Move over Darwin, there’s a new type of evolution in town.
Called ‘spatial sorting’, it’s a rapid form of evolution focusing on dispersal rather than fitness, which turns many of our ideas of Darwin’s natural selection on their head.
A new paper looking at Hurricane Harvey, a Category 4 hurricane that made landfall in Texas and Louisiana in August 2017 and the red-shouldered soapberry bugs (Jadera haematoloma) has been published in Nature Ecology and Evolution.
“With the profound and rapid changes we’re seeing with the environment, movement is becoming critical for species’ survival,” said Rice University evolutionary biologist and senior author of the study Dr Scott Egan.
“The takeaway from this study is that while natural selection is still incredibly important, there’s another form of evolutionary change that’s directly related to movement, and it could make a huge difference in the evolution of organisms.”
Natural selection is the idea of survival of the ‘fittest’ – in this case fitness means the longer you can stay alive and successfully mate – which over millennia creates species that are perfectly suited for their ecosystem.
Spatial sorting on the other hand is about how a creature can best expand or disperse into new areas. Those with the best dispersal abilities (for example large wings) are the creatures that end up colonising fastest. But this doesn’t happen over long timeframes – because the evolution is happening over space, not time, it can occur in just a few generations.
“This is an understudied and underappreciated form of evolutionary change that has emerged from the invasive species literature,” said Eden.
“For 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.”
The team was lucky enough to have been collecting data about soapberry bugs at 15 sites around Houston, Texas about 10 months before Hurricane Harvey. When the hurricane rolled through, it flooded 11 of the 15 sites and the bugs experienced local extinction events.
But what surprised researchers was that within three months bugs with longer wings and longer beaks had begun making it back to the sites.
While long wings makes sense for the spatial sorting theory, long beaks don’t add up.
“Looking 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,” said Dr Mattheau Comerford, who worked with Egan on this research but now works at the University of Massachusetts Boston.
“So 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.”
And because these long-winged, long-beaked bugs stuck around, it meant that “spatial sorting was having an effect that was much stronger than the effect of natural selection in that moment,” Comerford said.
“That is completely novel in this experiment.”
The researchers suggests that spatial sorting is only going to become more common as climate change causes more extreme weather events, but we can’t tell yet how exactly this will play out.
“The hallmark of the Anthropocene, and all of the changes that we’re 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,” says Comerford.
“This study shows there is a force out there that we haven’t been accounting for that could make a huge difference in the survival and evolutionary change of organisms on the move.”