It's a prime example of natural selection at work.

A species of fish in the New Bedford Harbor has adapted and is now thriving, even though the sediment has been contaminated with polychlorinated biphenyls (PCBs) for decades.

Mark Hahn, a senior scientist in biology at the Woods Hole Oceanographic Institution is the co-author of a new report on New Bedford's killifish population. Hahn tells WBSM's "Mid-Day Live" the large size and genetic diversity of the local killifish population allowed those fish with more resistance to toxins to survive over the years.

Hahn says the upside is the killifish population in New Bedford's harbor is very strong. The downside is the predators who eat the resistant fish are still exposed to the harmful toxins.

The entire report was published in the BioMed Central Evolutionary Journal.