According to the theory of natural selection, individuals who survive are the ones best adapted for their environment. This survival is due to favorable traits that give them an advantage in the struggle for existence.

Here's a breakdown:

* Variation: Individuals within a population exhibit natural variations in their traits (e.g., beak size in birds, fur color in mammals).

* Inheritance: These variations are often, at least partially, heritable, meaning they can be passed down to offspring.

* Survival and Reproduction: In a given environment, some traits provide an advantage in terms of survival and reproduction. For example, a bird with a beak better suited for cracking seeds might be more likely to find food and survive.

* Natural Selection: Over generations, individuals with advantageous traits will be more likely to survive, reproduce, and pass on those traits to their offspring. This process, called natural selection, gradually leads to a population that is better adapted to its environment.

In essence, natural selection is the process where individuals with advantageous traits are more likely to survive and reproduce, leading to the gradual evolution of a population that is better suited to its environment.