Flying is such an obviously useful skill it is constantly amazing to discover animals that have surrendered the power. Yet many birds and insects are unable to fly, despite being descended from creatures that could. Noting the frequency of these species on smallish islands, Charles Darwin proposed an explanation that has now been confirmed.
Darwin pointed out that islands can be windy places, and flying animals risk being blown out to sea, potentially never finding their way back. The less an island creature flew, he argued, the more likely it was to avoid this fate and survive to have offspring. Eventually, this would lead to a species abandoning the capacity altogether.
Joseph Hooker, director of the Royal Botanical Gardens, was considered Darwin's best friend and first prominent supporter of evolution. Yet even Hooker rejected this explanation, pointing out continents have flightless insects as well. Since then, many biologists have sided with Hooker, sometimes offering their own theories.
Noting the islands of the Southern Ocean are among the windiest places on Earth, Monash University PhD student Rachel Leihy recognized them as a perfect test case for the Darwin-Hooker debate. These islands are so small they present little obstacle to the Roaring Forties and even faster winds at higher latitudes that race around the globe with nothing to stop them. “If Darwin really got it wrong, then wind would not in any way explain why so many insects have lost their ability to fly on these islands," Leihy said in a statement.
In Proceedings of the Royal Society B, Leihy reports half these islands' insect species are flightless. “It's extraordinary,” Leihy told IFLScience. “There are moths there crawling around with wings too small to fly.” Moreover, lack of flight is most common where wind speeds are highest. By contrast, only 5 percent of the insects known worldwide can't fly."
Leihy also investigated alternative explanations, of which the most popular is that stable habitat removes the need to fly between locations for food. Although this has been confirmed as a cause of flightlessness in other environments, such as caves, Leihy told IFLScience the explanation doesn't hold up well in the Southern Ocean. Islands with low seasonal temperature variations don't have more flightless inhabitants, once wind is allowed for. Likewise, low temperatures, which create an incentive to conserve energy, were unimportant compared to wind.
On the other hand, as Hooker's point reveals, some species abandon flight for other reasons. In places like New Zealand, where many birds are flightless, Leihy said the lack of predators is the driving factor.
Along with his overarching theory of evolution by natural selection, Darwin came up with explanations for many phenomena that previously puzzled naturalists. Some proved immediately convincing, but others remained controversial for at least a century. In a few cases, Darwin's answer was eventually rejected, but this is one of many cases where history has vindicated him.