Evolution
Evolution is both a theory and a fact. We know that groups of organisms change over time. This is an established historical fact that has been overwhelmingly determined by the fossil evidence, the details of anatomy, molecular biology and the astonishing depth of new genetic information. Very substantial changes in organisms have been shown in recorded history with the breeding of pets, farm animals, crops and decorative flowers. There is the rapid evolution of bacteria that are resistant to antibiotics. We have seen the evolution of dark colored moths that are highly camouflaged in dirty urban environments. We know that organisms change. This is the fact of evolution.
The theory of evolution is the scientific description of the process by which organisms change. The term "theory" can be used colloquially to indicate an unconfirmed speculation. In science "theory" has a different meaning. Theory is used to indicate fully confirmed scientific laws such as in the "theory of gravity" and Einstein's "theory of relativity." A theory in science must make statements about the world that can be proven or disconfirmed by experimentation or objective observation. Scientific theory is confirmed by that evidence as evaluated by the preponderance of scientists.
What does the theory of evolution say and how has it been confirmed? Darwin described evolution as descent through generations with modifications that are then selectively enhanced in the population if those modifications contribute to the organism's ability to survive and reproduce. This is still an excellent one sentence summary of evolution. In Darwin's time this could be deduced from the externally obvious characteristics of organisms.
Darwin sailed to the Galapagos Islands in 1835. Islands start as rocks with almost no life. This means that when life appears there is often little competition for the available sources of food. The few organisms that arrive on the island or island system often have numerous choices concerning the type of food that they might prefer. If those that have similar preferences mate with each other they will over time form a different species from the others of their type. The ones that are best physically suited for the food choice made (ecological niche) will better survive and reproduce. This creation of multiple species from one starting species is called adaptive radiation. It happened with the finches on the Galapagos Islands, now named after Darwin.
There are at least thirteen species of Darwin's finches on the Galapagos Islands. They are highly related to each other with a major difference being the shape of the bill and whether they are tree or ground dwellers. Some specialize in eating insects, others eat seeds or fruit. Darwin noted, "One might really fancy that, from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."(1) Darwin recognized that all of these species were unique to the Galapagos but similar to a mainland genus. These finches provided major evidence for common evolutionary descent with modification.
There were once fifteen species of Giant Tortoise on the fifteen Galapagos Islands, another example of adaptive radiation. Darwin noted, "The inhabitants...state that they can distinguish the tortoise from different islands; and that they differ not only in size, but in other characters. Captain Porter has described those from Charles and from the nearest island to it, namely Hood Island, as having their shells in front thick and turned up like a Spanish saddle, whilst the tortoises from James Island are rounder, blacker, and have a better taste when cooked."(2) The length of the neck and the shape of the shell varies depending upon the need to reach higher to obtain food on a given island. They evolved from a small number (perhaps a single pregnant female) of a much smaller turtle species from the mainland. The large size came from the fact that there were no other large herbivores competing for that ecological niche.
These Galapagos examples are strongly suggestive of common descent given their extreme similarity. The theory of evolution generalizes this principle and says that all of life shares a common ancestry. This has been well documented with fossil and genetic evidence.
There is no intrinsic contradiction between religion and science unless a given religious believer asserts something about the physical world that a competent scientist can show to be untrue. Some people do assert for religious reasons that common descent does not occur. They are especially concerned about including humans among those that share a common descent with all other living creatures. They feel that if evolution accomplished the creation of human life then the role of God has been repudiated. They are quite correct that the findings of science do contradict all such religious conceptions of the origin of the species. Religious people who think that God used evolution to establish all life will have no problem with the evidence for evolution.
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