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Evolution

Palm Trees : So Close, yet so Different

A team from Montpellier has demonstrated different courses of evolution among palm trees in geographic proximity. This example of speciation provides support for a long-contested theory.

What could have possibly happened to the palm trees on Lord Howe Island in the Tasmanian Sea? On this tiny volcanic island, scarcely 12 km long and only 1 km wide, the trees have managed to evolve in two different ways. A single ancestor has given rise to two genetically different sister species. However, some members of these two species are no more than a few meters from each other and there is no geographic barrier to separate them into two distinct, isolated groups evolving separately.

“Speciation in the presence of a geographic barrier, or allopatric speciation, is certainly the best known and easiest to understand,” explains Marie-Charlotte Anstett, from the Evolutionary Sciences Institute of Montpellier (Isem).1 Indeed, in such situations, the flow of genes between the two groups is effectively broken. “However, it has been demonstrated that another type of speciation, sympatric speciation–with no geographic barrier–is theoretically possible,” she adds.

The first articles modeling this kind of speciation date back to the middle of the 1960s. Geneticists mathematically demonstrated that this kind of speciation should theoretically be frequent and expressed some surprise at the small number of clear examples found in nature. These examples include Amphilophus, a genus2 of round-headed fish that has given rise to two almost identical species, which abound in the waters of Lake Apoyo in Nicaragua. There is also an American parasitic insect that shares its territory with its almost identical “brother” species. We can now add the Howea palms of Lord Howe Island to this list.3

“As far as we know, this is the first example of sympatric speciation in plants that does not involve polyploidisation–an increase in the number of chromosomes,” points out Anstett, one of the authors of this international study. So how did they prove it? “We constructed the phylogenetic tree of all the genera of this type of palm tree found in the Indo-Pacific region. We were then able to date the divergence of the Howea genus and of its two species.” The results of this analysis clearly show that the two species separated after the emergence of the island. The reproductive barrier between the species is simply the result of a difference in flowering time, and this is sufficient for hybrids between the two species to be very rare.

It seems clear that these two species really did evolve in sympatry. But why? The answer may lie in the soil. Each species develops preferentially on a particular type of soil: calcareous for one and volcanic for the other. “This difference may have been the 'trigger' initiating speciation,” suggests Anstett. “We are going to try to test this hypothesis in the field, by transplantations and study of the reproduction of the two species on the two types of soil.” This discovery, adding to the complexity of evolutionary theories, should help us understand how species are created. With a bit of luck, they may even help to save the Howea palms, which are on the IUCN red list of endangered species4 and currently widely used as exotic decor in apartments and offices.

 

Charline Zeitoun

Notes :

1. Institut des sciences de l'évolution de Montpellier (CNRS/ Université Montpellier-II joint lab).
2. “Genus” is an intermediate category in the classification of species. For example, the genus Panthera includes the species Panthera tigris (tiger) and Panthera leo (lion).
3. Nature, 441 (7090): 210. 2006. In collaboration with researchers from the United Kingdom, Australia, Denmark, and Switzerland.
4. www.redlist.org/search/search-basic

Contacts :

Marie-Charlotte Anstett
Institut des sciences de l'évolution de Montpellier.
anstett@isem.univ-montp2.fr


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