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EVOLUTION   SELECTION – 1 SMOUSE – SPRING 2011 1 S ELECTION AND A DAPTATION Adaptive Complexes: One of the first things you notice about adaptations is that they are usually complex. Futuyma spends a bit of time describing particular adaptations, just to give you the idea. The orchid story is so beautiful that it can stand a bit of elaboration here, but read Futuyma . Recall that orchids, like other monocots, have a basic floral morphology in 3’s. Orchids - The orchid has 3 petals. One of the petals, the labellum, is greatly modified. The sexual organs are fused into a single structure, the column, usually involving a single anther with two pollen masses (pollinia), with the stigmatic surface just below the anther. The labellum is quite pretty and highly aromatic, basically luring the insects into the flower. In some cases, the insects are after nectar, in some cases pollen. In other cases, male insects are confused by the aromas, and attempt to copulate with the labellum (pseudocopulation). The insects are physically challenged to move in a particular route through the flower. They are so constrained that they first leave pollen on the stigmatic surface, and then are forced to pass by the anther, where their movements trigger the application of a pollinium, on a body part that is the point of contact with the stigmatic surface of the next flower visited. The Orchidaceae is the largest family of plants, with between 18,000 – 25,000 species, and they have several thousand variations on this theme. For most species, the insect gets what it came for (food), but in some cases (sexual deception), it just gets faked out. If we didn’t know better, we could read Divine Intent into the system. Futuyma – Figure 12.3
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EVOLUTION   SELECTION – 1 SMOUSE – SPRING 2011 2 The larger point is that this suite of adaptations is not due to a single-gene; it is the result of millions of years of adaptive radiation, and probably involves many genes. It evolved slowly, and it is also important to realize that the insects were evolving simultaneously. Ecological Aspects - I also need to remind you that natural selection (and adaptation) have to be viewed in broad ecological context. They don't occur in a vacuum. If you think about the setting within which orchids occur, it is easier to appreciate the floral adaptations. In the tropics, orchids tend to be a little spread out. Wind pollination is quite chancy. There are literally thousands of different pollinating insects (not to mention birds and bats), all vying for floral rewards, and hundreds of species of plants, all vying for pollinators. From the viewpoint of an orchid, anything that will ensure regular visits by particular pollinators will be beneficial. But pollinators come in all sizes and descriptions. A plant that can count on a pollinator of a particular size, with predictable behavior and
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