Somebody once me asked whether it was possible to come up with grand unifying analogy or quote to fully explain the theory of evolution and make it more understandable and accepted by whose who deny it. Unfortunately, while analogies may be useful in understanding the general concepts underlying evolution, I don't think they are much use when it comes to actually accepting the truth of evolution. And this is the case with most fields of science that attempt to explain things that are not, and cannot, be perceived directly and which may even appear to contradict our everyday experiences.
Relativity is truly weird, especially when you talk about curved space/time. Sure, comparing space/time to a rubber sheet and massive objects to a bowling ball rolling along that sheet may help me understand the general idea that somebody is talking about, but at the end of the day it doesn't really help me to understand what space/time really is or accept that it can be somehow distorted by massive objects. That will only come by learning a lot of complex mathematics and performing (or at least studying) tons of experiments. And if I insisted that all theories that describe reality must comport with my "common sense" view of the world, I would never be able to accept the validity of relativity, despite the fact that it is widely accepted among physicists and is actually used on a daily basis for such things as making adjustments to GPS satellites that are further away from the Earth's gravitational pull and therefore run at a slightly different speed than clocks on earth. Seriously weird stuff, but also seriously true.
Quantum mechanics is even worse. It has been said that nobody truly understands it, and yet its principals have been borne out by experimentation and physicists can make accurate predictions based on the various laws that have been discovered regarding it. Of course, the world we can observe with our eyes and ears does not operate on the quantum level, and once again my "common sense" experiences are not a reliable means of judging the validity of quantum mechanics.
Like relativity and quantum mechanics, the theory of evolution describes reality as it occurs on a scale not generally observable by our standard senses. In the case of evolution, the scale has to do with time rather than size or speed or distance. And, just like relativity and quantum mechanics, we cannot rely on our own "common sense" experiences as a guide to determining whether or not it is an accurate description of reality. Once again, however, just like the theories of relativity and quantum mechanics, the theory of evolution provides an explanation as to why the universe behaves the way it does and also lets us make falsifiable predictions as to what will happen in the future.
[As a side note here, let me point out that the word "theory," when used in a scientific sense (like the theory of gravity or the theory of relativity), does not mean an unverified guess or idea. In scientific terms, that would be a "hypothesis". Instead, the word "theory" is used to describe a system of interrelated laws and principles that have been tested, validated and confirmed and that are used to describe a particular area of observed reality. In other words, you don't get to call something a "theory" in science unless it has been proven to be be true.]
To understand evolution, all you really need to know (and I hope I'm getting this right) is that (a) small, random changes are occurring all the time within all biological organisms due to such things as random cosmic ray bombardment, (b) the environment in which most organisms live is constantly changing as well (either due to a change in the environment itself or because the organisms have moved to a different environment), and (c) these two factors frequently combine so that some members of any given species find themselves better suited to the current environment (and thereby survive to pass on their genes to future generations) while other members of that species find themselves less suited (and thereby do not survive to pass on their genes to future generations). Add to that a time span of billions of years for small changes to accumulate, et voila!
The best analogy I have read to help me accept the truth of the theory evolution is the one described in Dawkin's "Climbing Mount Improbable." It doesn't lend itself to a pithy quote, unfortunately, but the general analogy compares the evolution of, say, mammals from their ancient fish-like ancestors to a sheer-faced cliff hundreds (thousands?) of feet high. To somebody standing at the base of the cliff, the very thought of leaping to the top in a single bound is impossible to consider, just like it may be impossible to imagine a fish turning into a mouse. But, the analogy continues, what if you could look at the other side of the cliff and see a gradual slope extending for tens (or even hundreds) of miles in the distance, leading from sea level all the way to the cliff's edge? If you started a journey from the very beginning of the slope, the incline would be so gradual that at no point in your journey would you ever even notice you were rising. You could travel for days, weeks, months and still appear to be traveling on perfectly level ground. And yet, at the end of your journey you would eventually find yourself thousands of feet in the air despite never having made any perceptible leaps whatsoever. Replace "hundreds of miles" in the cliff analogy with "billions of years" in the theory of evolution, and the analogy is complete. The analogy only works, however, if you fully understand the processes involved with evolution in the first place.
Hopefully, this analogy provides with a framework to understand how evolution is even possible, similar to how the bowling ball on a rubber sheet analogy might help somebody understand the concept of warped space. It's not an exact analogy, but it should help (assuming, of course, that somebody actually wants to understand how evolution could possibly be true instead of just rejecting it out of hand). Having said that, let me just address a few of the most common criticisms I have seen and heard lobbed at evolution by those who clearly do not understand how it could be possible:
- If humans evolved from apes, why are there still apes around today? This is an easy one to answer -- humans did not evolve from apes! At least, not from the apes that are around today. Instead, humans and apes both evolved from a common ancestor species millions of years ago and we turned out different from modern apes because we moved to different locations than they did, encountered different challenges than they did, faced different environments over time, etc. It's sort of like asking, "If the English language evolved from Germanic roots, why are there still German speaking people today?"
- If evolution is true, why don't we ever find any "transitional" fossils that are clearly in between two other species? The answer to this is that scientists have actually found many different transitional fossils, especially in recent years. Numerous fossils have been found in the fossil record that show some characteristics of fossils found earlier in the fossil record and some characteristics of fossils found later in the fossil record. The problem is that some people either are not aware of these discoveries (willful ignorance, perhaps) or require impossible standards for "transitional" like a fossil that is half duck and half crocodile, despite the fact that the theory of evolution clearly states that evolution is a gradual process with no sudden leaps from one species to a wholly unrelated species on in a single generation. No duck ever gave birth to an animal that wasn't a duck, but over millions of years what is a duck now may be quite different from what was a duck back then.
- If evolution is true, that means we are just animals and therefore have no reason to act morally toward one another. Well, aside from the fact that this is basically arguing from the consequences (a logical fallacy where you try to disprove something simply by pointing out the possible negative consequences of that thing), I would have to take exception at the "just" part of this criticism. True, evolution means that humans are animals, but why do we have to be "just" animals? A dolphin is not "just" an animal -- it is an animal with a highly specialized, perhaps unique, ability to navigate underwater using sound. An eagle is not "just" an animal -- it's an animal with exceedingly keen vision and the ability to soar through the sky. And man is not "just" an animal, either -- he (or she) is an animal with a highly developed intelligence and moral sense that has evolved over time to help us better survive in our environment. The fact that we are animals doesn't mean we can't be different from other animals in significant ways, and it certainly doesn't mean that we have to act like other animals any more than you would expect an eagle to act like a dolphin (or to act like a penguin, for that matter).
- Evolution is just a "theory" that Darwin made up and scientists have blindly put their faith in it ever since! Actually, no. As mentioned above, the scientific use of the word "theory" (as in the "theory of gravity" and the "theory of relativity") used to describe a system of interrelated laws and principles that
have been tested, validated and confirmed and that are used to describe a particular area of observed reality. Darwin (and others like him) may have first proposed the idea of evolution, but it didn't become a scientific "theory" until it had been thoroughly tested, revised, expanded upon, and confirmed by generations of scientists looking at many different fields for corroboration.
- The odds of a complex organism like a human arising purely by "chance" are as ridiculous as a tornado whipping through a junkyard and assembling a complete, working jumbo jet airplane purely by chance! You're right, that would be rather ridiculous. But the theory of evolution doesn't actually state that everything happened purely by chance. Yes, it requires chance mutations to occur and accumulate over time, but that's just an ingredient in the recipe and not the recipe itself. The actual process of evolution is driven by the pressure of natural selection. It may be chance when one animal develops more hair than another member of the same species, but it's not chance when that hairier animal survives when the climate gets colder and the less hairier animal doesn't.
- Evolution can't explain how life got started in the first place. You are right, it can't. But, then again, neither can the theory of gravity or the theory of relativity. And that's because none of those theories actually claim to answer that question and their validity therefore does not rest on whether they can answer it or not. There is a completely separate field of biology called Abiogenesis that does try to come up with theories to explain how life could first arise (whether from inanimate matter or some other way, such as having been carried to Earth on a comet). Evolution, on the other hand, starts with the assumption that life exists and then explains how it became so diverse.
- But, what about [insert anomaly mentioned exclusively on creationist websites that seemingly "disproves" some tangentially related principal]? I don't have room to mention every single thing that creationists have come up with over the years in an attempt to "disprove" evolution. The important thing to remember, however, is that not only does evolution stand as the best explanation ever devised for every bit of observed biological phenomena, and not only has it shown again and again that it has strong predictive powers, it is also corroborated by many other branches of science. If the theory of evolution were just based on the observed fossil record, then maybe attacking the validity of the fossil record could be an attack on the theory itself. Instead, though, the theory of evolution is based on corroborating observations from the fossil record, from the genetic analysis of living species, from field examinations of species evolving in the wild, etc.
- But, there's no actual proof of evolution! Oh, go read a book. Preferably one written by an actual scientist with a degree from a real university with a degree in a field actually related to the study of evolution. That is, of course, if you actually want to learn all about the proof instead of just repeating what others have told you. Creationists have been shouting "there's no proof of evolution" for over a hundred years, ignoring or dismissing every single bit of evidence that comes along, as if simply stating that something isn't true will somehow make it not true. Or, in other words, yes there is actual proof of the theory of evolution. Lots of proof. So much proof that it could (and actually does) fill entire libraries. You just have to be willing to look at it.