In science, the term "evolution" refers to the change in the characteristics of a species over several generations and relies on the process of natural selection.
The theory of evolution is based on the idea that all species are related and slowly change over time and it relies on there being genetic variation in a population which affects the physical characteristics of an organism.
A few of these characteristics will possibly give the individual an advantage over other individuals within that population which they can then pass on to their offspring.
Genetic variation refers to the difference in DNA sequences between individuals within a population.
One example of evolution in nature is the peppered moth. Peppered Moths are normally white with black speckles across the wings. They went from being mostly light-colored to being mostly dark-colored during the industrialization of 19th-century England. This shift in color was attributed to natural selection: Dark-colored moths were apparently better camouflaged on pollution-darkened tree trunks and therefore more likely to survive predatory birds. Darwinism is the most famous and most accepted theory of evolution and it was developed by the English naturalist Charles Darwin. This theory states that all species of organisms arise and develop through the natural selection of small, inherited variations that increase the individual's ability to compete, survive, and reproduce.
In this theory, selective pressures include competition for food and other resources, disease, and other environmental factors.