What are the four main points of Darwin's natural selection?
Variation, inheritance, high rate of population's growth, differential survival and reproduction.
Within a population, some traits can be expressed in various ways and make individuals look and behave differently. It can be hair color, body size, eyes color, reaction while facing a danger, ...
Heritable traits are transmitted to the next generation.
High rate of population growth:
At each generation, the population produces more offspring than what the local environment can support. It leads to substantial mortality.
Differential survival and reproduction:
Individuals with the best combination of traits to survive in the actual environment will produce more offsprings for the next generation
Individuals with the best combination of traits will have a survival and reproductive advantage. As a matter of fact, their traits are more likely to be transmitted to the next generation. This process will change the frequency of traits within the population. This process is called natural selection.
Natural selection operate on a trait it it possess heritable variation and confer an advantage in the competition for resources.
See this link for more informations about evolution and natural selection