The term evolution is generally used to refer to the process by which life on Earth has changed – and continues to change – over time. More specifically, it’s used to describe the gradual alterations and tinkering of gene pools and organismal structures over generations.
A substantial body of evidence posits that all of the life forms that make up the ecosystem of the Earth share a common ancestor, and that all species are descended from other species. In other words, every organism on the planet is ancestrally connected.
In order to fully appreciate the process of evolution, one has to appreciate the mechanisms by which it works. A number of concepts are relevant in this respect, some of which include gene flow, genetic drift, mutation, and natural selection. Of these, the latter one is typically given the most attention, as it plays a unique role in shaping the ecosystem of the Earth.