An Essay on Evolution
This was originally written as a response to an email questioning evolution and many other things. I felt that it was important enough to put up here as a reference to those who might be interested. I don't claim it to be eloquent or complete, but I hope it does give greater understanding.
Update: December 2008. Apparently this essay is being plagiarized. Please don't do that. Plagiarism does not help you learn. And violation of copyright is against the law. You're welcome to cite it, even quote from it with credit, but don't copy it and claim it's your own. I worked hard on this. Plagiarism is THEFT. And yeah, I did write it about 1997, so it's not like it's not been around.
ARTICLE COPYRIGHT © 1997 Robert Hole Jr ALL RIGHTS RESERVED
Evolution is defined as genetically based change from generation to generation. IF and ONLY if you look exactly the same as both your parents and they look exactly the same as your grandparents and so on back to the beginning of life (however you define that beginning) can you say that evolution has not occurred.
Since I assume you are an organism born on this planet, and that you are a human being, rather than some clonal organism (one that reproduces by budding or similar method), I assume you do not look exactly like both your parents and that they do not look exactly like their parents, and that the differences are, at least in part, genetically based. Therefore, within your own family, evolution has occurred. You have seen evolution. Evolution is a fact, not a theory.
How long evolution has occurred, under what circumstances, and what drives it are theories. Evolution is not the theory, HOW life has been shaped by evolution is the theory. When a scientist says "the theory of evolution", it's not the "theory that evolution occurred," it's the "theory of how evolution occurred" that's being spoken of.
Even so, a scientific theory is a bit different than "I think the moon is made of green cheese." A scientific theory must be 1) falsifiable and 2) not disproved after some investigation.
Falsifiability means that you can disprove it without resorting to supernatural phenomenon. Show me a way to disprove that some god (any god) created the world, that could be done through natural investigation, and I'll say it meets one of the criteria. I haven't seen such a way even offered yet.
So far, the theory evolution by means of natural selection (which was Charles Darwin's theory) has been shown to be the best explanation for the path evolution has taken life. The hypotheses ("sub-theories") of the exact nature of how natural selection works are constantly being modified as we learn more and more about our world. Those hypotheses are constantly challenged and should be (that's how science learns more!). And much of what Darwin and Wallace wrote about in their early work has been found to be incorrect, but the basic premise that evolution occurs through natural selection has not been disproved.
The theory of natural selection says that organisms which do better in a particular environment will pass that ability to do better on to their offspring, and that those offspring will be more numerous than the offspring of organisms that fared poorly (at its most basic, really all it says is that {under natural conditions} the healthy leave more descendants than the unhealthy). So far, all evidence suggests this is true.
Can the effects of natural selection be seen? YES they can been seen. For instance, look in any major magazine or watch the evening news, and you'll see stories about disease causing bacteria becoming resistant to antibiotics. Those disease organisms are undergoing natural selection and evolving toward resistance.
In the case of antibiotic resistant bacteria, in your body, those which are not killed by antibiotics produce offspring even if you get over the disease. At least some of those offspring will have the ability to survive antibiotics, and will pass that on to their offspring, and so on down the years, until all (or most) of the bacteria that cause that disease that can be killed by a particular antibiotic are dead, and all (or almost all) that is left are antibiotic resistant bacteria.
There are many other examples of natural selection that have been observed, much to numerous to go into here, but look in the journals Ecology, Oecologia, or really, just about any scientific journal and you'll see them.
Another rather too common misconception is that evolution and natural selection are random. There is nothing random about either. Once life started down a path, it had to follow that path (though it's more like a superhighway than a garden trail). You're not going to have children that sprout wings. THAT would be random (and silly), because you don't have the capability of passing that characteristic on to your children (maybe in a few hundred generations....).
Mutation is somewhat random - but is strongly constrained by the ability to survive (an organism with a random mutation that leaves off the head of an organism, isn't going to live long enough to reproduce, which is a VERY strong constraint) as well as by what genes it has to work with (you CAN'T make feathers from fur in one mutation, too much has to change in physiology and anatomy to make them).
Within those limits, our children have to look something like us. They will be genetically different, but they must be similar. Each generation will be somewhat different than it's parents, but it will be different. Add up those changes - not random changes, but small changes, directed by natural selection (remember from above: the healthy produce more offspring, which produce more offspring, etc.) - over the course of 3.5 billion years and you can get a human being (or not, if there was not a major cataclysm 65 million years ago, then the dinosaurs may have lasted a lot longer and we would not have evolved).
Regarding complexity, we are not very complex, really. We just think we are. The basic differences between ourselves and an earthworm are in specialization of various parts. An amoeba carries out all the same biological functions as a human, and a few more that we can't mimic. And they have only one cell. Parts of the amoeba cell carry out all the functions that corresponding organs or organ systems in our body carry out.
When scientists work on a "family tree" of life on earth, there are some assumptions made about linking organisms at different points in the fossil record. Don't rely, however, on an encyclopedia to be able to give a complete explanation (look up your favorite religion, or anything else for that matter, and you'll find it's not complete either). That certain organisms are linked, yes. Direct ancestors of one another? Scientists rarely make any such claims. Rather scientists say they are (probably - until better evidence comes along) linked when the more recent organisms show an intermediate form between more recent (or modern) and older organisms.
The assumption made in linking fossil organisms is one that I think you might make as well. The assumption is that if two organisms look similar, they must be related. And that the closer the similarity, the more closely they are related. Do you look more similar to your siblings than to your third cousins? Of course you do. You are more closely related. Do crows look more similar to Blue Jays than to cattle? Yes. Therefore, we assume that crows are more closely related to Blue Jays than to cattle.
We do, of course, assume that all organisms are related in some manner. It is the most simple explanation. Science uses Occam's Razor, the idea that the simplest explanation is most probably the correct one. The alternative to the idea that all organisms are related is that not all organisms are related, and that life started more than once on Earth.
The idea that there is a lack of evidence for the age of the earth and the evolution of life is incorrect at best. There is much evidence, and it is quite conclusive. There is no scientific evidence that the earth is young.
And the evidence for an old earth is what gave rise to modern theories of natural selection. The major reason Charles Darwin and Alfred Wallace were able to come up with the theory of natural selection working over the course of long time periods as an explanation for how evolution had occurred, is because Charles Lyell (a geologist) came up with geologic evidence for the earths' great age. Evolutionary biologists don't assume an old earth, we are told by the geologists that it is a fact that the earth is old.
[Update 2010: It's recently been brought to my attention that there's an error in that last paragraph. While Charles Lyell was important in convincing many, including Charles Darwin, of the ancient age of the Earth, it was James Hutton who really deserves the credit as the founder of historical geology as a science and as the first strong evidence for an very old age for the Earth. I highly recommend a recent biography of Hutton (link to amazon below) that also does a lot to explain the geology and why it is important.
The Man Who Found Time: James Hutton and the Discovery of the Earth's Antiquity]