Toxic Avengers: Pollution Drove Fish Evolution

A mature Atlantic tomcod collected from the Hudson River. These tomcod evolved to handle excessive amounts of industrial pollutants, like PCBs and dioxin, in the water.

MELISSA BLOCK, Host:

Scientists have discovered a strange fish that lives in a soup of some of the industry's worst pollutants. The fish are found in rivers in New York and New Jersey.

Between 1947 and 1976, high levels of extremely toxic chemicals

were dumped into the Hudson River. By the 1980s,

about 95 percent of fish in some areas had liver tumors.

KATEY SAGAL: (As Leela) Look at that lake. One dip in that toxic muck and your DNA will be permanently mutated. Jump in and go for a swim.

JOYCE: In New York and New Jersey, it's not a lake full of toxic muck, but the Hudson and nearby rivers. The weird fish in question are called tomcod. They look like a regular cod but smaller. These tomcod are up to their eyeballs in dangerous chemicals - PCBs and dioxins that General Electric companies dumped into the Hudson from 1947 to 1976. By the 1980s, about 95 percent of those fish in some areas had liver tumors.

But toxicologist Isaac Wirgin at New York University found that some populations of the exposed fish were doing OK.

ISAAC WIRGIN: And it turns out that the more we were dealing with these things, it became apparent that they actually were very resistant to PCB and dioxin.

JOYCE: Here's what had happened. In some fish, pollutants entered the nucleus of cells, where they distorted the DNA instructions from one particular gene. The fish got sick. But some tomcod, just by chance, had a version of that gene that tolerates PCBs and dioxin. So over time, fish with the resistant gene did better than fish without it, and pretty much took over.

Technically they're not mutants - the chemicals just gave one genetic group an advantage over the rest. So, some survived. But Wirgin says there's a downside to that.

WIRGIN: Normally, these levels of PCBs or dioxins would kill these types of organisms, but here they survive and they're prime prey.

JOYCE: Prey for bigger fish, which absorb the pollutants in the tomcods and pass them up to whatever, or whoever, eats them.

The research appears in the journal Science. And a toxicologist who studies fish says it makes an important point.

RICHARD DI GIULIO: That pollution has driven evolution.

JOYCE: That's Richard Di Giulio at Duke University. He says it's happened in North Carolina, too, with something called killifish. They evolved resistance to another pollutant, polycyclic aromatic hydrocarbons, from wood preservatives that seeped into the Elizabeth River.

Di Giulio points out another drawback to the evolution of chemical hardiness.