The terms “evolution” and “natural selection” are often conflated, as the two concepts are closely related. They are not, however, synonymous. Natural selection refers to the process by which organisms better suited for their environment are more likely to survive and produce offspring, thereby proliferating those favorable genetics in a population. Evolution is defined more broadly as any change in the genetic makeup of a population over time. As expounded by Darwin, natural selection is a major driving force of evolution, but it is not the only one.
Genetic drift, for example, is another mechanism by which evolution may occurs. Genetic drift occurs when allelic frequency is altered due to random sampling. It is evolution by chance, and the smaller the population, the more significant the effects on genetic distribution due to sampling error. For example, a population bottleneck, which occurs when an event such as a natural disaster dramatically reduces the size of a population, can result in the elimination or significant reduction of a trait within a population, regardless of how beneficial that trait may be to survival or reproduction. Thus evolution can occur without natural selection.
Source: Boundless. “Misconceptions of Evolution.” Boundless Biology. Boundless, 26 May. 2016. Retrieved 12 Jul. 2016 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/evolution-and-the-origin-of-species-18/understanding-evolution-124/misconceptions-of-evolution-499-11725/