Podarcis hispanica

Kaliontzopoulou, Antigoni, Carretero, Miguel A. & Llorente, Gustavo A., 2012, Morphology of the Podarcis wall lizards (Squamata: Lacertidae) from the Iberian Peninsula and North Africa: patterns of variation in a putative cryptic species complex, Zoological Journal of the Linnean Society 164 (1), pp. 173-193 : 187-189

IS PODARCIS HISPANICA A SPECIES COMPLEX?

When describing phylogenetic variation in Iberian and North African Podarcis for the first time, Harris & Sá-Sousa (2002) suggested that P. hispanica is a species complex. Phylogenetic studies ever since have treated this group of lizards as such, uncovering high levels of cryptic diversity ( Pinho et al., 2006, 2007). However, the assumption that the Iberian and North African clade of Podarcis constitutes a complex of cryptic species has never been tested from a morphological perspective until now. Our investigation of a large number of external morphological characters in 15 of the 16 mitochondrial lineages present in this group paves the way for a formal evaluation of this question. However, the above question incorporates two aspects that should be distinguished, because they represent different biological issues: (1) are mtDNA lineages morphologically distinct, and can they be identified on the basis of morphological characters; and (2) do the evolutionary units corresponding to mtDNA lineages constitute species or not? The first half of the question is related to evaluating how ‘cryptic’ these lineages are ( Sáez & Lozano, 2005), whereas the second half corresponds to whether they are a ‘species complex’, and concerns the systematic decision of whether such units should be described as separate species ( Schlick-Steiner et al., 2007).

Considering the question of whether mtDNA lineages are cryptic, the answer is probably not. Our results indicate that although high levels of variability are present both at the population and lineage level, mtDNA lineages are statistically different considering body size, body shape, and pholidotic traits ( Tables 2 and 5). Whereas each of the examined data sets in isolation does not provide a good discrimination between lineages, the combined analysis using size, shape, and continuous pholidotic traits provided a much better classification ( Table 4). Moreover, when trying to ask questions that are more realistic in practical terms, such as whether we can discriminate one lineage from the rest, or whether we can discriminate between pairs of lineages, discrimination is visibly higher, exceeding 90% of correct classification (CVA; Appendix S4). In this sense, then, mtDNA lineages are morphologically different, and can be identified on the basis of morphological characters. However, the procedures to attain this objective are in practice very complicated: a very large number of individuals should be sampled to include the variation present in each group, and a large number of characters should be quantified (totalling 25 in this study). This makes the working scheme quite unrealistic for field identification of different lineages, but still some general lines can be drawn on the basis of characters most relevant for lineage differentiation. For example, the PHAM and PHGal lineages are distinguished from the rest by a remarkably smaller body size ( Fig. 2 ): PH 1A, PH 1B, and PHBat are visibly flatter (lower relative HH), and PVMA, PC, and PB are higher ( Fig. 2 ); PHJS has less femoral pores and supratemporal scales (FPN and STSN; Figs 4 and 5 ); PHTA, PHAM, and PHGal normally do not have a masseteric scale, whereas PB, PC, PVSCSp, and PVSSp normally have one (MASS; Fig. 6 ); and so on. Interestingly, our results indicate a much higher variability and morphological overlap between mtDNA lineages than that observed in another cryptic species complex of Podarcis investigated, in which different species could be effectively delimited and diagnosed on the basis of body size and pholidotic traits ( Lymberakis et al., 2008). The geological history of the two areas (Iberian Peninsula and Greece) may have played a role in determining this difference, as the geographical isolation between Greek taxa may have enhanced their morphological differentiation ( Lymberakis & Poulakakis, 2010), as is common for insular populations ( Meiri, 2007).

Considering the morphological identification of different lineages within Iberian and North African Podarcis , we should also note that one major type of external morphological characters has not been considered here. Colour variation is frequently used in lizard taxonomy, and can provide useful characters for both group delimitation and field identification. Both empirical observations (A. Kaliontzopoulou, M.A. Carretero & G.A. Llorente, pers. observ.) and the data available for some of the lineages examined here indicate that traits related to colour pattern could in fact be useful for identifying different groups (Sá- Sousa et al., 2002; Geniez et al., 2007). Moreover, colour characters are known to be used in partner recognition between overlapping mtDNA lineages in this group of lizards ( Barbosa et al., 2008). However, variation is again extreme (A. Kaliontzopoulou, M.A. Carretero & G.A. Llorente, pers. observ.), and the description of colour pattern using empirically constructed categorical variables would instead increase the already complex image. Novel methods for capturing colour pattern involving quantitative image analysis ( Anderson et al., 2003; Todd et al., 2005; Costa et al., 2009) could enhance the description of colour pattern variation in this group of lizards. Additionally, the implementation of techniques for quantifying colour characters invisible to the human eye, such as ultraviolet reflectance ( Font & Molina-Borja, 2004; Molina-Borja, Font & Mesa Avila, 2006; Font, Pérez i de Lanuza & Sampedro, 2009), could be very relevant for species delimitation, as they might function for intraspecific communication and may eventually be involved in reproductive isolation between different lineages. However, additional caution should be taken when examining colour traits, as these are known to vary ontogenetically, seasonally, and with reproductive stage ( Galán, 1995, 2000, 2008), and are frequently altered by specimen preservation in museum collections ( Geniez et al., 2007).

But, do the mtDNA lineages of the Iberian and North African group of Podarcis correspond to different species? Our results indicate that morphological investigation as traditionally approached cannot answer this question. Traditionally, marked morphological differences between groups of organisms have been used as indicators to define species and infer their phylogenetic relationships ( Wiens, 2007). Our results indicate that morphological variation is extensive both between and within lineages of Iberian and North African Podarcis , thereby entangling the detection of diagnostic characters ( Wiens & Servedio, 2000). Maybe in this sense we are at the limits of what the human eye can perceive, and what the human brain can register and describe ( Beheregaray & Caccone, 2007). Whereas most of the sensory information processed by the human brain is visual, other traits such as chemical or auditory might be more relevant for species delimitation if they are involved in mechanisms promoting reproductive isolation ( Sáez & Lozano, 2005; Bickford et al., 2007). Future research on the systematics of this group would benefit from focusing on the variation of such characters. For example, behavioural evidence already exists that chemical recognition mechanisms may be playing a crucial role in individual, intra-, and interspecific recognition, and may therefore be involved in reproductive isolation between the Iberian and North African lineages of Podarcis ( López & Martín, 2001; Barbosa et al., 2005, 2006; Martín & López, 2006). Additionally, the question of whether the mtDNA lineages of Iberian and North African Podarcis constitute different species should be approached through the investigation of the contact zones between them ( de Queiroz, 1998, 2005). The available evidence indicates that although signs of past introgression can be found, present hybridization is rare and does not affect the genetic and morphological cohesiveness of the species involved, at least as far as P. bocagei and P. carbonelli are concerned ( Pinho et al., 2009).

Put together, our results confirm that Iberian and North African Podarcis wall lizards are characterized by an extremely high level of morphological variation, but also indicate that such variation is not aleatory. Different mitochondrial lineages are morphologically distinct, although the high overlap of character ranges greatly increases the number of traits needed for correct identification. From a historical point of view, our analysis examining biometric and pholidotic traits routinely used in the past for species delimitation in lacertids ( Boulenger, 1920; Salvador, 1998; Arnold & Ovenden, 2002) indicates that when such traits are quantified in a large number of individuals, representing a large number of populations within each targeted group, the usefulness of these characters for direct species identification is overwhelmed by local variation. The recent development of molecular tools for studying phylogenetic relationships between organisms, as well as the increased capacity of sampling large areas and gaining access to large numbers of individuals, certainly change the way we explore and understand (morphological) diversity. This does not mean that morphological characters are useless for species delimitation, but rather that a shift of framework is necessary. In this sense, understanding how and why morphological traits evolve in closely related groups may shed more light on the evolutionary meaning and position of such groups than simple morphological comparisons between them.

Figures

Figure 2. Least-squares means for multivariate body size and size-corrected biometric variables in the different mitochondrial lineages examined. Only the characters most relevant for global biometric variation and group discrimination (after principle components analysis and canonical variates analysis, respectively; see Results) are presented. Error bars denote ± standard deviation. Females of each group are always presented first, denoted with a grey vertical bar, and males are in black. See Table 1 for group codes, Material and methods for variable abbreviations, and Figure 1 for symbols used to represent each lineage.

Figure 4. Least-squares means for continuous pholidotic traits in the different mitochondrial lineages examined. Vertical bars denote the observed range. Females of each group are always presented first, denoted with a grey vertical bar, and males are in black. See Table 1 for group codes, Material and methods for variable abbreviations, and Figure 1 for the symbols used to represent each lineage. Notice that no data are available for SCGN and SDLN in the PHJS lineage (Table 3).

Figure 5. Scatter plots of individuals scores (small symbols) and group means (big symbols) of the first three principal components of variation in continuous pholidotic traits for the mitochondrial lineages examined, considering males (top) and females (bottom) separately. The most highly (+, positively; -, negatively) contributing variables (Table 4) are indicated next to each axis. See Table 1 for group codes and Material and methods for variable abbreviations.

Figure 6. Observed frequencies for the different character states of the categorical pholidotic characters presenting sufficient variation across the sample examined, and multidimensional scaling (MDS) scatter plot of Manly’s overlap index between lineages. White always represents character state 0 and black represents character state 1, except for SL_SUBOC, in which white represents state 4 and black represents state 5. See Table 1 for group codes, Material and methods for variable abbreviations, and Figure 1 for the visual symbols used to represent each lineage.