Climate change spurs evolution, Rice biologists discover

By Jack Lee     10/24/23 11:08pm

A team of Rice biologists have observed the red-shouldered soapberry bug’s rapid evolution in response to Hurricane Harvey on campus. 

Evolution is typically understood to be a slow process occurring over the course of thousands of years. However, associate professor of biosciences Scott Egan said human influences can cause evolutionary adaptations to occur much faster. Such phenomena, Egan said, led his lab group to turn their attention toward the bug around 2016. 

“[Soapberry bugs] have been native and naturally fed on different members of the soapberry plant family for millions of years,” Egan said, “But as we humans do, we like to move things across the planet. One plant we’ve recently moved to the southeastern United States is a plant called the golden rain tree … this beautiful plant, which is from Southeast Asia, is a member of the soapberry family as well. The bugs that feed on native soapberry plants here saw this new resource, and they shifted onto it about 60 years ago.”

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To take full advantage of the new food source, Egan said the length of the beaks of soapberry bugs shortened in an evolutionary response to the golden rain tree’s short distance to the seed.

“These bugs are seed predators, they eat the seeds inside the fruits, and so if you have the right length of beak, you can get at the seeds inside,” Egan said.

Mattheau Comerford, then a graduate student in Egan’s lab and now a postdoctoral researcher at the University of Massachusetts Boston, said red-shouldered soapberry bugs could be found across Houston, even on campus.

“It was kind of like Darwin’s finches in our own backyard,” Comerford said. “Our first goal was to set out and document what the natural history of the insect was in the area.”

Comerford, who was lead author of the study, said the team established sites throughout Houston and routinely visited them to collect red-shouldered soapberry bug specimens. In addition to the changes in beak length, researchers observed variations in wing length, which Comerford speculated to be genetically linked to the beak length. While the study proceeded, in 2017, Hurricane Harvey struck. Comerford said the bugs were not spared.

“I'd spent about six months finding populations [and] measuring the populations when Hurricane Harvey happened, and it wiped out all the populations,” Comerford said.

Neither, Comerford said, was his car.

“I lost my car in the hurricane. I actually couldn't find another car in the area because everybody was buying cars,” Comerford said. “I ended up buying a car in Connecticut and driving it back down to Texas. But in doing so, I was able to sample soapberry bugs from all the way up to Pennsylvania.”

Comerford said the trip allowed him to observe that soapberry bugs outside their native range exhibited greatly shortened beaks.

“Historically, the soapberry bug is limited to Texas and the tip of Florida,” Comerford said. “[However], that exotic host plant from Asia that’s now invasive has spread all across the United States, so now the [soapberry bug] populations go all the way up to New York. ”

Upon his return to Houston, Comerford said he began to see the aftermath of the hurricane not as a setback but as an opportunity for his research. The team turned their attention from the pre-existing adaptations present in the Houston red-shouldered soapberry bug population to studying the soapberry bugs that resettled flooded areas and found a surprising result.

“The long winged individuals are the only ones that can leave their host plant and find a new host plant, so when we had these localized extinction events, only the long winged individuals were the ones recolonizing [flooded areas],” Comerford said. “As we monitored [the Houston sites], the bugs with longer wings [and] longer beaks stayed there.”

Whether the longer beaks caused by dispersion-motivated evolution, or spatial sorting, present in the post-hurricane Houston soapberry bugs will persist despite the broader trend of shorter beaks is an active area of inquiry, Comerford said.

“Those new populations are very interesting because there's very strong natural selection for shorter beaks,” Comerford said. “It'll be interesting to look at how potentially spatial sorting, which could be making beaks longer, is interacting with natural selection that's making beaks shorter.”

The team’s findings were recently published in the journal Nature Ecology & Evolution. In addition to Comerford and Egan, Scott Davis, a lecturer of entomology at the University of California Davis, and Tatum La, a then-high school student participating in a summer program and now an undergraduate at Stanford University, were credited as co-authors. Comerford said the opportunity to work with local students was one of the highlights of the project.

“One of the cool aspects of this study was that we got to include a high school student in the project, which was really fun and rewarding,” Comerford said. “We also got to work with students from Westside High School because they have a garden that survived the hurricane, and that's one of the sites that had soapberry bugs.”

The red-shouldered soapberry bug, Egan said, is an important model for understanding how human activity will continue to impact life on Earth.

“Under conditions of the Anthropocene, where the planet on average is incrementally warming, scientists predict that hurricanes, droughts, wildfires and other episodic weather events like [Harvey] should both increase in frequency and potentially increase in magnitude over the course of the next few decades,” Egan said. “Understanding what's going to happen to biodiversity is one of the reasons we were interested in studying this process.”

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