Does relaxed predation drive phenotypic divergence among insular populations?

Abstract

The evolution of striking phenotypes on islands is a well-known phenomenon, and there has been a long-standing debate on the patterns of body size evolution on islands. The ecological causes driving divergence in insular populations are, however, poorly understood. Reduced predator fauna is expected to lower escape propensity, increase body size and relax selection for crypsis in small-bodied, insular prey species. Here, we investigated whether escape behaviour, body size and dorsal coloration have diverged as predicted under predation release in spatially replicated islet and mainland populations of the lizard species Podarcis gaigeae. We show that islet lizards escape approaching observers at shorter distances and are larger than mainland lizards. Additionally, we found evidence for larger between-population variation in body size among the islet populations than mainland populations. Moreover, islet populations are significantly more divergent in dorsal coloration and match their respective habitats poorer than mainland lizards. These results strongly suggest that predation release on islets has driven population divergence in phenotypic and behavioural traits and that selective release has affected both trait means and variances. Relaxed predation pressure is therefore likely to be one of the major ecological factors driving body size divergence on these islands.

Ecological explanations to island gigantism: dietary niche divergence, predation, and size in an endemic lizard

“…Second, the sizes of most closely related species (e.g., Podarcis milensis and P. melisellensis, with maximum reported SVLs of 6.5 and 7.0 cm, respectively; Arnold and Ovenden 2002), are smaller than or similar to the largest mainland Skyros wall lizards (with SVLs of ;7.0 cm), whereas the largest islet lizards have SVLs of up to 9.6 cm. We have also confirmed that P ST for size is not significantly correlated with F ST (see e.g., Runemark et al 2014), and hence we can rule out that size differences between populations are a by-product of relatedness structure. Another explanation could be differences in relative contribution of age classes in the samples.…”

“…Both best models (diet only and diet plus predation, respectively) include diet, and hence we can conclude that this is an important factor. Based on the fact that predation is significant in the models (Table 2), that the diet-only model did not explain data significantly better, as well as on our evidence from a previous study (Runemark et al 2014), we suggest that predation and diet both are important components for the development of island gigantism. Reduced predation pressure is known to allow organisms to obtain larger body sizes (Blanckenhorn 2000, Herczeg et al 2009) and reduced predation can also mitigate selection for early maturation, with the result that organisms attain larger adult body sizes (Rodd and Reznick 1997).…”

“…In conclusion dietary niche is a major contributor to divergence in body size in the Skyros wall lizard, and predation probably also contributes to the body size divergence in this system (see also Runemark et al 2014). Although there is ample prior evidence suggesting that diet is a driver of divergence, it has mostly been studied in settings where a colonizing species diverges within a site to reduce competition for scarce food resources (Rundle et al 2000, Barluenga et al 2006, Grant and Grant 2008; but see Aubret andShine 2009, Aubret 2012 for exceptions).…”

The evolution of antipredator behaviour following relaxed and reversed selection in Alaskan threespine stickleback fish

“…The loss or reduction of predation is one of the most common contexts in which the consequences of relaxed selection has been evaluated (Berger, Swenson, & Persson, 2001; Bleakley, Martell, & Brodie, 2006; Blumstein & Daniel, 2002; Blumstein, Daniel, Griffin, & Evans, 2000; Fullard & Ratcliffe, 2006; Fullard, Ratcliffe, & Soutar, 2004; Runemark, Brydegaard, & Svensson, 2014). Variation in predation among prey populations is relatively easy to identify (particularly at the level of presence or absence of predators; but see Reimchen, 1994), as are adaptations whose primary function is to avoid predation.…”

Impact of sexually antagonistic genital morphologies on female reproduction and wild population demography

“…Reduced predator fauna in islands may influence the ecology and evolution of prey species (Runemark et al. ), but predator dropout expected in islands cannot explain observed decreases in island population size. Little is known about variation in predator fauna in our study area.…”