The Precambrian time refers to the time 4.5 billion to 542 millions year ago. This period is characterized by having reduced atmosphere and included reduced compounds. Scientists believed that atmosphere at that time was devoid of free oxygen and CO2 was present in excess. Before the complex life evolved on the Earth, the Precambrian Earth's ocean contained dissolved carbon dioxide.
Answer:
Positive natural selection.
Explanation:
The positive natural selection is a type of natural selection that increases the frequency of an allele or trait when it is advantageous for the population. What happened in the example is that the mouth with the slight change in morphology (trait) was more advantageous for the population in the south in relation to the ancestral morphology (still preserved in the population in the north), and therefore its frequency increased. This, in turn, is due to the fact that the food (prey) is not the same in the two habitats (north and south). The specific prey in the south, caused the new morphology to be selected, (increasing the frequency of individuals with the new mouth), becasue probably that trait allows the trouts in the south to hunt more effectively.
