Abstract
Co-occurrence of closely related taxa on islands could be attributed to sympatric speciation or multiple colonization. Sympatric speciation is considered to be rare in small islands, however multiple colonizations are known to be common in both oceanic and continental islands. In this study we investigated the phylogenetic relatedness and means of origin of the two sympatrically co-occurring Zosterops white-eyes, the endemic Zosterops ceylonensis and its widespread regional congener Z. palpebrosus, in the island of Sri Lanka. Sri Lanka is a continental island in the Indian continental shelf of the Northern Indian Ocean. Our multivariate morphometric analyses confirmed the phenotypic distinctness of the two species. Maximum Likelihood and Bayesian phylogenetic analyses with ~2000bp from two mitochondrial (ND2 and ND3) and one nuclear (TGF) gene indicated that they are phylogenetically distinct, and not sister to each other. The two subspecies of the peninsula India; Z. p. egregius of Sri Lanka and India and Z. p. nilgiriensis of Western Ghats (India) clustered within the Z. palpebrosus clade having a common ancestor. In contrast, the divergence of the endemic Z. ceylonensis appears to be much deeper and is basal to the other Zosterops white-eyes. Therefore we conclude that the two Zosterops species originated in the island through independent colonizations from different ancestral lineages, and not through island speciation or multiple colonization from the same continental ancestral population. Despite high endemism, Sri Lankan biodiversity is long considered to be a subset of southern India. This study on a speciose group with high dispersal ability and rapid diversification rate provide evidence for the contribution of multiple colonizations in shaping Sri Lanka’s biodiversity. It also highlights the complex biogeographic patterns of the South Asian region, reflected even in highly vagile groups such as birds.
Highlights
Speciation is the evolutionary process by which biological populations diverge into distinct species through reproductive isolation [1,2,3]
Based on the eigenvalues (S2 Table) and scree plot (S2 Fig) first three principal components (PCs) were selected for the Discriminant Function Analysis (DFA)
Z. ceylonensis did not cluster with sympatric Z. palpebrosus but is resolved as the basal lineage to all its congeners in the clade of Zosterops
Summary
Speciation is the evolutionary process by which biological populations diverge into distinct species through reproductive isolation [1,2,3]. Islands provide a unique opportunity to understand the evolutionary processes behind diversification and speciation [12,13,14]. Large islands (e.g. Madagascar) often have a greater diversity of terrain and ecological niches that may allow more intra-island diversification ([18,19,20] but see [21]). This diversity provides enough barriers for isolation, which results in intra-island allopatric speciation. When related species are found in small islands, they tend to be results of ‘multiple colonizations’ [17, 22, 23]
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