Species delimitation is a central issue in all fields of biology, and it is closely tied to the species concept employed. Thousands of pages have been filled with debates about competing species concepts; for instance, Coyne &amp; Orr (2004) counted no less than 25 distinct species concepts. Among these are several phylogenetic species concepts. The paper by Martin et al. (2013) on box turtles is one of many recent publications that raise formerly recognized subspecies to the species level, based on molecular genetic evidence and favouring implicitly or explicitly a phylogenetic species concept as the theoretical foundation. We wish to underline that there is growing concern with respect to this approach (see Zachos et al. 2013 for mammals), and this concern guided our recent taxonomic update for turtles (Fritz &amp; Havaš 2013). Without intending to enter the debate on species concepts, we will briefly explain why we are not convinced by the conclusions of Martin et al. (2013). Using sequence data of two mitochondrial genes (cyt b, COI) and one nuclear locus (GAPD), Martin et al. (2013) studied the relationships of box turtles (Terrapene) and recognized, like previous authors, the species Terrapene coahuila, T. nelsoni and T. ornata. However, with respect to the fourth generally accepted species, T. carolina, they proposed that this taxon should be split into two distinct polytypic species, T. carolina (containing the subspecies T. c. carolina, T. c. bauri and T. c. major) and T. mexicana (containing the subspecies T. m. mexicana, T. m. triunguis and T. m. yucatana). The three taxa referred to T. mexicana are fully allopatric, with the two subspecies from Mexico (mexican

Peter Havaš

Uwe Fritz

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On the reclassification of Box Turtles (Terrapene): A response to Martin et al.

Accepted by S. Carranza: 9 Jun. 2014; published: 10 Jul. 2014 
Licensed under a Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0 
 295
ZOOTAXA
ISSN 1175-5326  (print edition)
ISSN 1175-5334 (online edition) Copyright © 2014 Magnolia Press
Zootaxa 3835 (2): 295–298 
www.mapress.com/zootaxa/ Correspondence
http://dx.doi.org/10.11646/zootaxa.3826.2.10
http://zoobank.org/urn:lsid:zoobank.org:pub:8B2EFB73-DAE0-49C0-931A-6FCAC12CE8AC
On the reclassification of Box Turtles (Terrapene): A response to Martin et al. 
(2014)
UWE FRITZ
1 & PETER HAVAŠ
2
1Museum of Zoology, Senckenberg Dresden, A. B. Meyer Building, 01109 Dresden, Germany. E-mail: uwe.fritz@senckenberg.de 
2Titogradská 18, 040 23 Košice, Slovak Republic. E-mail: peter.havas@seznam.cz
Species delimitation is a central issue in all fields of biology, and it is closely tied to the species concept employed. 
Thousands of pages have been filled with debates about competing species concepts; for instance, Coyne & Orr (2004) 
counted no less than 25 distinct species concepts. Among these are several phylogenetic species concepts. The paper by 
Martin et al. (2013) on box turtles is one of many recent publications that raise formerly recognized subspecies to the 
species level, based on molecular genetic evidence and favouring implicitly or explicitly a phylogenetic species concept 
as the theoretical foundation. We wish to underline that there is growing concern with respect to this approach (see 
Zachos et al. 2013 for mammals), and this concern guided our recent taxonomic update for turtles (Fritz & Havaš 2013). 
Without intending to enter the debate on species concepts, we will briefly explain why we are not convinced by the 
conclusions of Martin et al. (2013).
Using sequence data of two mitochondrial genes (cyt b, COI) and one nuclear locus (GAPD), Martin et al. (2013) 
studied the relationships of box turtles (Terrapene) and recognized, like previous authors, the species Terrapene 
coahuila, T. nelsoni and T. ornata. However, with respect to the fourth generally accepted species, T. carolina, they 
proposed that this taxon should be split into two distinct polytypic species, T. carolina (containing the subspecies T. c. 
carolina, T. c. bauri and T. c. major) and T. mexicana (containing the subspecies T. m. mexicana, T. m. triunguis and T. m. 
yucatana). The three taxa referred to T. mexicana are fully allopatric, with the two subspecies from Mexico (mexicana
and  yucatana) occurring in completely isolated distribution ranges (Smith & Smith 1980; Ernst & Lovich 2009). 
However, in the southern USA, triunguis intergrades widely with other subspecies of T. carolina (Carr 1952; Ernst & 
Lovich 2009), a fact recently corroborated by microsatellite data together with evidence from morphology and mtDNA 
sequences (Butler et al. 2011). Based on microsatellite loci, Butler et al. (2011) found no population structuring in the 
contact zone, suggestive of a panmictic population with complete genetic admixture of the involved taxa. Furthermore, 
using extensive additional analyses of morphology and the rapidly evolving mitochondrial D-loop, Butler et al. (2011) 
concluded that “T. c. major is not a distinct evolutionary lineage but, instead, a mixture of extant taxa plus the extinct 
[subspecies] T. c. putnami.” In contrast, Martin et al. (2013) treated T. c. major as a valid subspecies, without discussing 
these obviously contradictory results. 
Martin et al. (2013) used their cyt b and GAPD data for phylogenetic inference. The COI sequences were used for a 
barcoding approach, which was inconclusive. It showed that several subspecies of T. carolina, including subspecies 
which are accepted as conspecific by Martin et al. (2013), differ by divergence values which exceed the universal 
barcoding threshold for distinct species (2%). This is in line with a recent review article on DNA barcoding using turtles 
as a case study, which pointed out that “no particular level of divergence can serve to identify species” (Shen et al. 2013), 
or in other words, whenever possible additional evidence is needed, and barcoding thresholds to be used as critical values 
for species delimitation need to be adjusted individually for smaller taxonomic groups (e.g., Fritz et al. 2012; Vargas-
Ramírez et al. 2012).
Based on cyt b and GAPD sequences, Martin et al. (2013) discovered within T. carolina sensu lato two major clades. 
In the mitochondrial tree (Fig. 1: top left), one well-supported major clade was comprised of sequences of T. c. mexicana, 
T. c. triunguis and T. c. yucatana; and the other major clade contained the sequences of T. c. carolina, T. c. bauri and of 
what Martin et al. (2013) called T. c. major. However, mitochondrial sequences of “T. c. major” were paraphyletic with 
respect to the allopatrically distributed T. coahuila, a taxon which is accepted as a species distinct from T. carolina since 
its description in 1944 (Schmidt & Owens 1944; Smith & Smith 1980; Ernst et al. 2000; Dodd 2001; Butler et al. 2011; 
Martin et al. 2013), and support values for monophyly of the second major clade were low. Support for the two major 
nuclear clades was also weak (Fig. 1: top right). Moreover, these two nuclear clades conflicted with the species FRITZ & HAVAŠ 296  ·  Zootaxa 3835 (2)  © 2014 Magnolia Press
delimitation proposed by Martin et al. (2013) in that sequences of T. c. carolina, T. c. bauri, T. c. major and T. c. triunguis
occurred in both clades. Consequently, we see no convincing phylogenetic evidence for splitting T. carolina sensu lato 
into two distinct species, regardless of the species concept applied.
Our conclusion is supported by the alternate phylogeny reported by Spinks et al. (2009), a study not mentioned by 
Martin et al. (2013, 2014). Using sequences of the cyt b gene and seven nuclear loci, Spinks et al. (2009) found T. c. 
bauri clustering with T. c. triunguis (Fig. 1: bottom). 
Like Martin et al. (2013), Spinks et al. (2009) found T. carolina to be paraphyletic with respect to T. coahuila. 
However, for the rapidly evolving D-loop Butler et al. (2011) found T. carolina not to be paraphyletic with respect to T. 
coahuila. Yet, they reported nearly identical haplotypes of a T. carolina from Escambia County (Florida) and T. coahuila, 
but these haplotypes were highly distinct from all other haplotypes of T. carolina. This confusing situation demands 
further research, and this issue is beyond the current discussion. 
FIGURE 1. Phylogenetic trees for Terrapene carolina and T. coahuila. Top: Trees based on Maximum Likelihood and 
Bayesian analyses, redrawn from Martin et al. (2013). Shown are all support values provided in the original publication 
(bootstrap values and posterior probabilities). Note the weak support values (red) for the clades within T. carolina, the 
paraphyly of T. carolina with respect to T. coahuila (red) and the shared taxa in the nuclear tree (blue). The missing 
support values on the basal nodes of the nuclear tree suggest weak support. Bottom: Maximum Likelihood trees redrawn 
from Spinks et al. (2009). Support values are Bayesian posterior probabilities, ML and MP bootstrap values. Asterisks 
indicate support values ≥ 0.95/90/90. Note the placement of T. c. bauri (red arrow) conflicting with the results of Martin 
et al. (2013). Zootaxa 3835 (2)  © 2014 Magnolia Press  ·  297 RESPONSE TO MARTIN ET AL. (2014)
To justify their species delimitation, Martin et al. (2014) now argue that many species of animals may hybridize, and 
hence that intergradation among T. carolina subspecies does not invalidate their splitting of T. carolina in two distinct 
species. Martin et al. (2014) cite, among others, a paper reporting that up to 14% of box turtles from eastern Texas are 
hybrids between T. carolina and T. ornata. However, if 14% are hybrids, 86% of the concerned populations must be pure, 
and this is evidence for the maintenance of largely distinct gene pools in sympatry. By contrast, T. carolina forms 
panmictic populations whenever different subspecies meet (Butler et al. 2011), and this is fundamentally different from 
the situation in T. carolina and T. ornata. Consequently, the recognition of T. mexicana as a species distinct from T. 
carolina, as proposed by Martin et al. (2013, 2014), is unwarranted.
We are not disputing that distinct species may hybridize, and there are many cases known, especially in turtles and 
tortoises, as well as in other taxa (e.g., Kraus et al. 2012). However, interspecific hybridization is different from 
intergradation among subspecies. The example of T. carolina and T. ornata shows that distinct species are capable of 
maintaining largely discrete gene pools, allowing them to occur together in widely overlapping distribution ranges. The 
region of sympatry for T.  carolina and T. ornata corresponds to a vast area including the eastern parts of Texas, 
Oklahoma and Kansas, most of Missouri, western Arkansas, western Louisiana, and parts of Illinois and Indiana (Ernst 
& Lovich 2009). In contrast, the genetic and morphological distinctness of the subspecies of T. carolina, including T. c. 
triunguis, dissolves in their contact zone (Butler et al. 2011), which constitutes a true genetic melting pot. As a legacy of 
the distinct subspecies, only the deeply divergent mtDNA lineages remain, which are inherited through the maternal line 
(without recombination). A similar case of genetic admixture is known for subspecies of spur-thighed tortoises (Testudo 
graeca) in the Caucasus region (Mashkaryan et al. 2013).
We respect the conservation-oriented motives of Martin and coauthors for their proposal to revise the taxonomy of 
Terrapene carolina. Martin et al. (2014) argue “because the Terrapene are of conservation concern throughout their 
range […], and because many conservation efforts are species-based and tend to ignore subspecies, it is imperative that 
their classification be correctly resolved”. However, we are also convinced that only well-founded taxonomic decisions 
serve conservation and science (see also Karl & Bowen 1999; Zachos et al. 2013), and will therefore continue to treat T. 
c. mexicana, T. c. triunguis and T. c. yucatana as subspecies of Terrapene carolina.
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