Opposites attracts? On the hybrids between two spectacular orchids

It is no coincidence that the Galapagos Islands inspired Charles Darwin. Isolated islands and archipelagos are often like an evolutionary playground. In late June, I visited Gotska Sandön with my family. The remote island is the setting for an natural experiment, isolated by 90 kilometers of sea south-east of mainland Sweden. Two orchid species have been crossing like there were no barriers between them. But there is something strange about this. These orchids have a very specialized pollination system that should make hybridization difficult. (Click on the slide-show for larger images)

When it comes to pollination, many orchids promise everything but give away nothing. They attract bees by looking and smelling wonderful but offer nothing in return. They have no nectar, and have no tasty pollen either. But there must be sucessful pollination for new orchids to be made. They lure pollinators with a game of deception, an intricate scam involving complex adaptations in order to be successful. But on Gotska Sandön, I got the feeling that the hybrids, were these adaptations have been blended away, were more common than its parental species.

The Red Helleborine, one of the two parent species, deceives Carpenter Bees into believing that it is a Bluebell. We humans do not find the flowers of the Red Helleborine and the Bluebell to be especially similar, but the bees have a different visual spectrum. When Professor L. Anders Nilsson was at the island of Gotland in the early eighties, he analyzed the Bluebell and Red Helleborine from the bee's visual spectrum, and they were practically identical. Anders also saw that the Red Helleborine had its flowering peek just before the Bluebells. If you should fool Bluebell pollinators, you do not only have to look right, you have to have your timing right as well. However, the story is a bit more complicated at Gotska Sandön where there are no Bluebell specialist pollinators. It seems like they haven't found their way out to the island.

The second parental species is the Sword-leaved Helleborine, a species which pollination ecology has not been investigated in Sweden. However, in Israel the species that the orchid is adapted to look like is a Cistus (Cistus salviifolius). In Sweden, there are no wild species of the genus Cistus. The closest wild relative of the same plant family is the Common Rock-rose, a species that hardly looks as the orchid, not even in a bee’s visual spectrum. If there even is a species that the swedish Sword-leved Helleborines are adapted to mimic, it remains to be discovered.

On Gotska Sandon something unusual has happened. Two barriers against cross-species pollination between these two orchid species have broken down. The first barrier is the flowering time. In Europe, the Sword-leaved Helleborine normally flowers before the Red Helleborine, but on Gotska Sandön sun-warmed sandy soils, the Red Helleborine flowers earlier than elsewhere in Sweden. In an increasingly warmer world, this type of species barriers might break down more and more often.

The second barrier is all about orchid deception. The Red Helleborine normally attracts Carpenter Bees of which there are none at Gotska Sandön, while the Sword-leaved Helleborine, at least in Israel, attracts bees of a different genus (the genus Halictus). But at Gotska Sandön, by whatever reason, it must happen that both species attracts the same pollinator, perhaps a bit just by chanse. Almost like garden plants, the colorful orchid hybrids are like spots of paint ranging from white to red against the pale green pine forest of Gotska Sandön; like living proofs of the pollinators that has been ignoring the barriers.

Today, the hybrids at Gotska Sandön are like a natural evolution experiment waiting to be investigated. Some hybrids were tall, others small, some had red flowers, others white and everything you could imagine in between. Some had finished flowering, while others were just starting to flower. Only time will tell whether these orchids is about to evolve into something new, or if the parental species will eventually evolve new barriers against hybridization.