The cover story of today's Nature is concerned with the first unambiguous finding of fossilised orchid pollen. This is very exciting for evolutionary botanists, who have long been "fascinated by the spectacular adaptations to insect pollination exhibited by orchids".
The discovery involves an extinct worker stingless bee (Meliorchis caribea) preserved in Dominican amber that carries an orchid pollinarium (the male reproductive structure that is transferred as a single unit during pollination). The researchers have analysed the morphology of this structure and assign it to "the extant subtribe Goodyerinae (subfamily Orchidoideae)". This means that an essentially modern orchid was living at the time when the amber was formed (Miocene, considered to be 15-20 million years old). Thus far, there is little here for evolutionary biologists to work with, because the pollinarium does not reveal anything about origins - only that a modern orchid was also present in the Miocene.
In this case, evolutionary theory is imported in order to frame the find and to allow a transformist interpretation. "We use the ages of other fossil monocots and M. caribea to calibrate a molecular phylogenetic tree of the Orchidaceae. Our results indicate that the most recent common ancestor of extant orchids lived in the Late Cretaceous (76-84 Myr ago), and [. . .] support the hypothesis of an ancient origin for Orchidaceae." The report by Ledford adds: "To investigate, the team used genetic information from 55 genera of living orchids to make a family tree, determining which plants are most related to each other today and then working backwards to see when they probably split apart. By dating the amber of their new fossil find to 15-20 million years ago, they could then start to put dates onto the various branches of this tree." The Late Cretaceous age was arrived at by "assuming a relatively constant rate of orchid evolution".
It is important to realise that that these conclusions have not emerged naturally from the empirical data, but are the product of an evolutionary framework adopted by the researchers. With different assumptions and a different methodology, it should not be deemed surprising if the same data can lead to contrasting conclusions. However, on the basis that the "rate of orchid evolution" exhibited by the subtribe Goodyerinae is almost zero, the comment of the lead author is probably correct: "The dinosaurs could have walked among orchids".
Dating the origin of the Orchidaceae from a fossil orchid with its pollinator
Santiago R. Ramirez, Barbara Gravendeel, Rodrigo B. Singer, Charles R. Marshall and Naomi E. Pierce
Nature 448, 1042-1045 (30 August 2007) | doi:10.1038/nature06039
Since the time of Darwin1, evolutionary biologists have been fascinated by the spectacular adaptations to insect pollination exhibited by orchids. However, despite being the most diverse plant family on Earth2, the Orchidaceae lack a definitive fossil record and thus many aspects of their evolutionary history remain obscure. Here we report an exquisitely preserved orchid pollinarium (of Meliorchis caribea gen. et sp. nov.) attached to the mesoscutellum of an extinct stingless bee, Proplebeia dominicana, recovered from Miocene amber in the Dominican Republic, that is 15-20 million years (Myr) old3. This discovery constitutes both the first unambiguous fossil of Orchidaceae4 and an unprecedented direct fossil observation of a plant-pollinator interaction5, 6. By applying cladistic methods to a morphological character matrix, we resolve the phylogenetic position of M. caribea within the extant subtribe Goodyerinae (subfamily Orchidoideae). We use the ages of other fossil monocots and M. caribea to calibrate a molecular phylogenetic tree of the Orchidaceae. Our results indicate that the most recent common ancestor of extant orchids lived in the Late Cretaceous (76-84 Myr ago), and also suggest that the dramatic radiation of orchids began shortly after the mass extinctions at the K/T boundary. These results further support the hypothesis of an ancient origin for Orchidaceae.
Ledford, H. Amber preserves rare orchid pollen,
email@example.com: 29 August 2007; | doi:10.1038/news070827-4
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