You study a small population of birds on one very isolated island and notice that the frequency of individuals with a green band on their wings increases over the course of 3 years. The green band appears heritable (offspring resemble parents), but, despite extensive experiments, you cannot identify any selective advantage of the green band. What would an evolutionary biologist be MOST likely to conclude?

A. The change in frequency of the band is most likely due to genetic drift.
B. This is not an evolutionary change because the increasing frequency of green bands is not due to natural selection.
C. The presence or absence of the green band is probably due to environmental rather than genetic differences.
D. There must be some as yet undetermined advantage to having the band to account for its increase in frequency.
E. Mutations causing the green band have increased in frequency over the past 3 years.
F. Gene flow from a mainland population is responsible for the appearance of the green band in the island population.