Darwin’s Orchids
This fascinating flower is perhaps the best example of an equally fascinating phenomenon called coevolution.
The Star Orchid, or Angraecum sesquipedale, was first discovered in 1798, found in low elevation, coastal regions, on the island of Madagascar, but its strange characteristics were not described until 1822.
The most notable of these characteristics is the flower’s exceedingly long nectar spur. The nectar spur is the long, green tubular extension that hangs off the flower and houses the nectar. Charles Darwin, upon seeing these strangely long spurs, hypothesized that this flower was pollinated by a moth with an unusually long proboscis, one so long that it could reach into the depths of the nectar spur.
The spur itself is usually somewhere between 27 and 43 cm long and only contains a small amount of nectar at the very bottom. For any organism to feed on this nectar, it would need a very long straw indeed, which is precisely why the long proboscis-ed moth hypothesis makes a lot of sense.
Unfortunately for Darwin, at the time of his surmising a moth with this sort of morphological feature had yet to be discovered. It wasn’t until 21 years after his death that the moth was finally found.
It turns out that the Sphinx moth is the culprit, it just took us a while to find it.
It’s also taken me a while to get back to coevolution, so let’s make the connection now, shall we?
If a Sphinx moth goes up to a Star Orchid and sticks its long proboscis into the flower’s shallow nectar spur the moth gets the food reward and goes on its merry way while the orchid goes un-pollinated. If that orchid is not pollinated, it will eventually die, never having passed on its genes.
Say a Sphinx moth goes up to a Star Orchid and sticks its long proboscis into the flower’s deep nectar spur. To get all the way down to the nectar, the moth will have to put its face on the flower. If the spur is long enough, it will force the moth get so close to the flower that it gets covered in pollen. Then the moth gets the food reward and goes to the next flower to get more food. As it sticks its proboscis in the second flower’s nectar spur, it will deposit the first flower’s pollen, thus fertilizing the second flower and passing on the first flower’s genetic material.
Imagine this process repeated numerous times over the course of thousands of years. As time goes on, the longer spurred Star Orchids would more frequently pass on their genetic information, increasing the frequency of the long spurred trait in the Madagascar Star Orchid population.
Longer nectar spurs confer reproductive success for Star Orchids and longer proboscises confer food acquisition and subsequent reproductive success for Sphinx moths. If the orchid has a longer nectar spur, it is more likely to be pollinated. If the moth has a longer proboscis, it is more likely to have the ability to eat the nectar at the bottom of the long pollen tubes allowing it to live long enough to reproduce.
So as time goes on nectar spurs and proboscises get longer and longer together, something of an arms race between the two organisms, and we have coevolution.
Life is beautifully interconnected and chocked full of patterns. That’s why evolution is so fascinating. Finding the patterns and weaving together the mechanisms through which life functions is a never ending, but ever fulfilling task.
“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
- Charles Darwin
