The definition of a theory is something that is not proven, but can be supported by evidence. There is such a large body of evidence in support of Darwin's theory of evolution that many scientists argue it should be considered the Law of Evolution. Seven types of evidence in support of evolution as a natural process by which species change are as follows:
1) The fossil record: The remains of ancient life forms has documented how species have changed and evolved from their ancestors to the modern forms living today. For example, we are able to trace our own roots from from primitive hominids to our Homo sapiens. The fossil record shows how the physical attributes of our ancestors changed over millions of years as we evolved into modern humans.
2) Homologous structures: Parallel physical structures seen in related species are evidence of descent from a common ancestor. The classic example given in many textbooks is the anatomy of the arms/forelegs/wings of vertebrates. All vertebrates are descended from a common ancestor that had a particular bone structure to it's forelegs. Though that structure has changed in different vertebrates through millions of years of adaptation to different selective pressures, the same basic skeletal structure is observed in all vertebrates. Though the modifications are different, even the flippers of whales and wings of birds have the same basic skeletal arrangement as the human hand.
3) Embryology: Embryos of animals give us a fascinating glimpse into evolutionary history. In all animals, embryos undergo the same basic initial stages of development. This is evidence that this process of development was inherited very early on in evolutionary history by the common ancestor of all animal life. Looking at the embryonic stages of an animal often reflects the stages of its evolutionary history. Often when a characteristic has been lost, that feature is still present in the early embryo. For example whale embryos develop hind legs, which are then reabsorbed as the embryo matures. Along the same lines, human embryos initially develop tails which are later lost.
4) Genetic evidence: Just as tracing the physical changes in animals over time can demonstrate the process of evolution, genetics can reveal similar information. By looking at the amount of difference in certain genes or even whole genomes of different organisms, we can show that those closely related species (those descended from a more recent common ancestor) have fewer differences in their genetic code than species more distantly related. Darwin observed the phenomenon of evolution but he did not propose a mechanism for how natural selection caused organisms to change in response to their environment. We now know that it acts through differential success and inheritance of different genes. Therefore we can use the genetic code to examine how organisms from different lineages differ from one another and infer how they radiated from their common ancestors.
5) Biogeography: Looking at the distribution of organisms across the globe and their physical and genetic traits also provides evidence of evolution. For example, we may observe that organisms in remote or isolated areas share unique traits not observed elsewhere, because their gene pool has been isolated and responding to the selection pressures of their particular environment without influence from other populations. Furthermore, both Darwin and Alfred Wallace noticed similarities in species now hugely isolated from one another, such as in South America and Africa. They concluded that these species shared common ancestors before continental drift separated the continents by the Atlantic Ocean.
6) Vestigial structures: Why does an ostrich have wings if it can't fly? This and other useless structures observed in nature are evidence of evolution. The ostrich evolved from a flying ancestor but over time evolved to defend itself from predators and escape using it's large size and ability to run. Because developing strong flight muscles is energetically difficult and flight may have limited the birds' size or speed on the ground, the ability to fly was selected against over millions of years. However, the vestigial wings are still present as evidence of the form of the ancestors from which they evolved.
7) Direct observation: For organisms with very short generation times, evolution can be observed happening in real time. For example, many studies have been done in which two groups of moths were reared in the lab. One group needed to fly to reach the food source, the other was fed on the ground. Over several generations, the group of moths fed on the ground lost their wings because there was no longer selection pressure for flight and wings are energetically expensive to develop. We can also directly observe evolution in the development of resistance in bacteria to antibiotics. The presence of an antibiotic creates a strong selection pressure on the bacterial community such that only those bacteria able to withstand the drug are able to reproduce and pass on their genes (including the genes for resistance).
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