Evolution is the empirical observation that living organisms do not exactly resemble their parents, and that after many generations, fundamental changes can take place at the genetic level. This is called microevolution. For example, dogs are essentially domesticated wolves (we know this by examining their physiology, morphology, genetics, fossil record, and even behaviour). Dogs, however, no longer resemble wolves (malmutes and huskies are similar to wolves, but not chihuahuas). Microevolution can be demonstrated quite easily in the laboratory. Many genetics courses have students observe the common fruit fly (Drosophila melanogaster) over several generations to record different mutations - microevolution at work.

Time flies like an arrow.
Fruit flies like a banana.
(Drosophila melanogaster)

  Macroevolution is the same thing, but on a much larger time scale. We can observe in the rock record (geology) species becoming new species over millions of years. Thus, evolution is an empirical observation. So, from observing the evidence of evolution, people like Charles Darwin concluded that all modern life evolved from a common ancestor (monophyletic) or a relatively small group of ancestors (polyphyletic). Evolution is as widely accepted as gravity and for good reasons. Evidence from the fossil record, embryology, morphology, taxonomy, genetics, and biogeography all lead to the conclusion that evolution has, and still is occurring. These subjects will be dealt with in greater detail.