The founding stock of UCSD birds was likely from the nearby mountain populations because some of those birds overwinter on the UCSD campus each year. Population sizes on the UCSD campus have been reasonably large, and there are significant habitat differences between the UCSD campus and the mountain coniferous forests; UCSD campus has a more open environment (making birds more visible) and a lower junco density (decreasing intraspecific competition) than that in the mountain forests. Given this information, which of the following evolutionary mechanisms do you think is most likely responsible for the difference between the UCSD and mountain populations?
a. natural selection.
b. sexual selection.
c. genetic drift.
d. gene flow.
e. mutation.

The UCSD birds which were likely from the nearby mountain and are largely increasing in population in their new habitat due to the more open environment and lower junco density are now different from the mountain populations.

The evolutionary mechanism responsible for this is called Natural Selection.