Which would least likely be a cause of natural selection?
adaptation
evolution
overpopulation
variation

The correct answer is Evolution.

Natural selection is a process by which organisms better adapted to their existing environment survive and make more copies of them through reproduction.

Evolution is the result and not a cause of natural selection.This is because natural selection brings change in the heritable characteristics of the population over many generations. This leads to evolution.

On the contrary, if there is variation in a population, the best trait will be selected by the nature and it will propagate. Similarly, if there is large number of population, there are more  chances of some adaptations becoming desirable over others. So adaptation, overpopulation, and variation are the important factors that cause natural selection.

Thus, evolution is the correct answer.

b. Evolution is the correct answer

Explanation: Natural selection could be a method by that organisms higher custom-made to their existing atmosphere survive and build additional copies of them through replica.  

Evolution is the result and not an explanation for survival. This is as a result of survival brings amendment within the transmitted characteristics of the population over several generations. This results in evolution.

On the contrary, if there's variation in an exceeding population, the most effective attribute is designated by the character and it'll propagate. Similarly, if there's a sizable amount of population, there are additional possibilities of some diversifications changing into fascinating over others. thus adaptation, overspill, and variation are the necessary factors that cause survival.

the sphincter muscles at the end of the digestive tract is the anus.

not only is it colder, it also becomes darker, less salty, and more dense.