What are some reasons why some organisms might be likely to survive more than others?

In a population of organisms all are variations and all are in the same immediate environment. Let's assume that this environment is a desert. Now, members of the population that are able to conserve scarce resources, such as water, better than other members of this population will have higher survivability and reproductive success than the other organisms of the population. This process will confer this trait on their descendents who will be of greater representation in the population leading to the evolution, the change in allele frequency over time, in this population.

Remember, it is not enough to survive, but one must couple that with reproductive success.