Darwin’s Idea of Evolutionary Link Between Plants, Insects Supported
Charles Darwin had a notion about 130 years ago that there was some sort of evolutionary link between the long nectar tubes of flowers and the tongues of insects that pollinated them.
This was not a notion that struck like a bolt out of the blue. The bearded naturalist obviously had given it considerable thought.
On the face of things, the evidence seemed clear: Here were flowers with long nectar tubes and insects with correspondingly long nectar-seeking proboscises, the tongue-like projections used to eke out a meal and inadvertently pass on pollen.
A perfect match, thought the naturalist--too close to be an accident and an apparent example of the symbiosis that is common in nature--two organisms, each needing the other to survive.
In something of a gambol through the world of flora and fauna, he saw it again and again. Bees and red clover, moths and orchids.
Evolved Together
This led Darwin to wonder if the long corollas of flowers and their pollinating insects somehow evolved together, perhaps in co-evolution.
On the flip side, flower without bug surely would have evolved differently and bug without flower, well, would have yielded an insect with a much shorter proboscis, or so Darwin reasoned.
Now studies suggest that he probably was right.
Generations of deep-tubed orchids might very well have driven the evolutionary lengthening of the proboscises of moths, the insects that pollinate them.
As the tubes of flowers lengthened, so did the tongues of moths, new studies maintain.
By manipulating flower length, L. Anders Nilsson, a botanist at the University of Uppsala in Sweden, has been able to take control of the process of evolution in the lab.
What May Occur
Such analysis allowed him to surmise through experimentation what may have occurred in the past, thousands of years ago, when flower and bug first encountered one another, and what may occur in the future as their relationship continues.
First, by measuring proboscis and floral tube, he found the average length of the Madagascar star orchid, the same flower favored in Darwin’s observations, was about 28 to 32 centimeters. There are about 2 1/2 centimeters to the inch.
The average length of the pollinating hawk-moth tongue is about 25 centimeters.
Moreover, the moths behaved just as Darwin had hypothesized, placing their proboscises no farther than necessary to obtain nectar, the botanist found.
Flower Length Reduced
But when Nilsson experimentally reduced the length of the flowers, either by taping them or tying them with cotton thread, the reproductive capacities of female flowers diminished dramatically. In response, shorter orchids increased their maleness.
This suggested that in the past, short tube flowers were at a distinct evolutionary disadvantage.
Because of a larger number of deep corollas and the food they offered, Nilsson’s studies suggest that these traits were more readily passed along by the moths with each pollinating visit to a flower. The object of which is to get the yellow, powdery substance containing male sex cells in pollen into the proper place in the female flower.
As a result, moths with longer proboscises also were at a greater evolutionary advantage because they were most likely to obtain food and survive.
Ideas Tested
Writing in a recent issue of the British journal Nature, Nilsson said he tested Darwin’s ideas using several moth and orchid species, the experimental results, each time bearing out the concept of co-evolution between insect and flower.
“Sexual encounters in animal-pollinated plants depend first on the deposition of the pollen grains on the pollinator, and subsequently on the appropriate area of the pollen-vehicle contacting receptive stigmas” in female flowers, Nilsson wrote.
In all instances tested, and in observations of all the flower-moth combinations, Nilsson found that the ratio between tube and tongue is determined by “the distance between tube extremity to pollen-attaching floral structure.”
“Such pollinators,” he said of nectar-seeking insects, “favor the evolution of longer floral tubes which in turn favors the evolution of still longer tongues in a reciprocal and escalating process.”
If long-tongued bugs become scarce, plants may begin developing shorter tubes, he said.
Premier Plant Breeders
Bugs such as the hawk-moth are among nature’s premier plant breeders, concluded Nilsson, who also pointed out that it is the moths that are carrying genetic information from flower to flower.
Even Darwin in “On the Origin of Species,” the book that set Victorian England astir, believed that the mutual growth of flower and bug would result in flowers yielding “the most seed, and the seedlings would inherit long nectaries.”
“Darwin’s ingenious conclusion,” Nilsson said of the co-evolution idea, “has received attention merely as an anecdote up till now.
“Data presented here show that he in fact provided the basic idea to a general principle about interactional evolution between animals and flowering plants.”
