A big question that originated from Darwin's theory was how the changes he observed came to be. Today, we know that these changes are ultimately caused by mutations. Mutations are changes in an organism's genes. Genes are instructions that all living organisms possess. Every gene is composed of a unique combination of four molecules called nucleotides: adenine, cytosine, guanine, and thymine. The order of these nucleotides, which is called the genetic sequence, determines the function of each gene. The total of all the genes in an organism is called the genome. The genome contains the information coding for all the characteristics of an organism, so a change in one or more nucleotides of a gene (called a genetic change) can disturb a feature of the organism, like the color of the eyes, its height or the way it processes food. This genetic information is inherited, meaning it is passed on from one generation to another.
Mutations in genes can happen spontaneously or in response to stress factors in the environment, but regardless of the cause, all mutations are random. This means that the genetic changes can occur anywhere in the genome of the organism.

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