Natural selection is the driving force of evolution in this scenario. The oil spill has directly impacted the tuna population, with those individuals that are less well-adapted to surviving in polluted waters being more likely to die. This results in the surviving tuna being mostly red, as this coloration may have provided some sort of advantage in the polluted waters. This process of natural selection, where certain traits are favored because they provide a survival advantage, is a key mechanism of evolution.
