Answer:
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All living cells need energy to function in order for the chemical reactions occurring in the cells to take place. In humans this energy is obtained by breaking down organic molecules such as carbohydrates, fats and proteins.
For evolution to occur:
There must be genetic variation in a population. This provides variation on which natural selection can act. Then natural selection can act by selecting those species which are better adapted to the environmental conditions. Over time those animals that are best suited to the environment only will survive and reproduce. Those organisms successful genes will be preserved and eventually the gene frequency in the population will change, changing the overall characteristics of a population. This is how evolution occurs.
