Wind Creates Evolutionary Changes In Flying Insects, Depriving Them Of Flight

About 95% of insect species worldwide can fly. The question of what environmental pressures determine whether or not an insect species evolves to have wings has long fascinated scientists. The first scientist known to be intrigued was the twenty-two-year-old Charles Darwin when, in January of 1831, he visited the Portuguese-held island of Madeira off the coast of Morocco.

The island of Madeira is part of a volcanic archipelago. On its dramatically rocky shoreline, northeast trade winds bring in huge ocean swells. Temperatures at sea level in Madeira average in the 50s and 60s (F) in the winter, and are only a little warmer in the summer. In the windswept mountains it can snow. Because of its microclimates and isolation, Madeira is home to a wide array of endemic species like the Madeiran long-toed pigeon with its six-note cooing and the endangered Zino’s Petrel, a seabird that lives exclusively at sea and only returns to the island to breed in the mountains. Madeira also has twenty-four species of endemic land mollusks, and it has lots and lots of beetles. Darwin was a beetle afficionado. On Madeira, he collected them, and noticed that they were wingless.  

Twenty-five years after his trip to Madeira, Darwin mentioned the wingless beetles to his best friend Joseph Hooker, a geographical botanist and an adventurer who, like Darwin, had voyaged to Madeira in his younger years. To Hooker, Darwin hazarded a guess about how the “apterous” (un-winged) beetles had evolved. Roughly, what he suggested was that Madeira is a small island with strong winds. Any flying insects would probably have gotten blown out to sea, leaving the flightless ones to dominate the gene pool. Hooker responded that he thought Darwin’s idea to be  “very pretty.”  At the same time, he pointed out that he had found wingless beetles in the Sahara Desert, which is nowhere near water.

And so a friendly argument began. Darwin and Hooker never resolved it, and it has persisted among scientists from 1855, the year of the two scientists’ discussion, to 2020. Publishing this December in the Proceedings of the Royal Society B, researchers from Australia’s Monash University have finally settled matters. Drawing on a comprehensive database of insect species that was compiled over the last three decades by other researchers, they have demonstrated that Darwin was right — at least in identifying wind as the cause of the evolutionary change.

The data used by the Monash University team had been culled by researchers working in Antarctica but also on twenty-eight Southern Ocean Islands, which are in the expanse of ocean surrounding Antarctica and were the focus of the Monash University study. According to study co-author Rachel I. Leihy, the group of islands is impressive for its geographic and geological diversity. “They are in the Southern Pacific, Southern Indian, and Southern Atlantic oceans, and in some cases they are isolated by thousands of kilometers of ocean. Some are relatively new, volcanic formations. Others are fragments of continents.”

To varying degrees, they are windy. According to the U. S. National Ocean Service, during the Age of Sail (the 15^th to 19^th centuries) the winds in the seas surrounding Antarctica “propelled tall ships across the ocean at breakneck speed.” Amazed sailors named the winds after the latitude lines near the southern tip of the world, and told tales of wild rides courtesy of the “roaring forties,” “furious fifties,” and “screaming sixties.”  

To the insect databases collected over three decades by other researchers, Leihy and collaborator Steven L. Chown added data about flight for each species. They also applied to each species every credible hypothesis since Darwin’s time about the evolution of flightlessness. These theories included:

·     Wind, freezing temperatures, or low air pressure might independently increase the energy required to fly, depriving insects of the energetic resources necessary to create or sustain fecundity. Only insects that opt not to fly in difficult environments might retain the ability to reproduce well.

·     Habitat fragmentation might deprive insects of the motivation to fly.

·     The disappearance of either predators or competitors from certain environments might make flying unnecessary.

·     Wind might blow flying insects off the island (Darwin’s idea).

And a few more.

Ultimately, Leihy and Chown found that nearly half (47%) of insect species that evolved on the Southern Ocean Islands are unable to fly, though some do retain small, remnant wings. According to Leihy, 47% is an exceptionally large number, representing nearly ten times the worldwide incidence of flightlessness among insect species.

Comparing multiple variables about environment with their identification of species that are flightless, the researchers found that wind speed, habitat stability, and summer land surface temperatures correlate with 77% of the Southern Ocean Islands species that have lost the ability fly. (The complete variable list also included seasonality, island area, island age, insectivore richness, and island isolation.)

Ultimately, Leihy and Chown recognized wind speed as the single strongest environmental contributor to the evolution of flightlessness in insects. Indeed, they noted in their paper that the windier the island, the more flightless insect species they identified.

While Darwin appears to have been right in identifying wind as the primary driver of the evolution of flightlessness among insects, Leihy and Chown’s study found that he was probably not correct in pinpointing precisely what it was about the wind that steered the evolutionary path. Darwin had suggested to Hooker that the wind tossed insects off of islands, but Leihy and Chown found that the proportion of flightless insects on an island doesn’t generally scale with island size. This would mean that insect populations are not decimated and their gene pools fundamentally changed because the bugs that fly get blown out to sea.

The researchers did suggest an alternate way in which wind might drive the evolution of flightlessness. In environments that are very windy (like Madeira, the Sahara Desert, and the islands near the southern tip of the world), the energetic cost of flying may be far too high. Darwin had specified that all organisms are driven by a biological imperative to ensure the survival of their particular genome into subsequent generations. If he was right, insects are better off investing energetically in the machinery of reproduction than they are trying to fly in impossible environments.

Indeed, according to Leihy, “Other researchers have looked at insect fecundity. They’ve found that species that are flightless have a higher reproductive output.”

Case closed, probably. Score one point (not two) for Darwin.