Sympatric speciation on oceanic islands

Is speciation possibly in sympatry? Using Lord Howe Island as a natural laboratory, this project has yielded important new insights into this controversial evolutionary question.

A collaborative team led from Kew has demonstrated that the two species of Howea, a palm genus endemic to Lord Howe Island (Howea forsteriana pictured here), have speciated sympatrically. The case study joins a small number of examples that indicate that sympatric speciation is actually possible. © Bill Baker

The origin of species diversity has challenged biologists for over two centuries. Allopatric speciation, species divergence resulting from geographical isolation, is well documented. However, sympatric speciation, divergence without geographical isolation, is highly controversial. Claims of sympatric speciation must demonstrate species sympatry, sister relationships, reproductive isolation, and that an earlier allopatric phase is highly unlikely; no empirical study has convincingly satisfied all criteria.
 

From the first phase of this project, funded by the Leverhulme Trust (2003-2005), we have published results in Nature (Savolainen et al. 2006) which provide clear support for sympatric speciation in a case study of two species of palm on an oceanic island. A large dated phylogenetic tree shows that the two species of Howea (Arecaceae), endemic to the remote Lord Howe Island (LHI), are sister taxa and diverged from each other well after the island was formed 6.9 million years ago. During extensive fieldwork, we found a substantial disjunction in flowering time that is correlated with soil preference. In addition, a genome scan (AFLP) indicates that only few genetic loci are more divergent between the two species than expected under neutrality, a finding consistent with models of sympatric speciation involving disruptive/divergent selection.
 

Additional funding was received by PI Savolainen (ERC Advanced Grant, NERC) to survey the entire flora of LHI and disentangle the underlying genomic architecture for speciation to occur in the face of gene flow. We found that sympatric speciation has occurred in several other taxa on LHI (published in PNAS; Papadopulos et al. 2011). This research in plant speciation provides a tantalizing opportunity for refining theoretical models on the origin of species and new impetus for exploring putative plant and animal examples on oceanic islands.
 

Key papers published since 2006.
1. Savolainen, V., Anstett, M.-C., Lexer, C., Hutton, I, Clarkson, J.J, Norup, M.V., Powell, M.P., Springate, D., Salamin, N. & Baker, W.J. (2006). Sympatric Speciation in Palms on an Oceanic Island. Nature 441: 210–213.
2. Savolainen, V., Anstett, M.-C., Lexer, C., Hutton, I, Clarkson, J.J, Norup, M.V., Powell, M.P., Springate, D., Salamin, N. & Baker, W.J. (2006). Sympatric plant speciation in islands? Reply. Nature 443: doi:10.1038/nature05217.
3. Babik, W., Butlin, R.K., Baker, W.J., Papadopulos, A.S.T., Boulesteix, M., Anstett, M. -C., Lexer, C., Hutton, I. & Savolainen, V. (2009). How sympatric is speciation in the Howea palms of Lord Howe Island? Molecular Ecology 18: 3629–3638.
4. Papadopulos, A.S.T., Baker, W.J., Crayn, D., Butlin, R.K., Kynast, R.G., Hutton, I. & Savolainen, V. (2011). Speciation with gene flow on Lord Howe Island. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1106085108.