Did postglacial sea-level changes initiate the evolutionary divergence of a Tasmanian endemic raptor from its mainland relative?

Abstract

Populations on continental islands are often distinguishable from mainland conspecifics with respect to body size, appearance, behaviour or life history, and this is often congruent with genetic patterns. It is commonly assumed that such differences developed following the complete isolation of populations by sea-level rise following the Last Glacial Maximum (LGM). However, population divergence may predate the LGM, or marine dispersal and colonization of islands may have occurred more recently; in both cases, populations may have also diverged despite ongoing gene flow. Here, we test these alternative hypotheses for the divergence between wedge-tailed eagles from mainland Australia (Aquila audax audax) and the threatened Tasmanian subspecies (Aquila audax fleayi), based on variation at 20 microsatellite loci and mtDNA. Coalescent analyses indicate that population divergence appreciably postdates the severance of terrestrial habitat continuity and occurred without any subsequent gene flow. We infer a recent colonization of Tasmania by marine dispersal and cannot discount founder effects as the cause of differences in body size and life history. We call into question the general assumption of post-LGM marine transgression as the initiator of divergence of terrestrial lineages on continental islands and adjacent mainland, and highlight the range of alternative scenarios that should be considered.

Isolation, marine transgression and translocation of the bare‐nosed wombat (Vombatus ursinus)

“…Lowered genetic diversity has also been observed when compared to mainland lineages (platypus, O. anatinus ; Furlan et al, ), which is a pattern commonly observed in island populations (Frankham, ). However, marine barriers have not influenced genetic structure for all species, such as the grey kangaroo, Macropus giganteus (Zenger, Eldridge, & Cooper, ), wedge‐tailed eagle, Aquila audax (Burridge et al, ), and white‐bellied sea eagle, Haliaeetus leucogaster (Shephard et al, ). Genetic structure (or lack‐there‐of) during comparisons of mainland and continental island populations may be influenced by several factors, including species dispersal capability and the environmental suitability of the land bridge.…”

Adaptive Genetic Divergence Despite Significant Isolation-by-Distance in Populations of Taiwan Cow-Tail Fir (Keteleeria davidiana var. formosana)

“…Geographic isolation since the marine transgression following the LGM may induce higher probabilities of allopatric isolation between island-mainland sister species (Otte and Endler, 1989 ; Losos and Ricklefs, 2009 ). However, assumption of allopatric divergence between sister species of island and adjacent mainland since the isolation caused by the last marine transgression has been challenged (Li et al, 2010 ; Burridge et al, 2013 ). The level of allopatric divergence may be lower than expected because of prolonged gene flow between geographically isolated closely related species (Li et al, 2010 ) and also due to the recent colonization from mainland to island (Burridge et al, 2013 ).…”

“…However, assumption of allopatric divergence between sister species of island and adjacent mainland since the isolation caused by the last marine transgression has been challenged (Li et al, 2010 ; Burridge et al, 2013 ). The level of allopatric divergence may be lower than expected because of prolonged gene flow between geographically isolated closely related species (Li et al, 2010 ) and also due to the recent colonization from mainland to island (Burridge et al, 2013 ). Species identification and characterization between closely related species have often been a challenge because of weak interspecific barriers.…”

Intrinsic factors drive spatial genetic variation in a highly vagile species, the wedge‐tailed eagle Aquila audax, in Tasmania

“…The strength of this cline was low (as indicated by regressions against easting and northing), but the existence of any significant pattern is indicative of some limitation to dispersal. Therefore, long-distance dispersal events, such as those implied by Ridpath and Brooker (1986) and Burridge et al (2013), are relatively uncommon within Tasmania, or are directionally biased.…”