Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life
Evolution only occurs when there is a change in gene frequency within a population over time. These genetic differences are heritable and can be passed on to the next generation — which is what really matters in evolution: long term change. Example:
Most of the beetles in the population (say 90%) have the genes for bright green coloration and a few of them (10%) have a gene that makes them more brown.

Some number of generations later, things have changed: brown beetles are more common than they used to be and make up 70% of the population. The changing color is definitely evolution: these two generations of the same population are genetically different. But how did it happen? Mutation, genetic drifts,migration and Natural selection-all of these mechanism can cause changes in the frequencies of genes in populations, and so all of them are mechanisms of evolutionary change. However, natural selection and genetic drift cannot operate unless there is genetic variation — that is, unless some individuals are genetically different from others. If the population of beetles were 100% green, selection and drift would not have any effect because their genetic make-up could not change.

Darwin Observation based on:
Observation 1: Members of a population vary in their inherited traits.
Inference 1: Those whose inheritated traits leave a higher % of surviving and reproducing in their environment, leave more offspring and this is an unequal reproductive success.
Observation 2: All species produce more offspring that can be supported by their environment. Many do not survive. (Remember T Malthus)
Inference 2: This unequal ability leads to an accumulation of favorable traits in a population over time, and this is called “natural selection’. This increases the match between organisms and their environment