Attacks on evolution from conservative religious believers often include the claim that there is no hard evidence for evolution ever actually occurring. Many people are swayed by such claims, in part because while the claim can be made dramatically and with ease, rebuttals are necessarily time consuming, academic, and far less dramatic. The truth, however, is that there exists abundant evidence for evolution. The difference between analogous and homologous structures provides an interesting way for atheists (and theists who accept evolution) to describe evidence of evolution coming from two directions.
Analogous / Convergent Structures
Some biological characteristics are analogous (also called "convergent"), which means that they serve the same function in different species but they evolved independently rather than from the same embryological material or from the same structures in a common ancestor. An example of an analogous structure would be the wings on butterflies, bats, and birds.
Another important example would be the development of a camera-type eye in both mollusks and vertebrates. This example of analogous structures is especially useful because one of most common claims made by religious creationists is that something as complex as an eye couldn't possibly have evolved naturally - they insist that the only viable explanation is a supernatural designer (which is always their god, though they rarely admit this outright).
The fact that eyes in different species are analogous structures proves not only that the eye could evolve naturally, but that it in fact evolved several times, independently, and in slightly different ways. The same is true of other analogous structures as well, and this is because certain functions (like being able to see) are just so useful that it's inevitable they will evolve eventually. No supernatural beings, whether gods or not, are necessary to explain or understand how eyes evolved multiple times.
Homologous structures, on the other hand, are characteristics which are shared by related species because they have been inherited in some way from a common ancestor. For example, the bones on the front fins of a whale are homologous to the bones in a human arm and both are homologous to the bones in a chimpanzee arm. The bones in all of these different body parts on different animals are basically the same bones, but their sizes are different and they serve slightly different functions in the animals where they are found.
Homologous structures provide evidence of evolution because they allow biologists to trace the evolutionary path of different species, linking them up in the larger evolutionary tree that links all life back to a common ancestor. Such structures are also strong evidence against creationism and Intelligent Design: if there were a deity who created all the different species, why use the same basic parts over and over in different creatures for different functions? Why not use completely new parts that are specially designed for specific and different purposes?
Surely a "more perfect hand" and a "more perfect flipper" could be created if based on parts designed for their specific purpose. Instead, what we have in reality are imperfect body parts — and they are imperfect in part because they are all derived from bones that originally existed for other reasons entirely. The bones were adapted, over long stretches of time, for new purposes that they needed to just barely succeed at. Evolution only requires that one be better than competitors, not that one be the best that's theoretically possible. This is why imperfect features and structures are the norm in the natural world.
As a matter of fact, the entirely biological world can be said to be composed of homologous structures: all of life is based on the same types of nucleotides and the same amino acids. Why? A perfect and intelligent designer could easily create life from a variety of animo acids and DNA structures, all specifically suited for particular purposes. The presence of the same chemical structures in all of life is evidence that all of life is related and developed from a common ancestor. The scientific evidence is unambiguous: no gods or other designers had a hand in the develop of life generally or human life in particular. We are what we are because of our evolutionary inheritance, not because of the desires or will of any deities.
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Evolution & Creationism...
Is evolution a science? Is creationism a science? What is science? Is there evidence for either? Religious fundamentalists often attack evolution, but rarely from a positions of really understanding what evolution is and how it works. This FAQ will not only teach you more about the nature of evolution and evolutionary theory, but it will also explain some of the more common complaints and where they go wrong.