Kauai, which carries the nickname “the garden island,” can seem like a paradise even when compared to the other islands of the Hawaiian archipelago. But a drama played out in its fields and rainforest over the space of just five years that tests the limits of evolution—a drama that balanced sex against “gruesome death.”
The story, covered in the latest edition of the Proceedings of the National Academy of Sciences, may on the surface seem like the most minor of puzzles involving nothing more than field crickets and tiny parasitic flies. But as author Amy McDermott and evolutionary biologist Marlene Zuk explain, the change that happened there has something to say about how animals, plants, and whole ecosystems respond when under extraordinary stress. And the limits of evolution’s ability to stay one step ahead of disaster.
When Zuk first began studying the crickets on the island, she had no problem finding them. Field crickets on Kauai did what field crickets do everywhere; they sing. Specifically, the male crickets “sing” by rubbing their wings together, providing an audio cue to attract females and lead to hot cricket on cricket action. But that was before a parasitic fly moved in. A fly with very good hearing. That fly used the same signals that were used by males to attract mates as their primary means of tracking down a cricket, planting some eggs on its back, and … yeah, it gets bad. Maggots eating the cricket alive bad.
By the mid-1990s, Zuk would arrive at locations where the fields had once been filled with singing crickets, only to climb out of her car into silence. Her initial assumption was that the flies had been triumphant, and the subjects of her study had been wiped out.
But then, she walked through a field and found that the crickets were still there. It was just that they had gone silent. The male crickets hadn’t exactly stopped singing, they were still moving their wings the same way they always had, but these crickets had a “defect” that prevented that movement from making the usual noise. A defect that had saved their lives, and maybe their whole species.
What silenced the crickets was a change, a mutation, in a single gene on the male chromosome. That mutation gave the crickets smooth wings that, when rubbed together, made a very pronounced … silence. Before the coming of the flies, that silence was an enormous disadvantage to any male that carried the mutation. They were almost certain to be beat out by singing males when it came ot securing the attention of females and passing those critical genes on to the next generation. But in the land of sound-hunting flies, the silent crickets were kings. What had been an evolutionary disadvantage, became an even greater advantage almost overnight.
Crickets carried on. Though they, and the sound of nights on Kauai, were certainly changed.
Scientists term what happened in the case of the silent crickets “evolutionary rescue.” It’s a 1990s term that hasn’t really gotten a lot of detailed examination until recently. Evolutionary rescue is what happens when conditions are driving a species rapidly toward extinction, but some genetic change appears to save the day. The crux of the article in PNAS (and yeah, scientists pronounce it that way, too) isn’t really so much about the crickets, but about the crickets as just one example of such a rescue and the limits on evolution’s ability to toss out a last minute lifeline to a species in peril.
After all, evolution isn’t directed. That doesn’t mean mutations are “random,” it just means that there’s no force sitting back to say “oh, flies a’coming, better put on the muffler.” The silent gene was something that was likely already present in a small percentage of the cricket population before the flies began their rampage. It just happened to be something that turned out to be helpful in the face of a new menace.
But what if there had been no advance contingent of silent crickets? Or what if developing that trait had required mutations to a number of genes? It seems unlikely that evolution would have coughed up just the right combination of mutations in time. Although … if each change addressed part of the problem. Or if each change at least didn’t make things worse … In any case, that’s just what the scientists in the article are studying.
In a series of experiments, experiments in which some flour beetles faced some pretty dire laboratory conditions, scientists tested not only evolution’s ability to save a population by finding a trump card hidden in genetic diversity, but also looked at another issue—the outsider. Could a small number of individuals who already had an edge on a specific environmental problem move into an area and “save” the local population, at least in the sense that the next generation preserved much of that locale’s own genetic history?
The answer is that evolution can “win” in both cases, but only within limits. The magnitude of change and the speed with which change occurs, has an enormous impact on the ability to find a “new normal” for survivors. An even bigger factor may be the diversity that already exists in a given population. Animals that have a deep genetic pool and a breeding population that regularly mixes up a robust set of options stand a much better chance in the face of change than species were diversity is so low that everyone might as well be clones (looking at you, cheetahs).
And of course all this research is incredibly valuable and immediately critical because of one thing: Us. That is, because of the way that human beings are radically altering the world’s environment through pollution, habitat destruction, and the climate crisis. What evolutionary biologists can discover about the ability of populations to survive and prosper in the face of change will go a long way to defining what kind of world we live in Real Soon Now.
I would encourage you to follow the link and read McDermott’s entire article. She’s done a masterful job of introducing the scope of the problem, the different approaches scientists are taking in trying to plumb evolution’s limits, and why this is very important stuff. And when it starts to get late tonight, go outside. Listen to the crickets. While you can.