Explain the concept of 'accumulation of variations' in a population during sexual reproduction, and how it contributes to evolution. Provide an example.

Accumulation of variations refers to the gradual build-up of small differences in traits within a population over generations. In sexual reproduction, variations arise due to processes like crossing over during meiosis, independent assortment of chromosomes, and random fertilization. Each new offspring inherits a unique combination of genes from its parents, leading to slight variations in their characteristics. 

These small, individual variations, when passed down and compounded over many generations, can lead to significant changes in the overall characteristics of a population. This continuous accumulation of variations provides the raw material for natural selection to act upon, driving the process of evolution. For example, consider a population of bacteria exposed to an antibiotic. Some bacteria might have a slight genetic variation that confers a small degree of resistance to the antibiotic. 

While this resistance might not be immediately obvious, if these slightly resistant bacteria survive and reproduce more effectively in the presence of the antibiotic, their offspring will inherit this variation. Over many generations, the accumulation of such advantageous variations can lead to a population that is largely resistant to the antibiotic, demonstrating evolution.