Martin Epstein writes: > I speculate that this makes cross pollination more common in orchids > than in most other flowering plants leading to a broadening of the > gene pool, rapid evolution and the fantastic diversity we see in > orchids today. > Great topic for speculation! Here's my comments.
It seems to benefit most plants to avoid self pollination, and most have evolved mechanisms to assure their pollen reaches others. These include separation of the sexes onto different plants (occasionally practiced by Orchids as well), having pollen from a flower ripen either before or after the stigma is receptive, and chemical markers that enable a plant to recognize its own pollen. Given the nature of most flowers, it is also quite possible to be pollinated both with their own pollen and pollen from another plant. Personally I think the characteristics of the Orchid family that have enabled them to spread and diversify so amazingly are, consolidated pollen, and massive quantities of tiny seeds. This assures a broad range of genetic variability in each seed pod, and a wide spread of the seeds so that each generation is subjected to great selection pressure. I think this seed quantity and mobility also helps the cleistogamous species, which seems to include a lot of pioneer plants (when considering all families, not just Orchids). They need to have seeds carried long distances as they quickly create conditions no longer favorable to the germination of their own seed, and have to find the next clearing, road cut, landslide, or exposed treelimb. But they don't necessarily need the genetic variability, as they prefer a rather specific environment to germinate.
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