Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource

Recent reviews have illustrated how rapid adaptive evolution is common and may be considered the rule rather than the exception in some cases (1, 2). Experimental introductions of populations in novel environments have provided some of the strongest evidence for natural selection and adaptive divergence on ecological time scales (3–6). However, little is known about the degree to which the observed changes in morphology may affect the population structure and behavioral ecology of organisms through the mediating effects of whole-organism performance (7, 8). Consequently, our understanding of how rapid phenotypic changes affect ecological processes at the population level is limited (2, 9). Moreover, despite the fact that microevolutionary responses to environmental changes have been well documented, the unpredictability and reversibility of changes of morphological traits in fluctuating environments (10, 11) have raised questions regarding how these microscale changes can lead to the emergence of novel structures as seen on macroevolutionary scales (2).

Here we address these issues by examining the outcome of a remarkable 36-year experimental introduction with the lizard Podarcis sicula. In 1971 five adult pairs of this species were moved from the small islet of Pod Kopište (0.09 km²) to the nearby Pod Mrčaru (0.03 km²) by Nevo and coworkers (12). Both islets lie in the middle of the South Adriatic Sea near the larger island of Lastovo and belong to Croatia. Although the islet of Pod Mrčaru was originally inhabited by another lacertid lizard species (Podarcis melisellensis), repeated visits (twice yearly over the past three years, beginning in 2004) show that this species has become extinct on Pod Mrčaru. Genetic mitochondrial DNA analyses indicate that the lizards currently on Pod Mrčaru are indeed P. sicula and are genetically indistinguishable from lizards from the source population [supporting information (SI) Fig. 5].

Morphometric data describing head size and shape show that both males and females of the two populations differ significantly in head morphology [MANOVA; males: Wilks's λ = 0.463, F_9,115 = 14.81, P < 0.001; females: Wilks's λ = 0.425, F_9,123 = 18.45, P < 0.001 (Table 1 and Fig. 1)] with lizards on Pod Mrčaru having longer, wider, and taller heads than lizards on Pod Kopište (Table 1 and Fig. 1). Differences between populations are not merely the result of differences in overall size but represent distinct changes in head shape [MANCOVA with SVL as covariate; males: slopes, Wilks's λ = 0.918, F_6,131 = 1.96, P = 0.08; intercepts, Wilks's λ = 0.387, F_6,132 = 34.88, P < 0.001; females: slopes, Wilks's λ = 0.983, F_8,122 = 0.25, P = 0.98; intercepts, Wilks's λ = 0.754, F_8,123 = 5.02, P < 0.001; juveniles: slopes, Wilks's λ = 0.969, F_6,39 = 0.21, P = 0.97; intercepts, Wilks's λ = 0.498, F_6,40 = 6.72, P < 0.001 (Table 1 and Fig. 1)].

Differences in head size and shape also translate into significant differences in bite force between populations (males: F_1,44 = 4.93, P = 0.03; females: F_1,38 = 16.94, P < 0.01). Whereas the difference in bite force is the result of overall head size differences in females (ANCOVA; slopes, F_1,36 = 0.02, P = 0.91; intercepts, F_1,37 = 1.55, P = 0.22), in males size variation does not explain the difference in bite force (ANCOVA; slopes, F_1,42 = 0.25, P = 0.62; intercepts, F_1,43 = 18.42, P < 0.01).

Our data show that P. sicula lizards consume more plant material on Pod Mrčaru compared with the ancestral population on Pod Kopište. Analysis of stomach contents shows marked differences in diet between populations in both spring (F_1,204 = 22.9, P < 0.01) and summer (F_1,74 = 103.13, P < 0.01) but no differences in diet between sexes in either population (F_1,202 = 1.36, P = 0.24). Seasonal differences in diet were significant in lizards from the introduced population (Pod Mrčaru, F_1,184 = 30.31, P < 0.01) with plants composing between 34% (spring) and 61% (summer) of the total volume of the food eaten (Fig. 2). In contrast, plant consumption was low (7% to 4%) and did not differ seasonally for lizards from the source population (Pod Kopište, F_1,94 = 0.33, P = 0.57). Moreover, ≈50% of the plant matter eaten year round by lizards from Pod Mrčaru consists of items with high cellulose content such as leaves and stems (Fig. 3).

This shift to a predominantly plant-based diet has resulted in the dramatic evolution of intestinal morphology. Morphological analysis of preserved specimens shows the presence of cecal valves (Fig. 4) in all individuals, including a hatchling (26.4-mm snout-vent length, umbilical scar present) and a very young juvenile (33.11-mm snout-vent length) examined from Pod Mrčaru. These valves are similar in overall appearance and structure to those found in herbivorous lacertid, agamid, and iguanid lizards (13, 14) and are not found in other populations of P. sicula (13) or in P. melisellensis. Cecal valves slow down food passage and provide for fermenting chambers, allowing commensal microorganisms to convert cellulose to volatile fatty acids (15, 16). Indeed, in the lizards from Pod Mrčaru, nematodes were common in the hindgut but absent from individuals from Pod Kopište. The fact that <1% of all currently known species of squamates have cecal valves (13, 14) illustrates the unusual nature of these structures in this population. The evolution of these structures has likely gone hand in hand with a novel association between P. sicula on Pod Mrčaru and a set of microorganisms assuring the digestion of cellulose as is suggested by the presence of nematodes in the hindgut of individuals from Pod Mrčaru.

Our data show that in only 36 years (≈30 generations) the experimental introduction of a small propagule of lizards (five males and five females) into a novel environment has resulted in large differences in external morphology with high phenotypic divergence rates (17) up to 8,593 darwins or 0.049 haldanes [Table 1; note, however, that these are synchronic rates (1) and assume no additional colonization of the island by P. sicula]. Moreover, the invasion of a novel environment has resulted in the evolution of a novel phenotypic character that is rarely observed in lizards and that cannot be quantified by such metrics. More importantly, the observed morphological changes appear adaptive because they result in an increase in bite performance in both sexes. Because plants are tough, fibrous materials, high bite forces may allow lizards to crop smaller pieces from larger plants (13, 18) and thus may help the breakdown of the indigestible cell walls (19, 20). Previous data show that lizards that include plant matter into their diet do indeed have higher bite forces (13, 18). Interestingly, phenotypic divergence rates are higher for females (the sex with the smallest heads and lowest bite forces) than males, suggesting that selection for high bite forces is directly related to the inclusion of tough and fibrous items into the diet. Additionally, functional components of the jaw system related to jaw opening (e.g., the inlever for jaw opening) show much lower divergence rates, again suggesting that morphological changes are specifically associated with the ability to bite hard and the increased consumption of plant matter (Table 1).

The relatively large fraction of leaves included into the diet of lizards in the introduced population of Pod Mrčaru has apparently also resulted in the evolution of cecal valves, a structure previously unreported for this species and rare in this family and scleroglossan lizards in general (13, 14, 18). Our data also add to the growing number of studies suggesting that the inclusion of plant matter into the diet of small temperate lizards may be more common than previously thought (21, 22). Moreover, our data show not only rapid, directional changes in quantitative phenotypic traits related to the inclusion of plant matter into the diet, but also the evolution of novel morphological structures on extremely short time scales. Although the presence of cecal valves and large heads in hatchlings and juveniles suggests a genetic basis for these differences, further studies investigating the potential role of phenotypic plasticity and/or maternal effects in the divergence between populations are needed.

The inclusion of plant matter into diet may have had profound effects on the population structure as well. Because of the larger food base available and the increase in the predictability of the food source, lizard densities on Pod Mrčaru are much greater (0.01 versus 0.05 lizards per trap per hour, caught in unbaited traps, on Pod Kopiste and Pod Mrčaru, respectively). This, in turn, likely affected the social structure, and lizards on Pod Mrčaru do no longer appear to defend territories. Moreover, changes in foraging style (browsing versus active pursuit of mobile prey) and social structure may also have resulted in the dramatic changes in limb proportions and maximal sprint speed previously documented for this population (23). Thus, our data show how rapid phenotypic changes may affect population structure and dynamics through their effect on behavioral ecology and life history of animals. They also show that rapid evolution can result in changes in both qualitative and quantitative characters.