Natural selection is a driving force of evolution. In Darwin’s view it was the primary force. According to the modern synthesis and later work, it is one of several important mechanisms.

Natural selection is the differential survival and reproduction of individuals based on the heritable characteristics they possess. Those with favorable traits that aid survival and reproduction are more likely to have offspring who inherit those traits. Those with unfavorable traits are less likely to pass them on because they are less likely to have offspring.

Units of Selection
It is important to understand that natural selection operates either directly or indirectly at several levels, from genes to populations.

Genes - Favorable and unfavorable traits are produced by genes and alleles. Acquired characteristics may influence an organism’s survival, but because they are not heritable, they aren’t significant to natural selection or evolution. Genes that code for favorable traits are more likely to be passed on to the next generation. Genes that code for negative traits may be passed on as well, if they are linked to positive traits or if they aren’t always expressed, as in the case of recessive alleles.

Individuals - Natural selection works directly on individuals. An organism either survives or it dies. It either reproduces or it does not. Every trait is bundled with all of the other characteristics an organism expresses. A favorable trait will not help an organism survive or reproduce if it is overwhelmed by the impact of other, unfavorable traits.

Populations – Although individuals succeed or fail because of their attributes, the results of natural selection are measured in populations, and it is populations that evolve, not individuals. Characteristics and the genes that regulate them become more common or less common in populations over time. Natural selection causes populations to change over time – in other words, it causes evolution.