Answer:
Darwin would have said that variation of sizes, tail colors, and tail shapes were due to the guppies developing some unique traits that had made them more preferable to breeders.
The breeders then used artificial selection to pick guppies with these desired characteristics.
These guppie then passed these favorable genes to their offspring.
The guppies without these traits were not selected and hence were wiped out.
These traits are particular to the breeders preferences.
