Hybrids between two species can produce “swarms” that flourish (RICHARD PALLARDY, 7/30/2024, Ars Technica)
When two related species overlap geographically, they may form what are called “hybrid zones.” Some of the most obvious hybrid zones occur at the boundaries of divergent ecosystems. A plant species adapted to one soil type may exchange genes with a related plant adapted to another, and their offspring thus develop a population that thrives in an intermediate area with characteristics of both soil types.
These hybrid zones are often quite stable over time, with insignificant introgression, or breeding back, to the parent populations. That’s because the genes that serve the organisms in the hybrid zone may not be particularly useful to those outside of it, so they do not spread more widely.
Sometimes, however, hybridization events become something more. They turn into swarms. The first instance of the term “hybrid swarm” occurred in 1926 in a Nature article about New Zealand flora.
“As far as biologically defining the difference between that zone and a swarm, I’ve been struggling to find a nice, clean definition,” Fant said.
“A hybrid swarm is the ultimate erosion of two species into some other thing that’s a combination of both,” suggested Scott A. Taylor, an associate professor at the University of Colorado who has worked on hybridization in chickadees.
Sic transit species.