In discussing (read: debating) evolution with one of my classmates, I used homologous structures as an example/proof of evolution. I know it's not the best argument, but I think it can work.
Anyway, the problem was that both of us were a little rusty in the nomenclature of evolutionary biology, and he didn't remember what homology was, and I couldn't give an explanation that didn't assume evolution to be true (which I know is often pointed out by creationists, as it's on the EvoWiki claims page).
So, I ask, what is a good definition of homology and analogy?
I propose this:
Analogy: similarity between two organisms that is due to adaptation to perform a function
Homology: similarity between two organisms that is not due to adaptation to perform a function
So, for example, the shape of the wings of a penguin are similar to the flippers of a seal, as both have adapted to perform the same function, so can be called analogous. But, the bones in the wings of a penguin are still similar to that of other birds, for no functional reason, indicating homology.
I think I accurately prevented circularity in my argument here. I seem to be using adaptation, which is pretty similar to evolution, so I worry I may not have the best wording. If I put the word 'design' in there instead, it doesn't change it much, so I think it's pretty good. I'm aiming for it to address the idea of God reusing designs when there was no functional need to do so.
However, I fear these example aren't as precise as they could be. And also, I'm not sure whether a structure could be considered both analogous and homologous to the same structure. I don't know whether they are usually, but my definitions don't seem to allow it.
So, do you agree with my definitions? If not, can you help me fine-tune these definitions, in any aspects?
(Evolution section would be a good place, but it could also fit in the ID section I guess)
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