NPR had a story yesterday about a fish called tomcod that are evolving to better survive the highly polluted environment of the Hudson River in New York. There are a lot of PCBs (stands for poly-chlorinated biphenyl) that end up in the Hudson. These molecules mimic hormones (substances in our bodies you can take pills for, like melatonin or estrogen), but don’t actually perform the roles of hormones. They are therefore called endocrine disruptors (or EDs).

Some of the fish in the population there are better at surviving the EDs than others because of the shapes of their proteins. These proteins are coded for by DNA, so it is called a genotype. Since DNA is what is passed on to offspring, and different forms of specific genes, or  alleles (like blue eyes vs. green eyes), make a difference in how well individuals survive and pass on their genes, we can measure evolution by the changes in how often one or another allele shows up in a population. (Actually, changes in allele frequency can also tell us about just random dumb luck – called drift or draft by evolutionary biologists – or small population effects, Allele frequency changes are cool.)

This story is a happy one for the tomcod – they’re surviving! There are downsides, though. The allele that makes them more resilient to PCBs makes them more sensitive to changes in oxygen or temperature. If you have ever had a pet goldfish, you know that matters to fish. It also means the PCBs can build up in the fish, which get eaten by other, larger fish, which find their way to sushi bars. Unfortunately, PCBs don’t seem to be good for people.

A new paper in the journal Science explains the molecular mechanisms of how these survivor fish do their thing.