mukulv wrote:
Loggerhead turtles live and breed in distinct groups, of which some are in the Pacific Ocean and some are in the Atlantic. New evidence suggests that juvenile Pacific loggerheads that feed near the Baja peninsula hatch in Japanese waters 10,000 kilometers away. Ninety-five percent of the DNA samples taken from the Baja turtles match those taken from turtles at the Japanese nesting sites.
Which one of the following, if true, most seriously weakens the reasoning above?
(A) Nesting sites of loggerhead turtles have been found off the Pacific coast of North America several thousand kilometers north of the Baja peninsula.
(B) The distance between nesting sites and feeding sites of Atlantic loggerhead turtles is less than 5,000 kilometers.
(C) Loggerhead hatchlings in Japanese waters have been declining in number for the last decade while the number of nesting sites near the Baja peninsula has remained constant.
(D) Ninety-five percent of the DNA samples taken from the Baja turtles match those taken from Atlantic loggerhead turtles.
(E) Commercial aquariums have been successfully breeding Atlantic loggerheads with Pacific loggerheads for the last five years.
OFFICIAL EXPLANATION
The argument uses the premise that Baja turtles and Japanese turtles share ninety-five percent of their DNA to conclude that Baja turtles hatch in Japanese waters 10,000 kilometers away. This sounds like convincing statistical evidence unless you realize that many organisms share DNA. For example, humans and chimpanzees share about 98% of their DNA (we share about 75% of our DNA with dogs, for that matter). Since Baja and Japanese turtles come from the same species, it is not surprising that they would share a high percentage of their DNA. Regardless of whether or not you saw this connection, you should have been skeptical of the reference to juvenile turtles traveling 10,000 kilometers. Such a lengthy trip by a juvenile animal is unlikely, and calls into question the soundness of the argument.
Answer choice (A): This answer does not impact the argument because no details—DNA or otherwise—are given about the turtles at these nesting sites off the Pacific coast of North America.
Answer choice (B): The fact that Atlantic turtles have nesting and feeding sites no more than 5,000 kilometers apart does not attack the argument because the argument is about Baja turtles.
Answer choice (C): This answer attempts to weaken the argument by inducing you to conclude that if the Japanese hatchlings are declining but Baja sites are constant, then the Baja sites cannot be supplied by the Japanese hatchlings. But, the answer choice moves from the number of hatchlings to the number of sites. Even with a declining number of hatchlings, the number of sites could remain constant, albeit with fewer turtles at each. Because of this possibility, the answer does not undermine the argument.
Answer choice (D): This is the correct answer. The answer shows that all turtles in the argument have the same ninety-five percent DNA, meaning that the Baja turtles did not have to take the 10,000 kilometer trip.
Answer choice (E): The breeding between species was not an issue in the stimulus.