Charles Darwin was a great british scientist and biologist who created the Theory of evolution. Actually, he published « On the origins of species » in 1859.
In the living being, the evolution is influenced by 4 major factors : the mutations, the natural selection, the genetic drift and the speciation.
The natural selection is one of the most important factor for the mechanism of evolution. It's a process by which some heritable traits that make it more likely for an animal/ organism to survive and then be able to reproduce become more common in a population over successive generations
For example,there are two butterflies on a tree. The first butterfly is white and the other is black. The bird sees the white one first so the white butterfly is eaten and the black one (which is hiding thanks to its color) survives. The black one will reproduce with other butterflies, he will give other black butterflies and this species will survive.
The speciation is a natural phenomenon that leads to the creation of new species. It happens when 2 different populations coming from the same specie can't reproduce themself. Thus, the population didn’t mix for a long time and it leads to that the 2 species can no longer have babies.
This was already observed in the Galapagos islands. A species of finches were separated when the sea level rises. In the end, new species appeared and they were different depending on their island environnement.