Final answer:
Mutation is the primary source of genetic variation, supplying the raw material upon which natural selection and genetic drift act. Sexual reproduction further increases genetic diversity by creating unique combinations of alleles. This variation is the foundation of the evolutionary process, driving the diversity and unity of life.
Step-by-step explanation:
The ultimate source of all genetic variation is mutation, which introduces new genetic material to a population's gene pool. This genetic diversity is critical for the processes of natural selection and genetic drift, allowing populations to adapt over time. Mutations in DNA can result in different traits for individuals within a population, and these traits can be advantageous, neutral, or even detrimental to fitness. Sexual reproduction also contributes to genetic diversity by combining alleles in unique ways during offspring formation. Without genetic variation, evolutionary forces such as natural selection would be ineffective, as there would be no differences upon which to act. It is essential to recognize that evolution does not aim for a 'perfect' organism but is a continuous process that shapes populations through the effects of mutation, gene flow, genetic drift, and natural selection. These forces of evolution contribute to the diversity of life and are driven by the base mechanism of mutation providing the raw material for evolution to occur.