In 1986, a nuclear power accident in chernobyl, ussr (now ukraine), led to high radiation levels for miles surrounding the plant. the high levels of radiation caused elevated mutation rates in the surviving organisms, and evolutionary biologists have been studying rodent populations in the chernobyl area ever since. based on your understanding of evolutionary mechanisms, which of the following most likely occurred in the rodent populations following the accident?
a) mutation caused the fixation of new alleles. b) mutation caused genetic drift and decreased fitness. c) mutation by itself led to major changes in rodent physiology over time. d) mutation led to increased genetic variation and a higher rate of evolutionary change.

D) Mutation led to increased genetic variation and a higher rate of evolutionary change.

Explanation:

A) Mutation caused the fixation of new alleles. If new alleles appeared one of the main mechanisms that would help the "fixation" of the allele would be natural selection. If the allele is not beneficial or harmful it would most likely disappear.

B) Mutation caused genetic drift and decreased fitness.  Even Though this could happen, due to the harsh environment caused by severe radiation less fitness individuals are less likely to survive.

C) Mutation by itself led to major changes in rodent physiology over time. The fact rodents are mutating this does not imply changes in physiology, mutation can be harmful, beneficial or neutral, and those changes are not necessarily physiological.  

D) Mutation led to increased genetic variation and a higher rate of evolutionary change. Radiation is known to be a potent mutagenic. This would lead to an increase of genetic variability at a faster rate. When this mutations are spread through the population, the population evolves. Remember that individuals change but the population is the one that evolves because the trait needs to be passed onto the next generations and individuals.

it would be c. xylem.