Scientists judge theories by their ability to a) explain observed natural phenomena, and b) predict future natural phenomena. Based on this criteria, the theory of evolution through natural selection is a sound scientific theory. For example, the theory of evolution through natural selection effectively explains why bacteria grow immune to certain anti-bacterial medication. Bacteria that are resistant to the medication survive and reproduce, thus spawning a new generation of resistant bacteria. The bacteria that are not resistant do not reproduce. Darwin's theory of natural selection also explains the emergence of new species and various physiological changes that occur within a species over time.
1.) All organisms differ in traits such as size and behavior.
2.) No two individuals are exactly alike.
3.) There was a gradual change in each species.
4.) Fossils were different from the living animals. However, these fossils had many similarities to modern forms of life.
5.) Species on the Galapagos Islands resembled species on the mainland but they were different in certain characteristics.
6.) Some organisms became isolated on many islands.
7.) Most organisms produce more offspring that can survive.