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Fantastical traits like foot-long tongues can arise from coevolution. Darwin guessed the existence of a new species based on the traits of one species! Coevolution is fairly common in nature, though not always as odd.
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The History of the Star Orchid
In London, one fateful day in 1862, Charles Darwin, the father of evolutionary biology, received a parcel from James Bateman (a famous orchid grower), containing a live specimen of a peculiar orchid.
Why was the orchid peculiar? All insect-pollinated flowers produce nectar from an organ known as a nectary. Nectar is bait for pollinators. In exchange for pollen moving from one flower to another, the insects get a sweet drink!
The peculiar aspect of this orchid is that the nectary is at the bottom of a tube that is over one foot long! “Good heavens…” Darwin exclaimed, writing to his friend Joseph Hooker. “What insect can suck it…”. The nectary was well out of reach of any insect documented at the time. Why would the Star Orchid make the reward for pollination impossible to reach?
Angraecum sesquipedale – now commonly known as Darwin’s Star Orchid is a plant found exclusively in Madagascar. White in color, the flower gives off a strong pleasant odor at night to attract pollinators active during that time.
Most nighttime pollinated flowers are quite distinct from daytime pollinated flowers. Rather than bright shades of yellow and pink, white is the most common color as this is the most visible in starlight and moonlight. Instead of bees and ants, bats and moths take the night shift!
Darwin’s book Contrivances included predictions that a moth with a long mouthpart that is able to access this deep nectary exists. How could Darwin have predicted the existence of a moth just by looking at the structure of a flower?
What is Coevolution?
In nature, we often see plants and animals depend on one another in a complex web of interactions. Sometimes, a few relationships occur where one organism is wholly dependent on another – and these organisms evolve to become ‘accustomed’ to one another. This is called coevolution: where over time, one species becomes more and more uniquely adapted to living in the presence of another.
The length of the tube to the nectaries of the Star Orchid flower had to have been within reach for at least one pollinator for the orchid to be able to reproduce and produce offspring. The nighttime release of fragrance and the lack of bright colors that other flowers use to attract daytime insects indicated that the Star Orchid caters to nocturnal pollinators.
So specialized this flower appeared to be, that Russell Wallace, a close friend to Darwin, was even able to suggest that the mystery Malagasy moth was related to the Hawkmoths found in Africa. “That such a moth exists in Madagascar may be safely predicted, and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune, – and they will be equally successful,” he wrote.
What did the moth actually look like?
Finally, in 1903, 20 years after Darwin’s death, Karl Jordan and Lord Walter Rothschild described a new subspecies of moth to science. This moth is found in Madagascar, with a mouth part that could extend up to 30cm (11.8 inches). Could this reach the nectaries of Darwin’s Star Orchid? An entire century after Darwin’s death, there was evidence of a Hawk Moth pollinating the Star Orchid. In honor of his prediction, Darwin’s Hawk Moth was named Xanthopan morganii praedicta.
The Darwin’s Hawk Moth, has the longest tongue of any known insect species – and is the only known species to pollinate Darwin’s Star Orchid. The orchid however is now facing threats from rampant habitat destruction in Madagascar and is threatened in the wild putting this distinctive duo at risk.
Species which coevolve together are often dependent on each other for survival. If one species is at risk, the other one may begin to die out.
Other Examples of Coevolution
Mutually helpful examples of coevolution include the ones between pollinators, like hummingbirds, and tubular flowers as well as the Star Orchid and Hawk Moth. The fig and fig-wasp also rely on each other for pollination. The fig fruit also provides a home for the new wasp larvae and eggs.
Coevolution does not need to be helpful! It can also be an antagonistic “arms race”!
Lodgepole pines are native to the Rocky Mountains. They do not want their seeds to be consumed by herbivores. The crossbill is a bird that gets around this, its unique beak breaking open the pinecones and harvesting the seeds within.
Coevolution needs a lot of time to take place. It mostly occurs when two specialized species are within the same environment for thousands, if not millions of years. Over time, the species become more and more closely linked. Relationships in nature can be intricate indeed.
Coevolution: A transition over time, where one species becomes more and more uniquely adapted to living in the presence of another.
Nectary: A nectar-producing organ
Nocturnal: An animal that is active during the nighttime
Pollinator: An agent that spreads pollen
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