Science Assessment

Atlantic Tomcod is a type of fish found in the coastal waters of the Northeast United States. Atlantic Tomcod has a gene AHR2 which has two possible alleles, allele 1 and allele 2.

Allele 1 is thought to be associated with resistance to toxins, such as PCBs (polychlorinated biphenyls). PCBs were formerly used as coolants, among other things, but were later found to be extremely toxic. The table below shows the frequencies of allele 1 and allele 2 in the Atlantic Tomcod populations in two different rivers in the Northeast United States.

The AHR2 allele frequencies in the Hudson River population used to look like the current allele frequencies of the Niantic river population.  Could natural selection have caused the allele frequency change in the Hudson River tomcod population?

1. Yes, heavy exposure to PCBs could have caused Hudson River tomcod to increase their toxin resistance in order to survive.
2. Yes, heavy exposure to PCBs could have killed Hudson River tomcod that had Allele 2, while those with Allele 1 were more likely to survive and reproduce. 
3. No, the allele frequency change could only have occurred after a natural disaster.
4. No, the change could only be explained by a new population of fish migrating into the Hudson.