Speaking of the evolution of orchids, I am familiar with the co-evolutionary pollination strategies which account for the structures of many orchid flowers. However, I have never seen an explanation for the twisting of the anther and lip in Ludisia discolor. I assume this is also a co-evolutionary feature relating to a pollination strategy. Could anybody explain the uncommon lack of symmetry in the flower of this species? Dave
I don't know much about Ludisia, but I doubt the asymmetry has much to do with coevolution of Ludisia and its pollinators.
Several years ago, while preparing a talk on orchid evolution, I planned to make a major point about the coevolution of orchids and their pollinators. However, I was informed by one of the best orchid taxonomists that there is little or no evidence that such coevolution has occurred. Most orchids deceive their pollinators by one of several strategies, usually without offering significant reward to the pollinator. They mimic nearby flowering plants that do provide a reward; they mimic female insects both by sight and smell to trick males into attempting to mate with them; they resemble rotting meat or fungi that insects often lay their eggs in; they produce aromatic oils that attract males and then momentarily trap them in elaborate cages or send them through pools of liquid or send them down slides - all these tricks to get the orchid pollinated with no reward to the insect. Under these conditions there is no selective advantage to an insect that pollinates the orchid and thus no coevolution.
The only example I know about that might represent coevolution is the Angraecum orchids and their hawk moth pollinators originally described by Darwin. This is a plausible example - a race between the orchid to continually lengthen its nectary and the moth to lengthen its proboscis. As you may know there was an article in Nature a few years ago giving experimental evidence that it is to the orchid's advantage to have as long a nectary as possible. In the field orchids with long nectaries are more frequently pollinated than are those whose nectaries have been experimentally shortened. The authors speculated that moths would have a nutritional advantage if they had a longer than average proboscis, but I don't know that this idea has been tested. So this is close to an experimental demonstration of coevolution, but falls a little short.
In summary, it seems clear that most orchid pollinators do not coevolve with the orchids they pollinate. This makes a lot of sense because orchids are rarely abundant in their habitat and would not be a sustaining food source even if they offered a reward. Of course a minor food source could make a selective difference, but as I mentioned above, most orchids offer their pollinators no reward. Despite this general statement, there is the Angraecum-moth interaction, not a conclusive example of coevolution, but quite tantalizing, just as it was for Darwin.
-- Steve Beckendorf Berkeley, California
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