Abstract
Populations of vertebrate species introduced onto islands regularly develop similar phenotypic changes, e.g., larger or smaller body size, shortened limbs, duller coats, as well as behavioural changes such as increased tameness and reduced flight-initiation distance. These changes overlap in part with those associated with the ‘domestication syndrome’, especially tameness and changes in coat patterns, and might indicate a similar neural crest involvement in the concurrent development of multiple phenotypic traits. Here I examine long-term data on free-living populations of wild Polynesian rats from seven mainland countries and 117 islands (n = 3,034), covering the species’ native and introduced range. Mainland populations showed no aberrant coat patterns, with the exception of one albino, whereas aberrant coat patterns were found in 12 island populations. Observed coat colour polymorphisms consisted of leucistic (including singular white patches), melanistic (darkly pigmented) and piebald (mixed) coat patterns. After isolation for at least seven centuries, wild Polynesian rat populations on islands seem to exhibit a trend towards a higher incidence of aberrant coat patterns. These phenotypic changes are here explained as a neutral, non-adaptive process, likely part of the ‘domestication syndrome’ (via the commensal pathway of domestication), in combination with genetic drift, little or no gene flow between the islands and/or the mainland and a relaxed selection (as a result of the weakening or removal of competitor/predator pressure) under commensality.
Highlights
Evolution on islands has often led to increased phenotypic variation and phenotypic novelty not observed in closely related mainland forms, inspiring studies on evolution ever since Alfred Wallace wrote his influential book (Wallace, 1880)
The perhaps most obvious phenotypic variation is observed in body mass, especially in island endemic lineages of the Pleistocene epoch, when rabbit-sized rats and pony-sized elephants and hippos were common elements of island faunas (Van der Geer et al, 2010)
E.g., lizards, show trait shifts that are consistent with the ‘island syndrome’, such as a higher degree of melanism (Runemark et al, 2010), increased tameness (Cooper Jr & Pérez-Mellado, 2012), shorter flight initiation distance and reduced sprint speed (Vervust, Grab & Van Damme, 2007), smaller clutch sizes (Huang, 2007) and larger bodies (Vervust, Grab & Van Damme, 2007; Pafilis et al, 2009)
Summary
Evolution on islands has often led to increased phenotypic variation and phenotypic novelty not observed in closely related mainland forms, inspiring studies on evolution ever since Alfred Wallace wrote his influential book (Wallace, 1880). The perhaps most obvious phenotypic variation is observed in body mass, especially in island endemic lineages of the Pleistocene epoch, when rabbit-sized rats and pony-sized elephants and hippos were common elements of island faunas (Van der Geer et al, 2010). E.g., lizards, show trait shifts that are consistent with the ‘island syndrome’, such as a higher degree of melanism (darkly pigmented skins) (Runemark et al, 2010), increased tameness (Cooper Jr & Pérez-Mellado, 2012), shorter flight initiation distance and reduced sprint speed (Vervust, Grab & Van Damme, 2007), smaller clutch sizes (Huang, 2007) and larger bodies (Vervust, Grab & Van Damme, 2007; Pafilis et al, 2009). Insular birds may have smaller clutch sizes (Cody, 1971), dull colours (Omland, 1997), increased tameness (Blondel, 2000), or may become flightless (Roff, 1994)
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