Radiating on Oceanic Islands: Patterns and Processes of
Speciation in the Land Snail Genus Theba (Risso 1826)
Carola Greve1*, France Gimnich1¤, Rainer Hutterer1, Bernhard Misof1, Martin Haase2
1Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany, 2Vogelwarte, Zoologisches Institut, Ernst-Moritz-Arndt-Universita ¨t Greifswald, Greifswald,
Island radiations have played a major role in shaping our current understanding of allopatric, sympatric and parapatric
speciation. However, the fact that species divergence correlates with island size emphasizes the importance of geographic
isolation (allopatry) in speciation. Based on molecular and morphological data, we investigated the diversification of the
land snail genus Theba on the two Canary Islands of Lanzarote and Fuerteventura. Due to the geological history of both
islands, this study system provides ideal conditions to investigate the interplay of biogeography, dispersal ability and
differentiation in generating species diversity. Our analyses demonstrated extensive cryptic diversification of Theba on these
islands, probably driven mainly by non-adaptive allopatric differentiation and secondary gene flow. In a few cases, we
observed a complete absence of gene flow among sympatrically distributed forms suggesting an advanced stage of
speciation. On the Jandı ´a peninsula genome scans suggested genotype-environment associations and potentially adaptive
diversification of two closely related Theba species to different ecological environments. We found support for the idea that
genetic differentiation was enhanced by divergent selection in different environments. The diversification of Theba on both
islands is therefore best explained by a mixture of non-adaptive and adaptive speciation, promoted by ecological and
Citation: Greve C, Gimnich F, Hutterer R, Misof B, Haase M (2012) Radiating on Oceanic Islands: Patterns and Processes of Speciation in the Land Snail Genus
Theba (Risso 1826). PLoS ONE 7(4): e34339. doi:10.1371/journal.pone.0034339
Editor: Mikhail V. Matz, University of Texas, United States of America
Received August 23, 2011; Accepted March 1, 2012; Published April 6, 2012
Copyright: ? 2012 Greve 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: This work was financially supported by grants of the Deutsche Forschungsgemeinschaft (DFG MI 649/7-1 and DFG HU 430/2-2) and the Alexander
Koenig Gesellschaft (AKG). 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.
* E-mail: email@example.com
¤ Current address: Museum fu ¨r Naturkunde, Institute of Systematic Zoology, Humboldt University, Berlin, Germany
A major problem for the study of speciation is that the
formation of new and genetically isolated species is in most cases a
slow and continuous process, lasting many generations. The direct
observation of the entire process of speciation is usually impossible,
except in the case of polyploid or hybrid speciation, which can
occur in one or a few generations . Speciation has yet to be
studied by comparing many snap-shots of this continuous process -
from the divergence of populations to fully reproductively isolated
species . Compared to continental regimes, islands provide
isolated systems often with less complex biotas, in which speciation
processes can be studied more effectively. Consequently, island
systems have been successfully used to study adaptive diversifica-
tion of ecologically and/or morphologically well differentiated
sympatric species as well as non-adaptive (cryptic) radiations of
ecologically or morphologically similar allopatric or parapatric
species [3–8]. The study of island radiations have thus helped to
establish our current understanding of allopatric, sympatric and
parapatric speciation, but the relative importance of these three
modes is still unclear. In this context, it is remarkable that species
divergence often correlates with island size. This phenomenon has
been interpreted as evidence for the important role of geographic
isolation (allopatry) in speciation .
Here, we analyzed the divergence of the land snail genus Theba
 on two of the Canary Islands: Lanzarote and Fuerteventura.
Theba populations on these two islands are found in allopatry,
parapatry and sympatry, displaying different levels of molecular
and morphological differentiation . The geological character-
istics of Lanzarote and Fuerteventura, including unusual longevity,
complex volcanic evolution and close vicinity to a continental land
mass  provide ideal conditions to study the interplay of
biogeography, dispersal ability and differentiation in generating
The volcanic archipelago of the Canary Islands is located in the
east Atlantic Ocean with Fuerteventura lying approximately
100 km off the West Saharan coast of NW Africa. The archipelago
has never been connected to continental landmasses. The eastern-
most islands, Lanzarote and Fuerteventura, emerged 24 million
years ago (mya) and form one large volcanic edifice separated by
shallow waters less than 50 m deep. During periods of glaciation
and low sea level in the Quaternary, they were periodically
connected by land bridges [12–14]. Due to intense erosion,
Lanzarote and Fuerteventura lack the high elevations seen on
other Canary Islands, which are exposed to the humid north-
eastern trade winds. Both islands are characterized by an arid
climate, enhanced by the continental influence of the Saharan
desert, and are dominated by xeric scrub vegetation. Local
hygrophilous fauna and flora can be found only atop a few
mountains (Jandı ´a on Fuerteventura and Famara on Lanzarote)
. Though the eastern islands exhibit less diverse habitat
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structure than the central and western Canary Islands, they are
characterized by complex geomorphology due to past and recent
volcanic and erosional activity [11–12].
Despite the close proximity of NW Africa to the Canary Islands,
the main speciation mode of the genus Theba on the islands has
been intra-archipelago diversification, rather than independent
colonization of the islands from the African continent . The
phylogenetic study of Greve et al.  was based on COI and
ITS1 sequence data and indicated that differentiation within Theba
species on Fuerteventura and Lanzarote is more extensive than
previously thought [15–17]. These molecular data helped to
identify three endemic species on Lanzarote and two on
Fuerteventura. Neither the species from Lanzarote nor those from
Fuerteventura appear to be monophyletic. These results, however,
were based on sequence information of only a few specimens. In
order to assess the extent and nature of population differentiation
within and between the Theba species of Lanzarote and
Fuerteventura, we expand existing COI sequence data and
additionally conducted comprehensive morphological and popu-
lation genetic (AFLP) analyses. As a null hypothesis, we assumed
that the long and dynamic history of both islands and the relatively
uniform ecological situation along with the low dispersal abilities of
land snails, led to isolation by distance (IBD) and eventually
allopatric speciation within and among islands. The greater
habitat diversity on the southern Jandı ´a peninsula of Fuerteven-
tura , however, might have supported ecological (adaptive)
diversification. Therefore, we sampled populations extensively
along a north-south transect on both islands hopefully covering
different levels of genetic differentiation. Additionally, we sampled
specimens from different ecological habitats on the Jandı ´a
peninsula. We then used a population genomic approach based
on the AFLP data to differentiate between different modes of
speciation within and among both islands, under the assumption
that allopatric speciation leads to random distribution of
differentiation within the genome, whereas sympatric/parapatric
speciation leads to a specific non-random differentiation of alleles
due to divergent selection [18–19].
Materials and Methods
Sampling and DNA isolation
Based on the preliminary results of Greve et al. , we
sampled Theba populations extensively on Fuerteventura and
Lanzarote. Snails were collected on these islands in December
2009. In total, forty-one populations were sampled along a north-
south transect, covering all known genetically distinguishable
clades of Theba on Fuerteventura and Lanzarote  (Fig. 1). In
order to cover the entire extent of differentiation, we included all
autochthonous species from the Canary Islands, viz. T. cf. arinagae
and T. grasseti from Gran Canaria and T. macandrewiana from the
Selvagens Islands (Table S1). T. macandrewiana was used as the
outgroup species. Snails were preserved in absolute ethanol and
total genomic DNA was extracted from foot muscle tissue of each
snail using the DNeasyH Blood & Tissue Kit (Qiagen) following the
manufacturer’s protocol. Vouchers are available at the Zoolo-
gisches Forschungsmuseum Alexander Koenig, Bonn, Germany.
Mitochondrial DNA sequencing
At least two specimens per population were sequenced for a
fragment of the mitochondrial cytochrome c oxidase subunit I
(COI). The COI fragment was amplified by the polymerase chain
reaction (PCR) using the primer combination LCO-1490 [59-
GGTCAACAAATCATAAAGATATTGG-39 ] and C1-N-
2191 [59-CCCGGTAAAATTAAAATATAAACTTC-39 ].
PCR reactions were carried out in a total volume of 10 mL using
the Qiagen Multiplex PCR Kit. Thermal cycling conditions were
as follows: 95uC for 15 min, 15 cycles of touchdown PCR (94uC
for 35 s, 55uC–40uC annealing for 90 s and 72uC extension for
90 s) followed by 25 cycles (94uC for 35 s, 40uC annealing for 90 s
and 72uC extension for 90 s) and a final extension step at 72uC for
10 min. PCR products were purified using ExoSAP-ITH (USB).
Double stranded sequencing was carried out by a sequencing
facility (Macrogen, Seoul, South Korea; using ABI 3730XL
sequencers). Sequences were deposited in GenBank (see Table S1).
The variation of standard nuclear genes in pulmonates does not
provide enough resolution to sort taxa below genus level . We
therefore used the amplified fragment length polymorphism
(AFLP) method to control for potential phylogenetically mislead-
ing lineage sorting of mtDNA. If possible, we sampled 20
individuals per population to achieve accurate results for
estimating population structure in the AFLP data .
AFLP markers were obtained with a slightly modified version of
the original protocol of Vos et al. . Selective amplifications
were performed using six different primer combinations: EcoRI-
EcoRI-ACT/MseI-CAG. The fluorescently labeled fragments were
separated by electrophoresis on a CEQTM
sequencer (Beckman Coulter, Inc., Fullerton, California), with
an internal size standard (CEQ DNA Size Standard Kit 600,
Beckman Coulter, Inc.). Signal detection, processing and binning
of the AFLP electropherograms were carried out using the
CEQTMSystem Fragment Analysis module of the manufacturer’s
software (Version 9.0.25, Beckman Coulter, Inc.). The fluores-
cence threshold for an accepted signal was set to 1% of the height
of the second largest peak detected in the AFLP profile. Choosing
a relative threshold instead of the frequently used fixed threshold
minimizes artifacts resulting from differences in total profile
strength among individuals as well as those resulting from unequal
detection among capillaries . Correct fit of the size standard
and fragment distribution was checked for all profiles. Low quality
profiles were discarded. Subsequently, fixed fragment categories
(hereafter also referred to as bins) were created between 60 and
550 bases (b). AFLP markers were automatically scored according
to the presence/absence of fragment peaks within each bin and for
each sample, setting the fluorescent signal detection threshold to
50 units. According to the accuracy of the CEQ sequencing system
(standard deviation=0.25 b; manufacturer’s specifications), the
maximum bin width for reliable fragment sizing was set to 0.75 b.
Monomorphic markers were excluded from the data set.
To ensure high reliability of AFLP genotyping, 11% of the
samples were genotyped twice for all primer combinations; these
replicates were taxonomically representative of the whole data set.
A perl script was written to fully automate the following marker
selection procedure and to estimate the average genotyping error
rate per marker (following [26–27]). Based on the replicated
samples, the repeatability of each individual marker was estimated
to control for scoring errors. Bins with less than 81% repeatable
markers were excluded from the data set. Furthermore, all bins
without any fragment peak present among replicates were
excluded, because shared fragment absences (null alleles) are
particularly prone to homoplasy due to the multiple and
independent ways in which a fragment can be lost [28–29]. Bins
without any confirmed (present/present) fragment peak among
compared replicate pairs were also excluded to avoid spurious
background noise in the data set. Finally, the remaining markers
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were used to estimate the average genotyping error rate per
marker. This value was 7.6% and was lower than the maximum
value of 10% recommended by Bonin et al. . The final AFLP
binary character matrix including all replicates is provided as
supporting information (Dataset S1).
Figure 1. Sampling localities of specimens of Theba on Lanzarote and Fuerteventua. The Jandı ´a peninsula is enlarged for better
recognition. *=,20 specimens collected, &=fewer than 20 specimens collected, #=empty shells.
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COI sequences were aligned with ClustalW  using default
parameter settings and obviously misaligned positions were
adjusted manually in Bioedit v7.0 . Homogeneity of base
frequencies among COI sequences was checked with the x2- test
implemented in PAUP* v4.0b10 . For phylogenetic recon-
struction Bayesian (BA) and maximum likelihood (ML) analyses
were performed. According to the results of the Akaike
Information Criterion in MrModeltest v2.3 , the GTR+C+I
model was selected for BA. BA was carried out with MrBayes v3.1
[34–35] using two parallel runs each with 6 simultaneous Markov
chains for 25,000,000 generations. Trees were sampled every
100th generation. Excluding the first 120,000 trees of each run as
burn-in, a 50% majority-rule consensus tree with posterior
probabilities was constructed from the remaining 260,002 trees.
Tracer v1.4.1  was used to determine the burn-in generation
number as well as to check convergence of parameter estimates by
inspecting effective sample size (ESS) values and traces of the
MCMC samples. ML analysis was conducted with RAxML v7.0.3
 using the GTR+C+I model. Node support for the best-scoring
ML tree was evaluated with 1000 rapid bootstrap replicates .
Phylogenetic reconstruction based on the AFLP data set was
performed with PAUP* v4.0b10  using neighbor-joining (NJ)
on Nei-Li  distances. This distance measure is best suited for
AFLPs, as it accounts for the sharing of presence alleles, while
absent alleles are ignored due to their more homoplasious
character . Internal node support was assessed using
nonparametric bootstrapping (1000 replicates).
We considered the criterion of reciprocal monophyly to infer
possible boundaries of molecular operational taxonomic units
(MOTU). We further tested accurate MOTU assignments among
specimens of the COI data set by using the cluster module of
TaxonDNA v.1.7.8 . This module groups sequences at
different similarity thresholds into clusters based on pairwise
uncorrected p-distances. We chose a clustering threshold of 3%, as
this threshold has been cited as sufficient genetic disparity for
species delimitation .
Samples used for morphometrics largely consisted of empty
shells (Table S1). However, only in the cases of the taxonomically
unproblematic flat shelled T. grasseti and T. impugnata did we
include samples from localities that remained genetically unin-
Shell morphology was analyzed using geometric morphometrics
[42–43]. Shells were balanced on a base of Styrofoam and the
apertural view photographed at the same scale using a Nikon D-
70s camera equipped with an AF-Nikkor 28–105 mm lens. Ten
landmarks were applied using tpsDig  after the images had
been transformed into tps format in tpsUtil . The procedure
has proved to be highly repeatable for depressed, globular and
conical shells as well as for shells of both coiling directions [46–47].
Multivariate statistical analyses were carried out based on
Procrustes superimpositions using the programs of the IMP suite
of H.D. Sheets and co-workers (http://www3.Canisius.edu/
,sheets/morphsoft.html) and PAST 2.0 . Centroid-size
(square root of the summed squared distances of each landmark
form the centroid of the landmark configuration ) was used as
a proxy for shell size. Morphometric comparisons were based on
MOTUs defined by the phylogenetic analyses (Figs. 2, 3).
Single population samples were treated as units only if they were
apparently morphologically different, for example FU23 and
FU25. Samples composed of two species (hereafter referred to as
‘‘mixed samples’’) were only used in morphological assignment
tests based on foregoing CVAs (see manual to CVAgen6 of the
IMP suite) to estimate the proportion of each species involved.
Similarly, FU17 was only used in a morphological assignment test
because of its ambiguous topological position. Except for the
genetically identified individuals of LZ23, shells from mixed
samples were not included in the overall analysis based on the
results of the morphological assignment tests, because of the high
rate of incorrect allocations.
Population structure based on the AFLP data set
We used Population graphs, a multivariate graph theoretical
approach , to examine the genetic structure among popula-
tions. This method is free of an a priori model of population
arrangement, unlike AMOVA, and generates a graph describing
the high-dimensional genetic covariance relationships among all
populations simultaneously. The Population graph contains the
minimal number of edges that sufficiently describe the among
population genetic covariance structure, with node size represent-
ing the within population genetic variance and edge lengths
representing the among population component of genetic
variation. As the sample size cut-off value per population is three
for Population graphs, populations with fewer than three
individuals were excluded from the analysis (Table S1).
We also used Structure v2.3.2 [50–51] to investigate patterns of
genetic structure. Analyses were conducted without a priori group
designation using a model allowing for recessive alleles, which is
best suited for dominant molecular markers such as AFLPs .
We chose an admixture model with correlated allele-frequencies
. We allowed for gene flow, thus avoiding inaccurate
presumptions about genetic barriers. The dirichlet parameter for
the degree of admixture (a) and the parameter of allelic
frequencies distribution (l) were set to be inferred from the data.
For all Structure analyses, we used a total run length of 250,000
generations, including a burn-in of 50,000 generations. According
to the sampled populations, K=1 to 46 was tested with ten
independent runs at each K (=number of populations or clusters).
We plotted the mean likelihood L(K) over 10 runs for each K and
used the statistic DK proposed by Evanno et al.  to determine
the optimal number of genetically differentiated clusters.
Genetic structure of the AFLP data was further investigated
using analysis of molecular variance (AMOVA) as implemented in
Arlequin v3.5 . For this analysis, all populations with fewer
than 5 individuals were excluded (Table S1). AMOVA was based
on unstructured as well as several subdivided data sets.
Hierarchical levels comprised within- and between-group com-
parisons (Table S2) according to geography and MOTUs.
Significance of variance components was tested with 20022
Isolation by distance
To test for IBD, a simple Mantel test  was conducted using
the software zt . The significance of the test was achieved by
permuting the matrices 1,000,000 times. Matrices of pairwise FST
values for all populations of Lanzarote and Fuerteventura were
calculated using Arlequin v3.5 . Geographical distances (in
kilometers) between sample sites were computed with AFLPdat
. All Mantel tests were based on the AFLP data set.
Outlier detection among AFLPs was performed with BayeScan
 using default parameter settings. It is based on the idea that
genetic differentiation among populations in contrasting environ-
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ments is expected to be different for loci under selection than for
the rest of the genome. BayeScan estimates a posterior probability
for each locus being under selection. All loci with a posterior
probability over 95% were retained as outliers. Compared to
alternative programs, BayeScan is less sensitive to false-positive
outlier detection and allows for different demographic histories
and different amounts of genetic drift between the populations
[59–60]. Inter- and intraspecific pairwise comparisons between
populations occurring in allopatry, parapatry and sympatry were
used to infer mechanisms of ongoing and past speciation [18–19].
The pairwise analyses allow the identification of loci that are
outliers in multiple population pairs of compared MOTUs. Only
outlier loci found in every population pair of compared MOTUs
(hereafter referred to as consistent outlier loci) were considered as
candidate markers most likely under selection, thus reducing type I
errors . Following Butlin , we refrained from a categori-
zation of loci into outlier and non-outlier groups.
In total, 125 specimens were sequenced for COI resulting in a
data set of 606 aligned positions. This included the 11 COI
sequences from the previously published data set . Base
composition among sequences was homogeneous (x2=127.11,
d.f.=372, p.0.999). Phylogenies obtained from ML and BA
analyses were highly consistent, with only slight topological
differences (Fig. 2). Both approaches supported ten MOTUs on
the eastern Canary Islands (see the results of the MOTU
assignment test). In the phylogenetic tree, T. impugnata and Theba
sp. 2 from Lanzarote formed a monophyletic group (Bayesian
posterior probability (BPP)=100%; bootstrap (BS)=95%) and
were sister to all other MOTUs (BPP=100%; BS=100%). The
monophyletic group of the remaining MOTUs (BPP=100%;
BS,50%) fell into two subgroups. One subgroup (BPP=100%;
BS=87%) comprised all populations of the Jandı ´a peninsula on
Fuerteventura with Theba cf. clausoinflata ‘‘Rock’’ sister to Theba sp.
5 ‘‘Sand’’. The other subgroup (BPP=100%; BS,50%) consisted
of six monophyletic lineages: T. geminata, Theba sp. 4, T. cf. arinagae,
T. grasseti, Theba sp. 1a and Theba sp. 1b. Population FU3 was split
with one individual belonging to Theba sp. 1a and the other to
Theba sp. 1b. Phylogenetic relationships in this subgroup were only
The AFLP data set consisted of 625 specimens, each scored for
1964 loci. Though the AFLP topology was different from that of
COI, it supported nearly the same MOTUs (Fig. 3). T. impugnata
and Theba sp. 2 formed a monophyletic group (BS=100%) and
were sister to all other MOTUs (BS=100%), as in the COI tree.
In the cluster comprising all remaining MOTUs (BS=90%), most
splits were only weakly supported with BS values ,50%. This
cluster comprised T. grasseti, T. geminata, Theba sp.1a, Theba sp. 1b,
Theba sp. 4, T. cf. arinagae, Theba sp. 5 ‘‘Sand’’ and Theba cf.
clausoinflata ‘‘Rock’’. In the AFLP tree, specimens of FU3 were
monophyletic and neither belonged to Theba sp. 1a nor to Theba sp.
1b. In contrast to COI, Theba sp.1b was paraphyletic as specimens
of FU7 formed an independent monophyletic lineage in the AFLP
phylogeny. In the Jandı ´a group, Theba cf. clausoinflata ‘‘Rock’’ was
not monophyletic. FU13, FU18, FU23, and FU25 formed a
separate lineage and were closely related to Theba sp. 5 ‘‘Sand’’.
Moreover, FU17 was placed within Theba sp. 5 ‘‘Sand’’ and did
not belong to Theba cf. clausoinflata ‘‘Rock’’ as suggested by COI.
Most of the basal nodes within the Jandı ´a subgroup had a BS value
The MOTU assignment test yielded 16 clusters, of which seven
(T. grasseti, T. geminata, Theba sp.1a, Theba sp. 1b, T. cf. arinagae,
Theba sp. 5 ‘‘Sand’’ and Theba cf. clausoinflata ‘‘Rock’’) fully agreed
with the a priori identified MOTUs of the phylogenetic
reconstruction of the COI sequences (Fig. 2). According to the
assignment test, Theba sp. 2 split into four clusters, but none of the
four clusters corresponded to any monophyletic subgroup of Theba
sp. 2 suggested by the COI or AFLP phylogeny (Figs. 2,
3).Therefore we conservatively treated Theba sp. 2 as one single
MOTU. Moreover, T. impugnata splits into three and Theba sp. 4
into two clusters. As in both cases additional clusters only
contained one specimen each, we refrained from over splitting
T. impugnata and Theba sp. 4 into several MOTUs.
In plots following a PCA of the ten landmarks with four
significant principal components explaining 87.65% of the total
variance, only the flat shelled but very variable T. grasseti and T.
impugnata – both overlapping extensively –, as well as FU25 could
be partially separated. The remaining globular-shelled MOTUs
overlapped considerably (Fig. 4). The results of a canonical
variates analysis (CVA) were similar (not shown). However, the
associated MANOVA was highly significant (Wilk’s lamb-
da=0.0183; df=176, 8545; p,0.0001) and in pairwise Ho-
telling’s comparisons (with four constraints) all MOTUs could be
distinguished from each other (p,0.0001 in all cases). FU23 and
FU25 had to be compared using Goodall’s F-test due to the
smaller sample sizes (F=7.52; df=16, 288; p,0.0001).
The F-test comparing size across all MOTUs was highly
significant (F=216; df=131, 4; p,0.0001). We preferred the F-
test over an ANOVA due to unequal variances. In Tukey’s
pairwise post-hoc tests, only T. cf. arinagae and FU23 as the
smallest and largest shelled samples, respectively, could be
distinguished from all other MOTUs. T. geminata and Theba sp. 2
as well as Theba sp. 1b and Theba sp. 4 could not be distinguished,
both pairs exhibiting parapatric distributions. Similarly, Theba sp.
5 ‘‘Sand’’, Theba cf. clausoinflata ‘‘Rock’’ and FU25 from the Jandı ´a
peninsula were not differentiated by size. In contrast, Theba sp. 1b
was significantly larger than its parapatric sister MOTU, Theba sp.
1a (Q=7.915; p,0.0001) (Fig. S1).
The assignment success based on shell shape ranged from
41.0% (Theba sp. 1a) to 95.2% (T. cf. arinagae). However, the
assignments were unambiguous and statistically significant in only
141 of the 961 (14.7%) cases, reflecting the high degree of
morphological similarity among the MOTUs (Table S3).
Among mixed samples, eight individuals of LZ23 were
measured and investigated genetically. Genetically, four snails
belonged to Theba sp. 2 and four to Theba sp. 1a. Morphologically,
the former were all assigned to Theba sp. 2, except one. Of the
latter, only two were correctly allocated. This indicates that even
on a small scale, assignment was ambiguous due to the very similar
Figure 2. RAxML tree based on partial COI sequences of Theba sampled on the Canary Islands. Bootstrap support (BS) values (1000
replicates) of the ML run are indicated below branches. Numbers above branches refer to Bayesian posterior probabilities (BPP) of the Bayesian
analysis (BA). &=BS support .50%, no BPP support; %=BS support ,50%, no BPP support;N=BS support and BPP .50%; #=BS support ,50%
and BPP .50%.
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morphologies of the MOTUs involved. LZ12 and LZ21 were
samples consisting of T. geminata and Theba sp. 2. In LZ12, 12 shells
were identified as T. geminata and 18 as Theba sp. 2. In LZ21, the
proportion was 21:8. In FU3, 13 shells were assigned to Theba sp.
1a and ten to Theba sp. 1b. As many specimens in these larger
samples were empty shells, these samples were excluded from
general analyses due the problems with molecular identification
and morphological assignment. According to COI, FU17
belonged to MOTU Theba cf. clausoinflata ‘‘Rock’’. In contrast,
AFLPs placed this sample in MOTU Theba sp. 5 ‘‘Sand’’.
Morphological assignment was again ambiguous with two shells
identified as ‘‘Rock’’ and three as ‘‘Sand’’. This sample also was
not included in the general analyses.
Population structure based on the AFLP data set
The Population graph of 34 Theba populations from Lanzarote,
Fuerteventura and Gran Canaria had 56 edges. It consisted of two
non-connected subgraphs (Fig. 5), indicating great genetic
differentiation between populations of Theba sp. 5 ‘‘Sand’’
(subgraph 1) and all remaining populations (subgraph 2).
Results of the Bayesian clustering analyses using Structure v2.3.2
were largely congruent with those of previous molecular analyses
of the present study. The mean likelihood L(K) increased from
K=1 to a maximum value at K=13 (2340259.27) and then
decreased to a minimum value at K=45 (24266627.79), whereas
the variance of L(K) between runs increased for larger Ks (Fig. S2).
The statistic DK described by Evanno et al.  showed multiple
peaks at K=2 (DK=935.36), K=3 (DK=550.49) and K=4
(DK=194.65). The highest mean likelihood value of L(K) at
K=13 was confirmed by a fourth peak of DK=20.26 (Fig. S2).
The graphical outputs of Structure (from the run at each K with the
highest likelihood) for K=2, 3, 4 and 13 are shown in Fig. 6. At
K=2, individuals of Theba sp. 2 and T. impugnata grouped into one
cluster and all remaining individuals into the other cluster. At
K=3, individuals of T. geminata were assigned to a separate third
cluster. At K=4, a fourth cluster comprised all individuals sampled
on the Jandı ´a peninsula. In the run that had the highest likelihood
at K=13, nine clusters corresponded to MOTUs identified by the
prior molecular analyses (above): Theba sp. 5 ‘‘Sand’’, T. cf.
clausoinflata ‘‘Rock’’, Theba sp. 4, Theba sp. 1a, Theba sp. 1b, T.
grasseti, T. geminata, T. impugnata and Theba sp. 2. Individuals of
LZ18 and of Theba sp. 2 of LZ23 were assigned to an additional
cluster, as were individuals of Theba sp. 1a of LZ23. Furthermore,
individuals of FU26 grouped into a separate cluster. Another
cluster did not correspond to any population or MOTU and
comprised two individuals of FU4 and one specimen of T.
Figure 3. AFLP neighbor-joining (NJ) tree of Canary Islands Theba based on Nei-Li distances. The AFLP data set consisted of 1964 loci.
Bootstrap support values (1000 replicates) are indicated below branches.
Figure 4. Principal component scatterplot (component 1 versus 2). The analysis was based on geometric morphometrics of ten shell
parameters. Convex hulls circumscribe the areas occupied by each MOTU.
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At K=13, most individuals were assigned to one cluster with
70% to 99% probability. Nevertheless, some individuals showed
high levels of admixture and had proportions much lower than
this. Individuals of the cluster T. impugnata were highly admixed
and showed proportions of up to 62% of the cluster Theba sp. 2.
Theba sp. 2 included populations (LZ16, LZ17, LZ21 and LZ20) in
which individuals had levels of admixture .49% with cluster
LZ18 & LZ23. The assignment of FU7 and FU5 to a single cluster
was not obvious. FU7 showed mixed ancestry with proportions of
the clusters Theba sp. 4 (24–48%) and Theba sp. 1b (24–75%).
Though individuals of the population FU5 were mainly assigned
to the cluster Theba sp. 1a, some individuals showed levels of
admixture of up to 68% of the cluster Theba sp. 1b. Mainly
assigned to Theba sp. 5 ‘‘Sand’’, FU17 were admixed and showed
proportions of the cluster T. grasseti. Corresponding to the COI
phylogeny, FU25, FU23, FU18 and FU13 were associated with T.
cf. clausoinflata ‘‘Rock’’, but additionally had proportions of the
clusters Theba sp. 5 ‘‘Sand’’ and T. grasseti. Both specimens of FU3
grouped into Theba sp. 1b, although one individual was admixed
and showed proportions of the clusters Theba sp. 1a and T.
geminata. Neither specimens of T. cf. arinagae nor those of T.
macandrewiana were assigned to a discrete genetic cluster. T. cf.
arinagae grouped with highest proportion into T. cf. clausoinflata
‘‘Rock’’, but it was highly admixed and showed proportions of the
clusters Theba sp. 4, T. grasseti and Theba sp. 1b. T. macandrewiana
was split with one individual assigned to the cluster T. grasseti and
the other to the cluster, which did not correspond to any
population or MOTU (cluster 9, see Fig. 6).
A non-hierarchical AMOVA of the whole data set indicated
great population differentiation (FST=0.24)  (Table S2).
Separate non-hierarchical analyses of Lanzarote and Fuerteven-
tura, respectively, showed that genetic variation among Lanzarote
populations (27%) was higher than that among Fuerteventura
populations (16%). A nested AMOVA of Lanzarote populations
indicated substantial genetic differentiation between populations of
T. geminata and populations of T. impugnata and Theba sp. 2
Figure 5. Population graph based on the AFLP data set. The graph represents the genetic covariance relationships among Theba populations
of the Canary Islands. Node sizes are proportional to within population genetic variability, whereas the edge lengths represent the among population
component of genetic variation.
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(FCT=0.28). On Fuerteventura, a hierarchical AMOVA showed
the highest among group variation between populations of T. cf.
clausoinflata ‘‘Rock’’ and Theba sp. 5 ‘‘Sand’’ (14.8%). Whereas
genetic differentiation between populations of Theba sp. 5 ‘‘Sand’’
was moderate (FST=0.05), it was low between populations of T. cf.
clausoinflata ‘‘Rock’’ (FST=0.04). A hierarchical AMOVA indicat-
ed moderate genetic differentiation between Jandı ´a populations
and populations of the main part of Fuerteventura (FCT=0.08).
Genetic divergence between populations of Theba sp. 1a and Theba
sp. 1b was low (FCT=0.04). A hierarchical AMOVA between
Lanzarote and Fuerteventura populations revealed that only 8.4%
of the variation was explained by between-group differences. The
variation among groups increased to 21.3%, by associating T.
geminata from Lanzarote with populations of Fuerteventura. The
AMOVA, structured according to the MOTUs suggested by the
phylogenetic analyses and the MOTU assignment test, attributed
19.2% of the global variation to differences among MOTUs, and
7.1% to differentiation among populations within MOTUs. All
fixation indiceswere significant.
AMOVA among populations of Theba sp. 2 and T. impugnata
revealed no significant genetic differentiation. In the light of the
results of the Population graphs and Structure analyses, we further
investigated genetic structure within Theba sp. 1a. LZ23 of
Lanzarote and FU26 of the Isle of Lobos (see Fig. 1) were each
compared with the other populations of Theba sp. 1a. The genetic
differentiation among populations of Theba sp. 1a was also non-
Figure 6. Results of the Structure analyses based on the AFLP data set. Structure graphical outputs for K=2, K=3, K=4, and K=13 are shown.
Each individual is represented by a vertical bar colored in proportion to their estimated ancestry within each cluster. X1=Cluster 8 comprising all
individuals of T. grasseti (GC3, GC4) and one individual of T. macandrewiana (SEL1); X2=Cluster 9 comprising one individual of T. macandrewiana
(SEL1) and two individuals of FU4; *=highly admixed populations, which also showed proportions of the cluster of the corresponding color;
m=mixed sample (i.e. samples are composed of two species).
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Isolation by distance
A Mantel test including all populations of Lanzarote and
Fuerteventura indicated evidence for IBD (Pearson correlation
coefficient r=0.41, p,0.001) (Fig. 7; Table S4). The correlation
between genetic and geographic distance increased, however, by
successively excluding populations of a) T. impugnata and Theba sp.
2 (r=0.82, p,0.001) and b) Theba sp. 5 ‘‘Sand’’ (r=0.84,
p,0.001) from the data set. Finally removing T. geminata induced
the highest correlation value (r=0.88, p,0.001) and indicated
strong patterns of IBD among populations of Theba sp. 1a, Theba
sp. 1b, Theba sp. 4 and T. cf. clausoinflata ‘‘Rock’’ on Fuerteventura.
A separate Mantel test of all populations from Lanzarote yielded
the lowest correlation value (r=0.27, p,0.001). Excluding T.
geminata from this data set, increased the correlation value to
BayeScan detected outlier loci in almost every population
comparison. The average number of outlier loci was highest in
MOTU pairs of Fuerteventura and lowest in those of Lanzarote
(Table 1). Consistent outlier loci were found in six MOTU pairs.
Theba sp. 4 and T. impugnata, however, were each represented by
only one population. Thus, consistent outlier loci detected in the
three MOTU pairs Theba sp. 4/Theba sp. 1b, T. impugnata/Theba
sp. 4, and T. geminata/Theba sp. 4 were not further considered, as
they were potentially linked to population-specific demographic
history, unrelated to ecological pressure. BayeScan identified three
consistent outlier loci (1199, 1488, and 1801) between populations
of Theba sp. 5 ‘‘Sand’’ and populations of T. cf. clausoinflata
‘‘Rock’’, indicating genotype-environment associations (GEAs) on
the Jandı ´a peninsula. Population comparisons within MOTU pair
Theba sp. 5 ‘‘Sand’’/Theba sp. 1b revealed one (1488) and within
MOTU pair Theba sp. 5 ‘‘Sand’’/Theba sp. 4 three consistent
outlier loci (788, 1488 and 1801). Though MOTU pair Theba sp. 5
‘‘Sand’’/Theba sp. 4 involved always the same population of Theba
sp. 4 in each comparison (as explained above), two of the three
consistent outlier loci were 1488 and 1801. These loci were
apparently linked to populations of Theba sp. 5 ‘‘Sand’’.
Currently, only two extant species of Theba are described from
Fuerteventura and Lanzarote - the flat-shelled T. impugnata and the
globular-shelled T. geminata [15–17]. In the present study, Theba
populations on Lanzarote and Fuerteventura displayed little
divergence in shell morphology. Apart from the flat-shelled T.
grasseti from Gran Canaria, a PCA based on ten shell parameters
largely separated only the flat-shelled T. impugnata (Lanzarote) and
snails from the Pico de la Zarza (FU25) from all remaining
globular-shelled snails (Fig. 4). Molecular data from these Theba
populations, however, showed extensive genetic differentiation
among globular-shelled snails of Theba on both islands. Aside from
the flat-shelled T. impugnata, molecular results supported two
endemic globular-shelled MOTUs on Lanzarote and five on
Fuerteventura (Figs. 2, 3; Table S2). Morphologically clearly
distinct snails from the Pico de la Zarza (FU25), however, were not
genetically differentiated from the globular-shelled group. Only in
the case of T. impugnata, did the distinct shell shape correspond to
its high genetic differentiation. In summary, we observed extensive
cryptic diversification among globular-shelled Theba populations
on both islands.
T. impugnata is restricted to the north of Lanzarote, whereas
Theba sp. 2 is found in the western parts of central and south
Lanzarote (Fig. 1; Table S1). Until now both forms have not been
found in sympatry. The AMOVA revealed that genetic differen-
tiation between T. impugnata and Theba sp. 2 was not significant
(Table S2). T. impugnata, however, is morphologically clearly
differentiated from Theba sp. 2 (Fig. 4) and Structure analyses
demonstrated allele frequency divergence between both MOTUs
(Fig. 6). Within the large range of Theba sp. 2, we observed a clear
differentiation between the most southern (LZ18 and LZ23) and
northern populations (LZ14 and LZ15). The populations in
between (LZ16, LZ17, LZ21 and LZ20) displayed mixed ancestry
suggesting gene flow among adjacent populations due to the lack
of reproductive isolation (Figs. 1 and 6). The discontinuous
distribution of genetic divergence and mixture is compatible with
Figure 7. Isolation by distance (IBD). A) Pairwise FSTvalues for all populations based on AFLP data versus geographical distances between all
sample sites in kilometers. B) Pairwise FSTvalues for all populations of Theba sp. 1a, Theba sp. 1b, Theba sp. 4 and T. cf.clausoinflata ‘‘Rock’’ based on
AFLP data versus geographical distances between corresponding sample sites in kilometers.
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documented historical (,500 years) volcanic activity on Lanzarote
 possibly eradicating snails in the central area of the island,
followed by allopatric differentiation of the most southern and
northern populations and secondary contact.
T. geminata is distributed in north and central Lanzarote, and its
southernmost populations occur sympatrically with either T.
impugnata or Theba sp. 2. There was no evidence of gene flow
(Fig. 6), suggesting that T. geminata is reproductively isolated from
both T. impugnata and Theba sp. 2. Due to the Pleistocene’s low sea-
level, Lanzarote and Fuerteventura were periodically connected by
land bridges, providing enhanced dispersal opportunities between
them [63–64]. We propose that Fuerteventura was colonized from
Lanzarote, with T. geminata closely related to MOTUs of
Fuerteventura (Figs. 2, 3 and 5; Table S2). This interpretation,
however, is tentative as phylogenetic relationships were not well
We found a clear pattern of isolation by distance (IBD) among
populations on Fuerteventura (Fig. 7; Table S4). Structure analyses
revealed that differentiation processes on the main part of
Fuerteventura were similar to those of Theba sp. 2 on Lanzarote.
Excluding populations with highly admixed proportions (FU3,
FU5 and FU7) (Fig. 6), the ranges of Theba sp. 4, Theba sp. 1b and
Theba sp. 1a are separated by mountain ranges possibly resulting
from the volcanic history of Fuerteventura [12,65]. In contact
zones (FU3, FU5 and FU7), however, the Structure analyses
suggested gene flow among adjacent populations. As with Theba sp.
2 on Lanzarote, this pattern of genetic differentiation is compatible
with allopatric divergence of Theba sp. 4, Theba sp. 1b and Theba sp.
1a, followed by hybridization on secondary contact due to a lack of
Jandı ´a populations were distinct from populations on the main
part of the island (Figs. 2, 3 and 6; Table S2), indicating that the
Isthmus of La Pared is probably a barrier to gene flow. The sand
Table 1. Outlier loci detection.
Average # of
outlier loci Consistent outlier loci
Jandı ´a, Theba sp. 5 ‘‘Sand’’ (3)vs.T. cf. clausoinflata ‘‘Rock’’ (4)7.921199, 1488, 1801
FuerteventuraTheba sp. 5 ‘‘Sand’’ (3)vs. Theba sp. 5 ‘‘Sand’’ (3)1.00–
T. cf. clausoinflata ‘‘Rock’’ (4) vs. T. cf. clausoinflata ‘‘Rock’’ (4) 1.00–
FuerteventuraTheba sp. 5 ‘‘Sand’’ (3) vs.Theba sp. 4 (1)10.33 788, 1488, 1801
Theba sp. 5 ‘‘Sand’’ (3)vs. Theba sp. 1a (5)4.07–
Theba sp. 5 ‘‘Sand’’ (3)vs. Theba sp. 1b (4)9.67 1488
T. cf. clausoinflata ‘‘Rock’’ (4)vs. Theba sp. 4 (1) 2.75–
T. cf. clausoinflata ‘‘Rock’’ (4) vs.Theba sp. 1a (5) 2.10–
T. cf. clausoinflata ‘‘Rock’’ (4) vs. Theba sp. 1b (4) 3.81–
Theba sp. 1a (5)vs. Theba sp. 1b (4) 1.25–
Theba sp. 4 (1) vs. Theba sp. 1a (5)2.60–
Theba sp. 4 (1)vs. Theba sp. 1b (4)7.25 1064, 1068, 1187
Theba sp. 1a (5)vs. Theba sp. 1a (5) 0.90–
Theba sp. 1b (4) vs. Theba sp. 1b (4)1.83–
Lanzarote Theba geminata (8) vs.Theba sp. 2 (5)0.80–
Theba geminata (8)vs. Theba impugnata (1) 0.88–
Theba sp. 2 (5)vs.Theba impugnata (1)1.00–
Theba sp. 2 (5) vs. Theba sp. 2 (5)1.00–
Theba geminata (8) vs. Theba geminata (8) 1.00–
Fuerteventura Theba sp. 2 (4)* vs.Theba sp. 1a (5)0.95–
LanzaroteTheba sp. 2 (4)* vs. Theba sp. 1b (4) 1.00–
Theba sp. 2 (4)* vs.Theba sp. 4 (1)1.75–
Theba impugnata (1)vs. Theba sp. 1a (5)1.00–
Theba impugnata (1) vs. Theba sp. 1b (4)0.75–
Theba impugnata (1) vs.Theba sp. 4 (1) 1.001064
Theba geminata (2)* vs. Theba sp. 1a (5) 0.90–
Theba geminata (2)* vs.Theba sp. 1b (4) 1.38–
Theba geminata (2)*vs.Theba sp. 4 (1)1.001064
Theba geminata (2)*vs.T. cf. clausoinflata ‘‘Rock’’ (4)0.86–
Theba geminata (2)*vs.Theba sp. 5 ‘‘Sand’’ (3)0.83–
Analyses were based on 1964 AFLP markers using BayeScan. It estimates a posterior probability for each locus being under selection. All loci with a posterior probability
over 95% were retained as outliers. For each MOTU pair, all possible pairwise comparisons between populations were tested.
() values in brackets indicate the number of populations of each MOTU.
*only a subset of populations of the corresponding MOTU was analyzed.
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dunes at the Isthmus of La Pared are a barrier to gene flow in
other organisms , which apparently leads to allopatric
divergence of species between the southern peninsula and the
main part of the island.
In all cases the BayeScan analyses did not found any consistent
outlier loci, suggesting that differentiation processes of Theba on
Fuerteventura and Lanzarote were probably driven mainly by
non-adaptive allopatric differentiation.
Speciation in non-adaptive radiations is often slow. It depends
on the accidental occurrence and fixation of different alleles in
allopatric populations in ecologically similar environments. Many
allopatric populations hybridize when they meet in secondary
contact zones [67–69]. In contrast, ecological speciation can be
rapid, developing reproductive isolation within a few thousands of
In contrast to the main part of Fuerteventura, the Jandı ´a
peninsula is characterized by high mountains and great habitat
diversity . Theba sp. 5 ‘‘Sand’’ and T. cf. clausoinflata ‘‘Rock’’
are parapatrically distributed on the Jandı ´a peninsula without any
obvious geographical barriers. Both formed stable genetic clusters
with great genetic differentiation from all other Fuerteventuran
MOTUs (Figs. 2, 3 and 6; Table S2). Whereas Theba sp. 5 ‘‘Sand’’
is restricted to sandy environments, T. cf. clausoinflata ‘‘Rock’’ is
exclusively found in rocky habitats. BayeScan analyses helped to
identify genetic loci that are more divergent between Theba sp. 5
‘‘Sand’’ and T. cf. clausoinflata ‘‘Rock’’ than expected under
neutrality, indicating genotype-environment associations (GEAs)
and possibly ecologically driven differentiation (Table 1). Since
both MOTUs are endemic to Jandı ´a, parapatrically distributed,
sister groups, and genetically differentiated, a sympatric/para-
patric speciation scenario involving divergent selection within
ecologically different environments seems plausible [18–19,72–
73]. This was also supported by the Population graph indicating
greatest genetic differentiation between Theba sp. 5 ‘‘Sand’’ and all
other MOTUs from Lanzarote and Fuerteventura (Fig. 5) mainly
restricted to compact-soil and rocky habitats.
Bierne et al.  argued, however, that endogenous (i.e.
environment-independent) genetic incompatibilities coinciding
with environmental boundaries rather than local adaptation are
often more likely to explain GEAs. The occurrence of outlier loci
would thus be an accidental by-product of demographic history
and present genetic isolation. This hypothesis would predict that
multiple independent comparisons across different ecological
environments would result in randomly distributed outlier loci
within genomes. As populations of Theba sp. 5 ‘‘Sand’’ are
discontinuously distributed on Jandı ´a and multiple comparisons
between ‘‘Sand’’ and ‘‘Rock’’ populations revealed a common set
of outlier loci, we considered this set as GEAs best explained by
ecological adaptation (Table 1). The presence of GEAs and strong
genetic differentiation, however, are not sufficient indications of a
sympatric/parapatric speciation scenario. Microallopatric specia-
tion might also play an important role in organisms with low
dispersal abilities like land snails [75–77]. Further evidence is
needed to test if Theba sp. 5 ‘‘Sand’’ and T. cf. clausoinflata ‘‘Rock’’
actually diverged in allopatry, parapatry or sympatry.
Though populations found on the top of Pico de la Zarza
(FU25) and the upper parts of Degollada de Vinamar (FU23) as
well as in rocky areas near the coast (FU13 and FU18) were
associated with T. cf. clausoinflata ‘‘Rock’’, they were admixed with
proportions of Theba sp. 5 ‘‘Sand’’ (Figs. 1 and 6). However, as
these populations were each represented by only two individuals,
we refrain from speculation on the reasons for this.
AFLP binary character matrix including
MOTU. The horizontal line represents the mean, box margins
are at the 25th and 75th percentiles, and bars extend to the 5th
and 95th percentiles.
Box plots of centroid size for each Theba
clusters in the AFLP data set. (A) K vs. mean L(K) with
standard deviation (SD) from 10 replicates for each K. (B) K vs. DK
(following Evanno et al. ).
Estimation criteria for the number of genetic
Summary of specimens used in the present
different hierarchies. Populations with fewer than 5 individ-
uals were excluded from analysis due to small sample size (see
Table S1). MOTUs corresponded to results of phylogenetic
analyses and the MOTU assignment test. Bold fixation indices
were statistically significant.
Summary of AMOVA with and without
assignment success. Theba MOTUs along rows, CVA groups
Canonical variates analysis (CVA) based
the analyses matrices of geographical distance (km) were compared
to genetic distances (FST).
Results of the simple Mantel test analyses. In
We thank Claudia Etzbauer for technical and laboratory support. Julia
Schwarzer and Fabian Herder gave valuable advices with regard to AFLP
technique. We would also like to thank Rodney Dyer for his help with
Population Graphs and Patrick Ku ¨ck for developing a perl script for
selecting AFLP markers automatically.
Conceived and designed the experiments: RH MH BM CG. Performed
the experiments: CG FG. Analyzed the data: CG FG. Wrote the paper:
CG BM. Carried out the field work: CG FG RH.
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