For centuries a group of indigenous people asked their gods for bountiful rain by stunning the cave's fish with a natural plant toxin. Once the fish have succumbed, the Zoque people scoop them into baskets for eating. Live Science scientists have found that the practice has impacted on the fish's evolution.
Perhaps unsurprisingly those fish that are resistant to the anaesthesia survive to pass on their genes, while the others simply die
The religious ceremony is held in the Cueva del Azufre at the end of the dry season during the holy week before Easter. The Zoque grind up the toxic, carrot-shaped roots of the tropical barbasco plant and mix them with lime to form a paste, which they wrap in leaves. They place the bundles about 110 yards (100 meters) into the cave to poison its waters and anesthetize fish, which the Zoque believe are gifts from gods that inhabit the underworld. The collected fish supplement the meals of the Zoque until crops are ready for harvest.
Michael Tobler, an ecologist from Oklahoma State University Tobler and his colleagues were in the area investigating the Atlantic molly (Poecilia mexicana), to figure out how these fish made their way from the surface all the way underground. And once in the dark reaches of the cave, Tobler wondered how they survived in the cave system despite the presence of toxic hydrogen sulphde there.
They learned about the ceremony in 2007 and so to see whether it influenced the evolution of these fish, the researchers collected specimens from the annually poisoned waters as well as areas upstream that hadn't been affected by the ritual. They next placed barbasco root toxin into tanks holding the fish.
Fish exposed to the annual ritual indeed proved more resistant to the toxin than fish that lived elsewhere, able to swim in poisoned waters for roughly 50 percent longer. As such, the poison from the ceremony apparently has over time helped select fish that can tolerate it — fish that cannot get captured and killed by the Zoque.
Ironically the fish do not seem to be good eating: "We actually got to eat some of these cave fish," Tobler "They're not very good, by the way.”
Religion aiding evolution who’d a thunk it?