyperboreus from
Kulusuk have European haplotypes, three A12 and one
H2. Larus smithsonianus from N. America have
haplotypes that differ by one substitution from H1,
and Larus glaucoides (also N. American) has been found
to share both H1 and G1.
Table 1. Numbers of individuals (n) sampled from each location in three time periods. (The numbers in sample names refer to
the different periods: 1, 1964–1973; 2, 1985–1986 and 3, 2005–2006. The third letter in sample names refers to east (e) and
west (w). Numbers (no) present locations in figure 1, the same number is given to locations in close vicinity to each other.
Asterisks, samples were not included in population comparisons.)
species samplesnolocationlatitude (N)longitude (W)
Hjo ¨rleifsho ¨fði?
Bu ´landsho ¨fði
Hybridization of glaucous and herring gulls
F. Vigfu ´sdo ´ttir et al.
Phil. Trans. R. Soc. B (2008)
The two clusters I and II are connected through C1,
of the 26 haplotypes in clusters I and II, 4 are shared
between L.hyperboreus andL.argentatus,A20inclusterI
and MA1, A12, and Hy2 in cluster II. These four are
other European L. hyperboreus. The wide occurrence of
the A12 inbothspecies inEurope(Liebers etal. 2004) is
reflected in the IGs. IG values from samples from
northern Europe are in the range of 0.53–0.75, being
highest in the sample from northern Norway; in
Germany, it is 0.33 and the IG values are equal to zero
A12 is absent. In Iceland, the IG values are high for
overlap with L. hyperboreus sampling sites. A high IG
value is also observed for the h1w from the first time
period,where two individualsshare two haplotypes from
cluster I (A20).
Cluster I is formed by 54 individuals, 80% from
Iceland and western Europe (France, Netherlands and
Faroe Islands), representing the Larus argentatus
argenteus group (Barth 1968). Sixindividualsrepresent-
ing the L. a. argentatus group are from more southern
range of the groups’distribution, Germany, Finland and
Estonia. The proportion of cluster I among Icelandic
L. argentatus increases significantly from time period 1
(7%) to periods 2 and 3 (42 and 32%; p!0.0039,
Fisher’s exact test). Cluster II holds 14 haplotypes
among 249 individuals, 138 L. hyperboreus and
Figure 2. Network of the cytochrome b haplotypes obtained in this study and the study by Liebers et al. (2004). The size of the
pies reflects the frequency of a particular haplotype. The length of the lines connecting the pies, measured from their centres, are
in proportion to the number of base pair substitutions (1–3) separating the haplotypes. Clusters I and II are found within the
European samples and cluster III represents the American clade. Species and geographical regions are noted with different
shadings: hyp I, L. hyperboreus from Iceland (10 out of 114 specimens from Liebers et al. 2004); arg I, L. argentatus from Iceland
(15 out of 100 specimens from Liebers et al. 2004); hyp K, from Kulusuk, Greenland; hyp E and arg E are specimens from other
sites in Europe; and hyp A are from Canada (Liebers et al. 2004).
Table 2. Mitochondrial DNA haplotypes, nucleotide diversity (p) and haplotype diversity (h) for each sample at a given time
period in all locations. (Numbersof the haplotypes are givenin brackets.IGswith standard errors in parenthesis are givenfor the
samples from Iceland. Population names correspond to table 1. Asterisks, not calculated.)
A12(13), A6(1) A25(1), A26(1), MA1(7), MA2(1), H2(4)
A12(7), A6(1), A14(1), A20(4), A23(3), MA1(7), MI3(3)
A12(9), A6(3), A20(2), A23(2), A24(1), MA1(7)
A12(2), A6(1), A21(1), H2(3)
A12(16), A20(2), H2(8), H8(1)
A12(23), H1(1), H2(29), H6(1), H9(1)
A12(3), H1(7), H2(1), H7(1), G1(4), G3(2)
2854F. Vigfu ´sdo ´ttir et al.Hybridization of glaucous and herring gulls
Phil. Trans. R. Soc. B (2008)
111 L. argentatus. Most individuals in this cluster
originate from northerly areas, where the distribution
of the two species is adjacent or overlapping. The
majority are represented by haplotype A12 (59%) or
the derivative H2 (29%). The H2 group is represented
by almost exclusively Icelandic samples (99%), 60
L. hyperboreus from west Iceland and 8 L. argentatus
only from Iceland (a1 and a3w).
Excluding h1e owing to the low sample size, the
overall nucleotide and haplotype diversity was higher in
L. argentatus groups compared with L. hyperboreus.
Interestingly, L. hyperboreus from Kulusuk are more
divergent than all hyperboreus groups from Iceland.
Haplotype diversity is generally stable over time
among the species. Nucleotide diversity changes
through time; in the first time period, it is lowest in
L. argentatus, whereas in L. hyperboreus, it is largest
(table 2). The high diversity in L. hyperboreus during the
first period results from the L. argentatus haplotype A20,
present in two individuals.
(b) Microsatellite analysis
The effective numbers of alleles (ne) are similar for both
1.6 to 2.3 in hyperboreus. The total number of alleles per
locus ranges from 3 to 11, with all populations
combined. Most alleles are shared by both species,
and only one allele (170 bp), at locus HG16 in
L. hyperboreus, is in a frequency higher than 5% but
zygosities were also similar across samples, 0.38–0.57 in
L. argentatus and L. hyperboreus. The most variable loci
areHG25(HeZ0.70) andHG18(HeZ0.66), whilethe
least variable locus is HG16 (HeZ0.31). The
differences in allele composition between populations
are mainly presented by frequency changes and not
many low-frequency alleles were detected. In Iceland,
a3w displays the largest variation (HeZ0.569 and
neZ2.3). Populations from period 1, a1, h1e and
h1w, have the lowest genetic variability (HeZ0.448,
0.377 and 0.510, neZ1.8, 1.6 and 2.0). FISacross loci
is significant and positive in three populations after
Bonferroni correction, in a1, h1w and h2. The largest
deviation is seen for one locus (HG25), in a1, h1w and
h3. Out of 100 tests of linkage disequilibrium, 7 give
permutation values larger than or equal to the observed
p values in the range of 0.8–3.1%. This weak signal of
linkage disequilibrium can be explained by the random
chance for unlinked markers.
(c) Population genetic differentiation
Hierarchical analysis of the genetic variance shows a
significant differentiation among and within popu-
lation. Differentiation among species is in all cases
non-significant, representing an intraspecific variance,
although it is close to significance when based on the
mtDNA (table 3). The extent of differentiation among
populations within species is larger when the variance
in genetic differentiation among haplotypes is taken
into account (increases from 3.4% to 7.4%). Similar
results are obtained for microsatellites, the variance
among populations within species increases to 40.21%
in calculations based on RST.
Pairwise comparisons among samples generally
distinguish the L. argentatus and L. hyperboreus in
Iceland, whether based on mtDNA haplotypes
(figure 3a), the microsatellites (figure 3b) or both using
the Bayesian clustering (figure 4). The population
comparisons of the mtDNA haplotypes, including
the samples from Liebers et al. (2004), locate the
Icelandic samples between the L. argentatus and
the L. hyperboreus and from rest of Europe. The figure
shows also a close relationship between some geographi-
cally adjacent populations of the two species, e.g. in
Iceland (a1 and h1w), Russia (R and NS) and Norway
(SN and S), and the distinct grouping of the
L. hyperboreus samples from Greenland and Canada.
The composition of mtDNA frequencies within
species in the Icelandic samples are not signifi-
cantly different from each other. The L. hyperboreus
samples differ from all but one L. argentatus population,
a3w, and h1w is not significantly different from a1.
Interestingly, h1w is significantly different from h2 and
h3 (p!0.05). Pairwise differences based on the FST
method are larger than the conventional FST, and this is
observed both in comparisons of conspecifics at
different periods (h3 and h2 versus h1w and h1e; a3e
and a2 versus a1) and also between different species at
the same locations (a3w versus h3 and h2).
nine groups, using F-statistics, are more often significant
for the microsatellite data (32 out of 36) than for the
cytochrome b (18 out of 36). Non-significant pairwise
comparisons based on microsatellites are between a1, a2
and h1e, and between h1w and h1e (table 1).
Table 3. Hierarchical analysis of molecular variance (AMOVA) of mtDNA and microsatellite variation among L. argentatus and
L. hyperboreus in Iceland. (The p-values are based on 1000 permutations of the data, which resulted in equal or larger value than
the observed statistic.)
source of variation% variation fixation index
mtDNA (FST) among species
populations within species
populations within species
populations within species
Hybridization of glaucous and herring gulls
F. Vigfu ´sdo ´ttir et al.
Phil. Trans. R. Soc. B (2008)
Based on both the mtDNA and the microsatellite
data, the Bayesian clustering method (BAPS) gives an
optimal partition of four clusters with a probability
of 99.9%. The log likelihoods for different numbers of
clusters k from 2 to 4 were K3673.6, K3656.5 and
K3655.3, respectively. Larger numbers of initial groups
(5–8) resulted in four clusters. All L. argentatus groups
cluster together in cluster 1, plus the h1e group. All
L. hyperboreus from the west of Iceland are in clusters 2
(h1w, h2) and 3 (h3), and they are equally distant
from cluster 1. L. hyperboreus from Kulusuk (cluster 4)
clusters further apart from the previously mentioned
clusters. The same partitioning was obtained among the
Icelandic samples when the Kulusuk sample was
omitted from the analysis (figure 4), showing a clear
evidence of admixture. A considerable proportion of the
individuals have admixture coefficients (q1) which are
intermediate (figure 4a,b). This is especially evident for
a1, a3w and h1e. The frequency of individuals per
sample, which have a higher proportion of their genome
originating from the other species, is largest at the first
time period in eastern Iceland where it decreases with
time for the L. argentatus samples, and an asymmetry is
observedinthe introgression (figure 4c). Inaddition, the
two L. hyperboreus individuals sampled in eastern
Iceland during the second and third periods (see
table 1) have the largest genetic similarities to
L. argentatus, the second non-significant (q1Z0.94 and
Iceland and Hjo ¨rleifsho ¨fði during period 1 have a
significantly higher proportion of the L. hyperboreus
out of 143) have a significantly larger proportion of
the genetic markers that are characteristic for the
L. hyperboreus samples. However, only 4% (6 out of
150) of L. hyperboreus individuals have a significantly
larger proportion of the genetic markers that are
characteristic for the L. argentatus samples. Four of
these are L. hyperboreus from h1e. The association is
a1a2 a3e h1e h1w h2h3
a1 a2a3e a3wh1e h1wh2h3
a1a2a3e a3wh1e h1w h2h3
Figure 4. Admixture analysis by BAPS (Marttinen et al.
2006) for samples from Iceland. (a) Larus shadings: dark
grey, white and grey vertical lines represent the admixture
coefficients for each individual, i.e. the proportion of the
genome of each individual (qi) which is traced to clusters 1–3,
respectively. (b) Boxplot of the admixture coefficient q1
for each sample. (c) Proportion (p) of individuals per sample,
Figure 3. Multidimensional scaling plot based on pairwise FSTs. (a) Comparisons based on mtDNA data from this study
(denoted with small letters, see table 1) and from Liebers et al. (2004) in capital letters.Larus argentatus (black): N, Netherlands;
Fr, France; G, Germany; Fa, Faroe Islands; E, Estonia; Fi, Finland; SN, southern Norway; NN, northern Norway; R, Russia;
IA, Iceland. Larus hyperboreus (grey): S, Svalbard; NS, Novaja Semlija; IH, Iceland; C, Canada Baffinland. (b) Comparisons
based on the microsatellite data.
2856 F. Vigfu ´sdo ´ttir et al. Hybridization of glaucous and herring gulls
Phil. Trans. R. Soc. B (2008)
significant (pZ2.2!10K8, Fisher’s exact test). When
considering the samples from Greenland, 2 out of 19 are
assigned to cluster 3 (H3).
A previous study of mtDNA phylogeography by Liebers
et al. (2004) shows that L. argentatus and L. hyperboreus
do not correspond strictly to distinct monophyletic
mtDNA haplogroups within the phylogeny of large
white-headed gulls. Whereas L. hyperboreus appear
biphyletically in an American and an European clade,
L. argentatus presents a large ramificated tree consisting
of two distinct clusters (I and II) within the European
clade, separated by haplotypes that are shared by other
gull species, namely L. cachinnans, L. fuscus and
is mainly found in one of these clusters (II), sharing
haplotypeswithL. argentatus.The observed polyphylyof
the twospeciescould bedue tothe incompletesortingof
mtDNA lineages from a polymorphic ancestral gene
pool but, in some cases, is more likely as a result of
hybridization, as discussed below.
The mtDNA of L. hyperboreus is characterized by low
diversity, as it has been observed for several Arctic
species (Hewitt 2004). The low diversity and shape of
the phylogenetic network may reflect fluctuations in
population size which may have been more severe for
the mtDNA than the nuclear locidue to smaller effective
population size. The Icelandic L. hyperboreus population
belongs to the European L. hyperboreus clade, but is
clearly distinct from the populations sampled elsewhere.
Only 1 L. hyperboreus out of 117 sampled in Iceland and
20 from Svalbard and Novaja Zemlija shared an
American haplotype (H1) with L. hyperboreus from
Kulusuk, Greenland and Baffinland, northeastern
Canada. This points to little contact between the
Icelandic and the East Greenland population, reflect-
ing the division between the Palaearctic and the
Nearctic L. hyperboreus. This division is also sup-
ported by the microsatellite data, although the most
recent samples are most similar, possibly reflecting
Overall, L. argentatus harbour larger number of
haplotypes and higher diversity in mtDNA in Iceland
compared with L. hyperboreus, in agreement with the
previous genetic studies of L. argentatus in Europe
(Crochet et al. 2002, 2003; Liebers et al. 2004). The
Icelandic population differs from the rest of Europe,
showing greatest similarity to the samples from
northern Europe. Interestingly, comparisons with
other samples from Europe, obtained by Liebers et al.
(2004), show that the earliest L. argentatus samples in
Iceland were actually more similar to populations of
L. hyperboreus from northern regions (Norway, Russia)
than to other L. argentatus populations. Samples of
L. argentatus from time periods 2 and 3 are more similar
to populations with more southern and western
distribution, sharing haplotypes with individuals
from, for example, France, Faroe Islands and
Germany. A closer look at the composition of the
haplotypes in Icelandic L. argentatus shows that the
proportion of the clusters I and II changes over time.
Cluster II is dominant during the first time period
(92%), whereas haplotypes in cluster I increase in
frequency with time, up to 40% during 1985–1986.
This change is also reflected in the nucleotide and
haplotype diversity. This could be explained by a high
frequency of hybrid individuals or indication of intro-
gression during the first period among L. argentatus, or
alternatively by later waves of immigration of
L.argentatus toIceland. This may also explain a bimodal
mismatch distribution observed for L. argentatus, which
and II (data not shown). Bimodality is generally
interpreted as a sign of stable evolutionary population
size; here it may have resulted from an admixture of
L. argentatus populations, or alternatively hybridization
over a long period, where cluster II has spread from
L. hyperboreus to L. argentatus by introgression.
Genetic variation in large white-headed gulls appears to
be shaped by both the retention of ancestral poly-
morphism and introgression. Liebers et al. (2004)
suggested that the haplotypes, here presented in cluster
II, were ancestral in L. argentatus and that cluster I had
resulted from hybridization with descendants from the
Aralo-Caspian refugium. The argument was that
cluster II harboured more variation in L. argentatus
and that it had been supported by nuclear AFLP
(de Knijff et al. 2001). This is also supported by the
recent study by Gay et al. (2007) on introgression
between L. cachinnans (a descendant from the Aralo-
Caspian refugium) and L. argentatus.
A characteristic result of the study on the mtDNA in
European L. hyperboreus is a lack of haplotypes that are
not shared by L. argentatus. Most of the L. hyperboreus
haplotypes are found in cluster II and only three of
them have not been found in L. argentatus. These three
are singletons. The most common haplotype (A12) is
shared by L. argentatus in northern and northwestern
Europe, and is the only haplotype shared between
L. hyperboreus and L. argentatus outside Iceland. The
widespread occurrence of A12 results in high IG values
and reflects possibly past introgression or incomplete
lineage sorting. Sharing of other haplotypes among the
species is, however, only found in Iceland, including
the common haplotype H2. As L. argentatus is a recent
settler in Iceland, this suggests recent introgression
rather than shared ancestry. Three instances are
observed where widely distributed L. argentatus haplo-
types (MA1 and A20) are found only in L. hyperboreus
in Iceland, all in samples from the first time period
(1964–1973), which had high IG values. One of these
haplotypes is from cluster I (A20), and it caused the
high nucleotide diversity seen in L. hyperboreus during
the first time period. If shared ancestry would be the
case, these haplotypes would most likely have been
found in other L. hyperboreus elsewhere. Less sharing of
the haplotypes in cluster I, among the two species, may
indicate that the introgression of mtDNA from
L. hyperboreus to L. argentatus has been more common
than that from L. argentatus to L. hyperboreus, or that
the L. argentatus with haplotypes found in cluster I have
arrived on Iceland more recently. As concluded by
Liebers et al. (2004), a study including more nuclear
Hybridization of glaucous and herring gulls
F. Vigfu ´sdo ´ttir et al.
Phil. Trans. R. Soc. B (2008)
markers will be needed to solve the question of which
direction the cytonuclear replacement occurred. The
genetic difference between the European and the
American L. hyperboreus remains to be explained; it
might reflect a parallel morphological evolution or
ancient introgression from L. argentatus.
The lack of significant differences between the
species in the hierarchical analysis points to a high
intraspecific variability, agreeing with gene flow
between the species and the retention of ancestral
polymorphism. This also suggests that a larger number
of nuclear markers may be needed to distinguish
between the two species. It is also possible that there
are only few genes that are responsible for the species
differences. The similarity of sympatric groups of the
two species was, however, clearly seen in all pairwise
comparisons based on microsatellites, implying gene
flow. Bayesian clustering analysis (BAPS) based on
mtDNA and microsatellite results gave similar findings
as the FST’s analysis, where the L. hyperboreus group
(h1e) sharing the L. argentatus types, clustered among
the L. argentatus. Supporting these signs of hybrid-
ization, admixture analyses revealed a number of
L. argentatus individuals that were more likely to be
characterized with L. hyperboreus genotypes. The
admixture analysis indicated that 27% of individuals
showing L. argentatus plumage pattern should be
classified as L. hyperboreus based on genetic markers
and only 4% of the L. hyperboreus as L. argentatus,
indicating that introgression occurs more frequently
from L. hyperboreus to L. argentatus. In agreement with
these findings is the fact that the L. hyperboreus that
were breeding in a colony in southeast Iceland, which
also harboured a large number of L. argentatus, were
clustered with all L. argentatus groups. Similarly, most
of the L. argentatus that were assigned to L. hyperboreus
clusters were sampled during the first time period in
southeast Iceland. The observed directional introgres-
sion may possibly reflect the different numbers of
L. argentatus and L. hyperboreus studied from this area,
where hybridization appears to have been common. It
seems that the hybridization during this early period
has not had a lasting impact on the genetic composition
in eastern Iceland. Today, the main hybridization
appears in western Iceland, where L. argentatus,
although in low numbers, are highly admixed.
A monitoring of the populations in western Iceland
may provide further insights into the dynamics of this
secondary contact zone.
One caveat about the results presented in this study
is that the species classification is originally based on
morphology. Although the classification is well sup-
ported by the Bayesian analysis, some individuals may
be wrongly identified due to introgression of genes
behind the morphological trait. A further analysis of
these individuals may show that the extent of
hybridization can be even more common than that
revealed in this study. Such admixed individuals may
cause admixture in the pool of the reference group. To
solve this problem, it would be good to include just
‘pure’ populations for reference for the putative hybrid
individuals. However, such populations may be hard to
find, considering the population structure, recent
origin of the species, incomplete lineage sorting and
even widespread hybridization.
Pairwise comparisons based on microsatellites were
in general similar to comparisons based on mtDNA,
although comparisons between populations within
species based on the microsatellites are more often
significant. The difference may result from a larger
statistical power obtained when studying the micro-
satellites, as they are based on more markers and on 93
more individuals than the mtDNA analyses. Interest-
ingly, the extent of the differentiation varies in some
cases based on the markers studied, for example, the a2
and h3k samples are more differentiated from the
L. hyperboreus in Iceland for the mtDNA than the
microsatellites (figure 3). Similarly, a larger genetic
differentiation for the mtDNA was observed in the
study by Gay et al. (2007) on L. argentatus and
L. cachinnans, where a complete lineage sorting was
observed for the mtDNA, despite shared poly-
morphism in microsatellites. Such differences may
result from stronger drift on mtDNA due to lower
effective population size, behavioural effects and/or to
different post-zygotic isolation in male and female
hybrids (e.g. Orr 1997; Crochet et al. 2003; Gay et al.
2007). The observation of larger differences for the
mtDNA is in accordance to the prediction of Haldane’s
rule (Haldane 1922); the genetic markers transmitted
by the heterogametic sex should show more differen-
tiation. A previous study of the L. hyperboreus and
L. argentatus in Iceland (Ingo ´lfsson 1970) did not
detect any clear evidence of assortative mating.
The result of this study concurs with numerous
reports of introgression in areas of sympatry among
various species of plants (e.g. Palme ´ et al. 2004) and
animals of recent origin (e.g. overview on studies on
birds in McCarthy 2006). The hierofalcons provide
one recent example where variation reflects both
hybridization and incomplete lineage sorting (Nittinger
et al. 2007). In such studies of closely related species, it
has become evident that both phylogeographical
patterns as well as genetic information from the
different species are needed in order to disentangle
the impact of incomplete sorting of lineages, and
current and past hybridizations among taxa. A more
common haplotype sharing in areas of sympatry points
to hybridization. If a biphyly is due to shared ancestry,
there should be no correspondence to geographical
distribution. A biphyly has been detected in a previous
study of gulls by Liebers et al. (2004) where
introgression was implied along with shared ancestry.
As suggested by Crochet et al. (2002, 2003), sharing of
the most divergent mitochondrial lineages clearly
results from introgression. Where more than one
haplotype is found in a species and the less frequent
haplotype is identical to the common haplotype in
another species, the geographical distribution of
haplotypes is indicative of interspecific horizontal
transfer. Such events of a specific haplotype sharing
between a recent colonizer (here L. argentatus) and a
settled species (here L. hyperboreus) was seen in the North
American L. marinus (a recent colonizer), which acquired
the L. smithsonianus (the settled one) haplotypes,
presumably through hybridization (Crochet et al. 2003).
2858F. Vigfu ´sdo ´ttir et al.Hybridization of glaucous and herring gulls
Phil. Trans. R. Soc. B (2008)
5. CONCLUDING REMARKS
Earlier claims of hybridization between L. hyperboreus
and L. argentatus in Iceland (Ingo ´lfsson 1970), based
on morphology, were questioned by Snell (1991a,b)
who argued that the observed intrapopulation varia-
bility in L. argentatus resulted from a founder effect.
Snell suggested that the claimed hybrids in Iceland
Scandinavia and that no hybridization between the
two species in Iceland occurs. Along with all the
previously mentioned cases of hybridization in gulls,
genetic research strongly suggests that gene flow among
large white-headed gulls is in fact extensive in many
parts of the world (Crochet et al. 2002, 2003; Liebers
et al. 2004; Gay et al. 2007). Thus, the haplotype and
observed in this study should not be surprising. The
genetic sharing in Iceland follows a geographical pattern
and is most obvious in an area where both species were
after the arrival of L. argentatus on Iceland.
We thank Guðmundur A. Guðmundsson, Gunnar 3o ´r
Hallgrı ´msson, Jo ´hann Brandsson and Pa ´ll Leifsson who
provided samples or assisted in the field, and Guðmundur
assistance with museum samples. We also want to thank the
editors and three anonymous reviewers for their useful
Council and the University of Iceland Science Fund.
Bandelt, H. J., Forster, P. & Rohl, A. 1999 Median-joining
networks for inferring intraspecific phylogenies. Mol. Biol.
Evol. 16, 37–48.
Barth, E. K. 1968 The circumpolar systematics of Larus
argentatus and Larus fuscus with special reference to the
Norwegian populations. Nytt Mag. Zool. 15, 1–50.
Barton, N. H. & Hewitt, G. M. 1985 Analysis of hybrid
zones. Annu. Rev. Ecol. Syst. 16, 113–148. (doi:10.1146/
Belahbib, N., Pemonge, M.-H., Ouassou, A., Sbay, H.,
Kremer, A. & Petit, R. J. 2001 Frequent cytoplasmic
exchange between oak species that are not closely related:
Quercus suber and Q. ilex in Morocco. Mol. Ecol. 10,
Belkhir, K., Borsa, P., Chikhi, L., Raufaste, N. &Bonhomme,
F. 2004 GENETIX version 4.05, Logiciel sous Windows
pour la ge ´ne ´tique des populations. Laboratoire ge ´nome,
populations, interactions. CNRS UMR 5000, Universite ´
de Montpellier II, Montpellier, France.
Bell, D. A. 1996 Genetic differentiation, geographic variation
and hybridization in gulls of the Larus glaucescens–
occidentalis complex. Condor 98, 527–546. (doi:10.2307/
Coulson, J. C. 1991 The population dynamics of culling
herring gulls and lesser black-backed gulls. In Bird
population studies, relevance to conservation and management
(eds C. M. Perrins, J. D. Lebreton & G. J. M. Hirons),
pp. 479–497. Oxford, UK: Oxford University Press.
Cramps, S. & Simmons, K. E. L. 1983 Handbook of the birds of
Europe, the Middle East and North Africa. The birds of the
Western Palearctic. Oxford, UK: Oxford University Press.
Crochet, P. A. 2000 Genetic structure of avian populations—
allozymes revisited. Mol. Ecol. 9, 1463–1469. (doi:10.
Crochet, P. A., Lebreton, J. D. & Bonhomme, F. 2002
Systematics of large white-headed gulls: patterns of
mitochondrial DNA variation in western European taxa.
Auk 119, 603–620. (doi:10.1642/0004-8038(2002)119
Crochet, P. A., Chen, J. Z., Pons, J. M., Lebreton, J. D.,
Hebert, P. D. & Bonhomme, F. 2003 Genetic differen-
tiation at nuclear and mitochondrial loci among large
white-headed gulls: sex-biased interspecific gene flow?
Evolution 57, 2865–2878. (doi:10.1111/j.0014-3820.2003.
de Knijff, P., Denkers, F., van Swelm, N. D. & Kuiper, M.
2001 Genetic affinities within the Larus argentatus
assemblage revealed by AFLP genotyping. J. Mol. Evol.
52, 85–93. (doi:10.1007/s002390010137)
Excoffier, L., Smouse, P. E. & Quattro, J. M. 1992 Analysis of
molecular variance inferred from metric distances among
DNA haplotypes—application to human mitochondrial-
DNA restriction data. Genetics 131, 479–491.
Excoffier, L., Laval, G. & Schneider, S. 2005 ARLEQUIN ver
3.0: an integrated software package for population genetic
data analysis. Evol. Bioinform. Online 1, 47–50.
Gay, L., Neubauer, G., Zagalska-Neubauer, M., Debain, C.,
Pons, J.-M., David, P. & Crochet, P.-A. 2007 Molecular
and morphological patterns of introgression between two
large white-headed gull species in a zone of recent
secondary contact. Mol. Ecol. 16, 215–227. (doi:10.
Grant, P. R. & Grant, B. R. 1992 Hybridization of bird
species. Science 256, 193–197. (doi:10.1126/science.256.
Guðmundsson, F. 1955 I´slenskir fuglar XI. Hvı ´tma ´fur (Larus
hyperboreus). Na ´ttu ´rufræðingurinn 25, 24–35.
Haldane, J. B. S. 1922 Sex-ratio and unisexual sterility in
hybrid animals. J. Genet. 12, 101–109.
Hewitt, G. M. 1996 Some genetic consequences of ice ages,
and their role in divergence and speciation. Biol. J. Linn.
Soc. 58, 247–276.
Hewitt, G. M. 2000 The genetic legacy of the Quaternary ice
ages. Nature 405, 907–913. (doi:10.1038/35016000)
Hewitt, G. M. 2004 Genetic consequences of climatic
oscillations in the Quaternary. Phil. Trans. R. Soc. B 359,
Ingo ´lfsson, A. 1970 Hybridization of glaucous gulls Larus
hyperboreus and herring gulls Larus argentatus in Iceland.
Ibis 112, 340–362. (doi:10.1111/j.1474-919X.1970.tb00
Ingo ´lfsson, A. 1982 Ma ´far, kjo ´ar og sku ´mar. In Fuglar
(ed. A. Garðarsson). Reykjavı ´k, Iceland: Landvernd.
Ingo ´lfsson, A. 1987 Hybridization of glaucous and herring
gulls in Iceland. Stud. Avian Biol. 10, 131–140.
Ingo ´lfsson, A. 1993 The variably plumaged gulls of Iceland.
Auk 110, 409–410.
Liebers,D., de Knijff, P. & Helbig, A. J. 2004 The herring gull
complex is not a ring species. Proc. R. Soc. B 271,
Marttinen, P., Corander, J., To ¨ro ¨nen, P. & Holm, L. 2006
Bayesian search of functionally divergent protein sub-
groups and their function specific residues. Bioinformatics
22, 2466. (doi:10.1093/bioinformatics/btl411)
McCarthy, E. M. 2006 Handbook of avian hybrids of the world.
Oxford, UK: Oxford University Press.
Newton, I. 2003 The speciation and biogeography of birds. San
Diego, CA: Academic Press.
Nittinger, F., Gamauf, A., Pinsker, W., Wink, M. & Haring,
E. 2007 Phylogeography and population structure of the
saker falcon (Falco cherrug) and the influence of hybrid-
ization: mitochondrial and microsatellite data. Mol. Ecol.
16, 1497–1517. (doi:10.1111/j.1365-294X.2007.03245.x)
Olsen, K. M. & Larsson, H. 2004 Gulls of Europe, Asia and
North America. London, UK: A & C Black Publishers.
Hybridization of glaucous and herring gulls
F. Vigfu ´sdo ´ttir et al.
Phil. Trans. R. Soc. B (2008)
Orr, H. A. 1997 Haldane’s rule. Annu. Rev. Ecol. Syst. 28,
Palme ´, A. E., Su, Q., Pa ´lsson, S. & Lascoux, M. 2004
Extensive sharing of chloroplast haplotypes among
European birches: Betula pendula, B. pubescens and B.
nana. Mol. Ecol. 13, 167–178. (doi:10.1046/j.1365-294X.
Panov, E. N. & Monzikov, D. G. 1999 Intergradation
between the herring gull Larus argentatus and the southern
herring gull Larus cachinnans in European Russia. Zool.
Zh. 78, 334–348.
Randler, C. 2002 Avian hybridization, mixed pairing and
female choice. Anim. Behav. 63, 103–119. (doi:10.1006/
Rogers, A. R. & Harpending, H. 1992 Population-growth
makes waves in the distribution of pairwise genetic-
differences. Mol. Biol. Evol. 9, 552–569.
Slatkin, M. 1995 A measure of population subdivision based
on microsatellite allele frequencies. Genetics 139, 457–462.
Snell, R. R. 1991a Interspecific allozyme differentiation among
Snell, R. R. 1991b Variably plumaged Icelandic herring-gulls
reflect founders not hybrids. Auk 108, 329–341.
Sokal, R. R. & Rohlf, F. J. 1995 Biometry. New York, NY:
W. H. Freeman and Company.
Spear, L. B. 1987 Hybridization of glaucous and herring-
gulls at the Mackenzie Delta, Canada. Auk 104,
Venables, W. & Ripley, B. 1994 Modern applied statisticswith
S-plus. New York, NY: Springer.
Weir, B. S. 1990 Genetic data analysis. Sunderland, MA:
Weir, B. S. & Cockerham, C. C. 1984 Estimating F-statistics
for the analysis of population-structure. Evolution 38,
Yesou, P. 1991 The sympatric breeding of Larus fuscus,
L. cachinnans and L. argentatus in Western France. Ibis 133,
2860F. Vigfu ´sdo ´ttir et al.Hybridization of glaucous and herring gulls
Phil. Trans. R. Soc. B (2008)