Complete the passage by writing a suitable word in each of the spaces.

• The primary source of new alleles in a gene pool is genetic ______ which introduces variation.
• When selection pressures are present, individuals with advantageous phenotypes are more likely to ______ and breed.
• These individuals are then able to pass on their beneficial ______ to their offspring.
• Over many generations, this process of natural selection changes the allele frequencies within the ______.