Continue reading...1. A Theory
Not “just a theory”, but a proper scientific Theory. The problem with saying “Evolution is just a theory” is that in everyday usage theory really means hypothesis or hunch, as in “I have a theory about his motives.” But in science a Theory is an explanatory framework that is tested over and over again and found to be accurate. Proponents of Evolution often compare saying “Evolution is just a theory” to “gravity is just a theory”, because their about the same.
2. Evolution: fact vs. Theory
Evolution is not only a Theory, it is also a fact. The fact of evolution is descent with modification, or changes in genetic variation of the gene pool over time. There is absolutely no doubt in any unbiased mind that this occurs. The evidence is so overwhelming that it’s simply undeniable to all but the most committed dogmatists. But the Theory of Evolution covers the mechanisms of how it occurs, whether via natural selection, sexual selection, genetic drift, or others; the Theory explains how the fact happens. When there is a scientific controversy over evolution, as there are and always will be, it’s about the “how”, not the fact.
3. Some evidence for the fact
Because the good people at Talk.Origins do a better job of explaining the evidence than I ever could, here’s 29+ evidences for Macroevolution. But a short list of the reasons are:
- Biogeography: islands are only inhabited by creatures that can reach them, regardless of how well others might do, etc.
- Vestiges: coccyx, appendix, leg bones in whales, etc.
- Atavisms: the reappearance of a lost traits, such as tails in humans
- The nested hierarchy of the phylogenic tree.
- The relative abundance of transitional forms (reptile to mammal, ape to human, reptile to bird, etc).
- Genetic data.
There are many more than that, and all of those are explained more thoroughly at the link provided.
4. Some discussion of the Theory
Again, Talk.Origins does far better than I could hope to, so here’s a good article introducing evolutionary biology. But the long and short of it is that there are three main ways of describing how evolution happens, that is why gene pools change. They are natural selection, sexual selection, and genetic drift.
Natural selection is the most famous, and is usually phrased as “survival of the fittest”. But this has been misconstrued all sorts of ways, the worst of which being Eugenics (no, the Holocaust did not happen because of Darwin’s theory, Hitler hated it as much as any other creationist). What “survival of the fittest” means is simply that the organisms that have the most successful children will pass on more of their genes, and over time those genes will dominate the gene pool. Any gene that gives an organism an advantage will be “selected for”, and any one that hinders and organism will be “selected against”, increasing or decreasing its representation in the gene pool. Of course, there is no actual force doing the selecting, all that’s happening is a change in an organism’s capability to leave behind descendents, it’s just far easier to phrase it as selective pressures.
Sexual selection is the famous theory of the peacock’s tail, and it attempts to explain all sorts of traits that are disadvantageous to the survival of an organism, but have come to dominate the gene pool nonetheless. It also looks at the consequences of sexual reproduction on a species, and how that species has been shaped by it. This theory is rather nuanced and changes on a species-to-species basis, but most of the time one sex has some kind of trait that makes it worse at surviving to prove how good its other genes are. For example, some female birds are attracted to males with long tails, and a male with an extremely long tail will get the most mates. But he also won’t be able to survive as well, so there’s a balancing act between reproductive and survival success. Again, this is very nuanced, and Matt Ridley’s The Red Queen is a very good discussion of it.
Genetic drift is essentially that gene pools can change all on their own, without any selective pressure, simply by chance. That a gene can “drift” to the point where it is common in the gene pool, even if it confers no survival advantage. This plays a far bigger role in smaller populations, where it wouldn’t be as improbable, but exactly how important it is in evolution as a whole is debated.
It’s important to note that these mechanisms can only act on existing variation, and in fact they destroy variation. But it’s constantly being replenished by mutations. That’s how Evolution works, mutations create new variation, selection picks the good ones. In principle any trait can evolve that comes from any functional, useful (or at least non-deleterious) trait or traits. Once there is something for selection to work upon, all sorts of amazing things can evolve; this world is the evidence for that.
That wraps it up for this short discussion of evolutionary theory. If you want to know more, check out Talk.Origins, or some of the link on my blogroll such as The Panda’s Thumb. Or there’s always books like The Origin of Species by Darwin, or something by Richard Dawkins or any other Evolutionary Biology popularizer.