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Climate Catastrophe Produced Instantaneous Evolutionary Change

Spatial Sorting Dominated Bug Evolution After Hurricane Harvey Flooding

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Dense white cloud seen from space spirals outwards from the center of the image. A coastline is barely visible through the cloud.
In 2017, unprecedented rainfall from Hurricane Harvey caused local extinctions of populations of soapberry bugs. This allowed researchers from Rice University and 嘿嘿视频 to observe evolution at work as area were re-colonized by different strains of the same species. (NOAA/NASA GOES project)

With species the world over on the move due to climate change, a unique experiment in the wake of 2017鈥檚 Hurricane Harvey has revealed a way that species can instantly evolve when they move in response to a climate catastrophe.

鈥淲ith the profound and rapid changes we鈥檙e seeing with the environment, movement is becoming critical for species鈥 survival,鈥 said Rice University evolutionary biologist , senior author of  in . 鈥淭he  is that while natural selection is still incredibly important, there鈥檚 another form of evolutionary change that鈥檚 directly related to movement, and it could make a huge difference in the evolution of organisms.鈥

, the  in U.S. history, stalled over southeast Texas and dropped more than three feet of rain over thousands of square miles in a matter of days. Record flooding in and around Houston produced 鈥渕ini extinctions鈥 of insects and other species in areas that remained inundated for 10 or more days.

Study lead author , Egan and co-authors  and Tatum La discovered evolution played out across space rather than time when the insect species  recolonized flooded areas.

Comerford, a former graduate student in Egan鈥檚 lab, is now  at the University of Massachusetts Boston. Carroll is a biologist at the 嘿嘿视频 Department of Entomology and Nematology and La is a Stanford University undergraduate and former high school intern in .

The researchers showed that in the wake of the storm, a form of evolution called  exerted a greater influence on the evolution of J. haematoloma, commonly known as the red-shouldered soapberry bug, than did natural selection.

鈥淭his is an understudied and underappreciated form of evolutionary change that has emerged from the invasive species literature,鈥 Egan said of spatial sorting. 鈥淔or example, cane toads in Australia have exhibited these patterns. And up until the last decade or two, spatial sorting has not really been considered a significant form of evolutionary change.鈥

Left-hand panel shows bugs evolving over time. Right-hand panel shows bugs evolving by hopping from island to island.
Illustration by Mattheau Comerford, Rice University

Natural selection and spatial sorting

Natural selection and spatial sorting each change the likelihood that specific traits will be passed to future generations. With natural selection, change plays out over decades or even millennia as traits become more common or less common depending on which individuals survive and reproduce. With spatial sorting, change occurs when species move into new territory, and traits become more common or less common depending on which individuals are at the leading edge of range expansion.

鈥淚f everybody is moving at different rates, the ones that move the fastest will always tend to end up at a higher density at the range edge,鈥 Egan said. 鈥淪o right at that range edge, you have just a subpopulation of the fastest dispersers. And when they mate, there鈥檚 a tendency for them to . So there鈥檚 this kind of accumulative, increasing rate of dispersal as the range front moves outward.鈥

Prior to Harvey, Comerford spent about 10 months gathering field data about soapberry bugs at 15 sites around the Houston metropolitan area. Native to Texas and the southwestern United States, the bugs are a textbook example of rapid evolution. They feed on seeds by plunging needle-like beaks deep into the fruit of their host plants.

When the  was widely introduced in the United States as an ornamental plant after World War II, it proved an opportune host for soapberry bugs. The trees鈥 seeds are not as deep as those of the soapberry鈥檚 native host plants, and the bugs rapidly evolved shorter beaks to access them. Through decades of study,  this is one of the fastest recorded examples of ecological adaptation.

Southeast Texas is one of the few places in the United States where short-beaked soapberry bugs coexist alongside two native varieties with longer beaks that are adapted to the seed depths of their host plants.

At field sites, Comerford sampled soapberry bugs from the three host plants every two weeks. Prior to the storm, the bugs found on each host were the varieties whose beaks were adapted to match the host鈥檚 seeds. This was expected, given the 鈥渆xtremely strong selective pressure to rapidly adapt beak length to suit the seed pods of whatever host plant they are living on,鈥 he said. 

Two black insects with red patches on the forward sides of their bodies on a finger.
Red-shouldered soapberry bugs (Mattheau Comerford)

Harvey flooded 11 of the research team鈥檚 15 sites. At nonflooded sites, the three varieties of soapberry bugs continued to thrive. At flooded sites, Comerford found soapberry bugs began returning within three months, but only bugs with longer wings reached these sites. It took more than two years for all the sites to be recolonized by the long-winged bugs.

鈥淟ooking at our data and past publications, we saw that beak length and wing length were genetically linked, meaning insects with longer wings also had longer beaks,鈥 Comerford said. 鈥淪o now that we had better dispersers driving spatial sorting, they were dragging along these other traits, the longer beaks, that had nothing to do with dispersal.鈥

The persistence of long-winged, long-beaked individuals on all of the host plants at the recolonized sites showed 鈥渟patial sorting was having an effect that was much stronger than the effect of natural selection in that moment,鈥 Comerford said. 鈥淭hat is completely novel in this experiment.鈥

Egan said the study demonstrates the significance of spatial sorting, because similar scenarios could play out after many types of climate catastrophes, including wildfires, droughts, heat waves and winter storms.

鈥淎ll these episodic types of events are predicted to increase in frequency and magnitude in the coming years,鈥 Egan said. 鈥淎s populations move in response to these events, and as they are moving poleward to get into the envelope to which they are adapted, they鈥檙e going to be subject to spatial sorting.鈥

Comerford said, 鈥淭he hallmark of the Anthropocene, and all of the changes that we鈥檙e seeing with the environment, is that the ability to move and migrate to new locations that are more hospitable is critical for the survival of organisms. This study shows there is a force out there that we haven鈥檛 been accounting for that could make a huge difference in the survival and evolutionary change of organisms on the move. We need to be cognizant of its potential, because this is a phenomenon that is probably going to come into play more as events unfold.鈥

The research was supported by grants from the National Science Foundation (1802715), the American Museum of Natural History鈥檚 Theodore Roosevelt Memorial Fund, the American Philosophical Society鈥檚 Lewis and Clark Fund and the Society for the Study of Evolution鈥檚 Small Grants for Local and Regional Outreach program.

Media Resources

(Nature Ecology & Evolution)

Jade Boyd is Associate Director of Communications at the Wiess School of Natural Sciences, Rice University. Adapted from an by Rice University. 

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