Limits on population - Individuals can t all be supported by the limited resources of the environment. Predators and disease also limit populations. Survival of the fittest - differential reproductive success. Most favorable combination of traits is likely to be passed on to offspring. Over many generations, population becomes better adapted to the environment.
Fittest organisms reproduce more.
Two groups that are geographically separate will adapt uniquely to their surroundings and after time will form separate species. Variation comes from mutations, and this variability is acted on by natural selection. The sediments harden to form sedimentary rock and minerals replace the remains, thus preserving details Teeth and bones are common, and can be used to deduce size, gait, where muscles attached, etc. Rock layers have oldest on bottom, but the layers can be disturbed, so geologists age by mineral content and index fossils, which are common in rocks of a certain age. Can also compare layers that are in different areas.