Throwback: Here’s How Darwin Predicted The Existence of the World’s Weirdest Pollinator

A lot of things are named for Charles Darwin.  A glacier, an asteroid, a mouthbrooding frog, the Darwin Awards for people who perish in unfortunate and foreseeable ways.  The list goes on.

But the coolest thing named for Darwin is a flower.

This magnificent specimen is Darwin’s orchid, or Angraecum sesquipedale.

It has one especially unusual feature.

That extremely lengthy tube is a nectar spur, a hollow extension of the flower filled with sugar-rich nectar to attract pollinators.

The flower gives sugar as a reward, and the insects help the flower reproduce.  That makes the flower a literal sugar daddy.

Surprisingly, these nectar spurs extend a full 11.5 in (29.2 cm), and only the bottom few inches contain nectar.

How could any insect reach that nectar and pollinate the plant?

In 1862, James Bateman, an orchid grower, mailed Angraecum sesquipedale to one of the prominent scientific figures of the time.

Charles Darwin had just published On the Origin of Species in 1859.   In it, he argued that individuals within a species differ in their traits, and compete with each other for survival.

Because individuals with more useful traits are more likely to survive and reproduce, those traits are “selected” and passed on to future generations.  This mechanism leads to change within species and the formation of new species.

Darwin was fascinated by Angraecum sesquipedale and wondered why it had evolved the trait of having a long nectary.  He tried to remove its pollinia (pollen grains that stick together as single units) using brushes and needles, but failed.  The only way he could remove the pollinia was by sticking a 1/10th inch rod all the way down into the nectary.

He wrote a letter to his best friend, J.D. Hooker:

Darwin predicted that the orchid was pollinated by a moth with an extremely long proboscis.  The flower must have evolved long nectar spurs for a reason!

Darwin died in 1882.  Twenty-one years later, in 1903, people realized that Darwin’s mystery moth had already been described in 1832 by Cuban zoologist Felipe Poey y Aloy and sloppily named in 1856 by British entomologist Francis Walker.

Here it is:

Xanthopan morganii‘s 6-inch (15 cm) proboscis is far longer than its body!  And it’s long enough to reach Angraecum sesquipedale‘s nectar.

Darwin had predicted an otherwise improbable proboscis.

Darwin?  More like Dar-WIN!

 

Here’s a video of Xanthopan‘s nectar-sucking feat as proof:

 

Scientists have since taken a closer look at the evolution of ludicrously long nectar spurs.   In columbine flowers (Aquilegia), nectar spurs actually have no choice but to get longer.

Researchers reconstructed the genus’ evolutionary history of “pollinator shifts”. The ancestor of all columbines was pollinated by bees.  They found that once a bee-pollinated lineage shifted to hummingbird pollination, its lengthened spurs never switched back to bee pollination.  And once a hummingbird-pollinated lineage shifted to being pollinated by hawkmoths, its extra-extra-lengthened spurs never switched back to bee or hummingbird pollination.

 

The logic behind this is that bees and hummingbirds can’t reach the nectar in a very long spur, but a hawkmoth’s long tongue can reach the nectar in a short spur.  Short-spurred species visited mostly by hawkmoths evolve longer spurs because when hawkmoths have to strain for nectar they end up with more pollen attached.  But no bee would ever visit a long-spurred species, so long-spurred species don’t evolve shorter spurs.

Therefore, Aquilegia nectar spurs always evolve to be longer, never shorter.

Who knows how many other bizarre natural-selection stories are waiting to be discovered?  Maybe you’ll have an orchid named after you one day.

Or we’ll just have another 100000000000 species named after Darwin.  Probably that one.

 

Sources & Further Reading:

Arditti, J., Elliott, J., Kitching, I.J., Wasserthal, L.T.  2012.  ‘Good Heavens what insect can suck it’- Charles Darwin, Angraecum sesquipedale and Xanthopan morganii praedicta.  Botanical Journal of the Linnean Society 169: 403-432.

Whittall, J.B., Hodges, S.A.  2007.  Pollinator shifts drive increasingly long nectar spurs in columbine flowers.  Nature 447: 706-709.

Note: There’s an unresolved debate on whether coevolution or pollinator shifts drive floral evolution.  Look into it if you dare!