We also observe that children tend to look like their parents, puppies like their parents, and so forth. There is some mechanism for inheritance of appearance. Since organisms vary, some of this variablity is inherited, and those organisms best suited to their local environments are the ones to survive to produce offspring, traits that improve fit to local environmental conditions will tend to be passed on to future generations. Conversely, those traits that are locally disadvantageous will tend to be lost. This is evolution by natural selection.
Natural selection is easy to study - it occurs over fairly short periods of time.
The classical textbook example of natural selection at work is observation of changes in the wing color of English moths as soot from industrial polution darkened the trees they lived on. Moths that spent their days hiding on the bark of trees had wings that were camouflaged, colored to match the color of the bark of the trees. As soot darkened the trees, moths that happened to have slightly darker wings tended to be less likely to be picked off by birds, since their darker wings stood out less agaist the newly dark background. This variablity happend to be heritable, thus through natural selection, the typical wing color of these moths shifted over time. Conversly, later in the 1900s, as polution from soot was reduced in England, tree bark became lighter, and natural selection shifted moth populations back to lighter wing colors.