Island colonisation and the evolutionary rates of body size in insular neonate snakes

Abstract

Island colonisation by animal populations is often associated with dramatic shifts in body size. However, little is known about the rates at which these evolutionary shifts occur, under what precise selective pressures and the putative role played by adaptive plasticity on driving such changes. Isolation time played a significant role in the evolution of body size in island Tiger snake populations, where adaptive phenotypic plasticity followed by genetic assimilation fine-tuned neonate body and head size (hence swallowing performance) to prey size. Here I show that in long isolated islands (46000 years old) and mainland populations, neonate body mass and snout-vent length are tightly correlated with the average prey body mass available at each site. Regression line equations were used to calculate body size values to match prey size in four recently isolated populations of Tiger snakes. Rates of evolution in body mass and snout-vent length, calculated for seven island snake populations, were significantly correlated with isolation time. Finally, rates of evolution in body mass per generation were significantly correlated with levels of plasticity in head growth rates. This study shows that body size evolution occurs at a faster pace in recently isolated populations and suggests that the level of adaptive plasticity for swallowing abilities may correlate with rates of body mass evolution. I hypothesise that, in the early stages of colonisation, adaptive plasticity and directional selection may combine and generate accelerated evolution towards an 'optimal' phenotype.

Convergence in reduced body size, head size, and blood glucose in three island reptiles

“…The gigantic island scrub-jay fossils date back to 1 million years on the islands (McCormack, Heled, Delaney, Peterson, & Knowles, 2011), but there is evidence of more recent adaptive divergence in beak morphology within island populations associated with the postpleistocene shift from coniferous to oak forest (Langin et al, 2015). Similar to that morphological evolution on other island systems has been shown to occur relatively quickly after colonization (Aubret, 2015;Vartanyan, Garutt, & Sher, 1993). This study of morphological evolution on both the Channel Islands and other islands worldwide indicate that the timeframes estimated for reptile colonization in our system would allow for ample opportunity for evolutionary divergence in body size from mainland populations.…”

Ecological specialization and morphological diversification in Greater Antillean boas

“…; Keogh et al. ; Boback ; Natusch and Lyons ; Aubret ), though some plasticity in these characteristics is also likely (Queral‐Regil and King ; Madsen and Shine ; but see Schuett et al. ).…”

“…Snakes exploit diverse island habitats and prey resources (Rodríguez-Robles and Greene 1996;Boback 2006;Boback and Carpenter 2007) and exhibit a wide variety of morphological and behavioral adaptations to them, even resulting in substantial differentiation among closely related species (Vitt and Vangilder 1983;Guyer and Donnelly 1990;Cadle and Greene 1993;Lillywhite and Henderson 1993;França et al 2008). Head shape and body size in snakes are thought to contribute to a greater degree of trophic specialization than is typical of other reptilian groups (Savitzky 1983;Voris and Voris 1983;Henderson et al 1988;Martins et al 2002;Vincent et al 2004;Hampton 2011), and both aspects of morphology can evolve rapidly on islands in response to divergent substrate preference and prey size (Arnold 1993;Madsen and Shine 1993;Rodríguez-Robles and Greene 1996;Forsman and Shine 1997;Vincent et al 2004;Keogh et al 2005;Boback 2006;Natusch and Lyons 2012;Aubret 2015), though some plasticity in these characteristics is also likely (Queral-Regil and King 1998;Madsen and Shine 2000; but see Schuett et al 2005).…”

Inconsistent patterns of body size evolution in co‐occurring island reptiles

“…Anderson and Handley (2002) suggested that, where over-water dispersal is unlikely (as in the case of Aegean Sea reptiles; Foufopoulos & Ives, 1999), body sizes on close and far islands would not differ. However, accelerated trait evolution on recently isolated islands has also been suggested (Aubret, 2015). However, accelerated trait evolution on recently isolated islands has also been suggested (Aubret, 2015).…”