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Item EM055003: Environmental changes can lead to the increase of individuals of the same species with certain alleles and the decrease of others (cod fish that have an allele that gives them resistance to a toxin).

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.

Allele Frequency

Niantic River

Hudson River

Allele 1 

0.06

0.99

Allele 2 

0.94

0.01

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.
Distribution of Responses
Chart showing distrubtion of responses for Item 055003
Points Earned
Group Correct Total Percent
Overall 299 558 54%
Grades
  6–8 11 25 44%
  9–12 266 498 53%
Gender
  Male13124354%
  Female14827454%
Primary Language
  English27050254%
  Other61932%

View data table

Distribution of Responses
Chart showing distrubtion of responses for Item 055003
Points Earned
Group Correct Total Percent
Overall 178 294 61%
Grades
  6–8 12 23 52%
  9–12 151 247 61%
Gender
  Male6810962%
  Female9917158%
Primary Language
  English16026461%
  Other6967%

View data table

Distribution of Responses
Chart showing distrubtion of responses for Item 055003
Points Earned
Group Correct Total Percent
Overall 193 281 69%
Grades
  6–8 1 3 33%
  9–12 177 257 69%
Gender
  Male9213270%
  Female9414067%
Primary Language
  English17325468%
  Other101191%

View data table