Some populations of Atlantic tomcod fish have an allele that makes the fish resistant to toxic pollutants called PCBs. Tomcod populations in several rivers were analyzed for the presence of this allele. Each river had varying levels of PCB pollution. Which of the following results would best support the conclusion that natural selection is influencing the presence of this allele in the tomcod populations?
A. All of the tomcod in each of the rivers have this allele.
B. The percentage of tomcod with this allele remains the same from year to year in each river.
C. The rivers with high PCB levels have larger percentages of tomcod with this allele than the rivers without PCBs.
D. Eggs from tomcod without this allele can hatch in rivers with or without PCBs, and eggs from tomcod with this allele can only hatch in rivers without PCBs.
