Like the last installment of this series, this article is formatted like a Q&A. If you don't want to read all of it, feel free to glance at the questions and read whatever interests you. For the previous article, see here.
Q) But isn't evolution just a theory!
A) This is probably my favorite (by which I mean least favorite) claim against evolution. This claim comes from a misunderstanding of what "theory" actually means in science. Unlike your average detective novel, a theory is not a guess (in science that would be more along the lines of a hypothesis). In order for something to be considered a theory, it has to have a large body of evidence gained through repeated observation and testing. Scientists aren't allowed to throw words like theory around for nothing (though it has happened. *cough cough* string theory *cough*).
Still not sold on theories? It might surprise you then to learn that a lot of other things taken as facts are "just a theory". Like the Earth orbiting around the sun (heliocentric theory), relativity (general/special theory of relativity), the existence of atoms (atomic theory), ect.
Q) How could complex systems arise by evolution?
A) This is commonly known as irreducible complexity. Because the question is somewhat vague, I'll explain a bit more. This is when a complex system cannot have a part removed and still function. To use ourselves as an example, if our lungs are removed the rest of our bodies will fail. Irreducible complexity is easy to explain. The idea is that evolution works by addition of parts. So according to this a system should be able to be "reduced" to previous evolutionary (and still working) levels by taking away a gradual succession of parts. Therefore, if a system cannot have any of its parts removed it could not have evolved.
The logical flaw in this reasoning is apparent. Evolution certainly can add parts, but this argument is completely forgetting that it can also remove and alter parts. A system may appear irreducibly complex because evolution got rid of a part that is no longer needed, but was required in the past. Though we discovered this through the field of genetics quite some time ago, examples of things that appear irreducibly complex can be seen even outside of biology.
Take an arch for instance. There are certainly arches that developed completely naturally, yet if you take out a part of that arch the rest of it will collapse. This is because the arch wasn't made by adding material, but by taking it away.
Q) Doesn't evolution violate the second law of thermodynamics?
A) No. The second law of thermodynamics has to do with entropy, which for our purposes I can refer to as disorder. Using that (simplified) definition, we can say that the second law is that a closed system will not become more organized, and must either stay the same or become more disorganized. Because evolution leads to systems that can be considered more organized, some think that this violates the second law. However this is missing something very large, like the sun for instance. As long as there is energy being put into the system, the second law does not prevent things from becoming more organized.
The sun is an obvious example of energy being put into the planet from an outside source, and this energy is fundamental in our food chain. However the sun is not the only source of external energy that an organism has access to. For starters, we are all in the habit of eating other organisms. Because of the abundance of external energy sources to an organism, the second law in no way prevents development of any that particular organism.
To see part three, visit this link.