It is genes that make evolution possible. All eukaryotic organisms are related in a massive tree of life that includes organisms familiar and unfamiliar, linked through the transmission of genes from generation to generation over the last 2 billion years. The differential transmission of genes over time produces evolutionary change, and ultimately, with speciation, the branching of the tree of life. In addition to being fundamental to biology, this realization of our linkage through genes to every other living thing on the planet, past and present, has profound religious and philosophical implications.
Genes produce traits that either do or do not suit an organism for the environment in which it finds itself. This brings us to the second basic concept, adaptation. Natural selection is adaptive differential reproduction. Some genes produce traits that enable an organism to survive better and reproduce more in a particular environment. Those traits and the genes that affect them will become more prevalent over time, as long as the environment favors them. Those same genes, in a different environment, might not be adaptive, and because environments change, “perfect” adaptation neither occurs nor is expected—but nobody said the concept of natural selection was simple. Natural selection is best understood as survival of the fit enough.