Charles Darwin's first diagram of an evolutionary tree Click to enlarge image
from his ‘First Notebook on Transmutation of Species’ (1837) - on view at the Museum of Natural History Manhattan, NYC. Interpretation of handwriting: "I think case must be that one generation should have as many living as now. To do this and to have as many species in same genus (as is) requires extinction . Thus between A + B the immense gap of relation. C + B the finest gradation. B+D rather greater distinction. Thus genera would be formed. Bearing relation" (next page begins) "to ancient types with several extinct forms". Image: Charles Darwin
Public Domain

Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift.

If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome.

Natural selection is defined as a process by which species of animals and plants that are best adapted to their environment survive and reproduce, while those that are less well adapted die out.

Natural selection ensures only the fittest survive to pass their genes on to the next generation.

In evolution, the process by which those individuals (of a species) with characters that help them to become adapted to their specific environment tend to leave more offspring and transmit their characters, while those less able to become adapted tend to leave fewer offspring or die out, so that in the course of generations there is a progressive tendency in the species to a greater degree of adaptation.