So here are the standard arguments for creationism, and the standard rebuttals from the scientific consensus, starting with my favorite:
Evolution is just a theory, not a fact. This is an easily digestible sound bite intended to show that evolution is just an unproven hypothesis, like any other, and thus should not be taught in schools as if it were fact. Actually, evolution is both a theory and a fact. A fact is something we observe in the world, and a theory is our best explanation for it. Stephen Jay Gould famously addressed this argument by pointing out that the fact of gravity is that things fall, and our theory of gravity began with Isaac Newton and was later replaced by Einstein's improved theory. The current state of our theory to explain gravity does not affect the fact that things fall. Similarly, Darwin's original theory of evolution was highly incomplete and had plenty of errors. Today's theory is still incomplete but it's a thousand times better than it was in Darwin's day. But the state of our explanation does not affect the observed fact that species evolve over time.
The next argument you're likely to encounter states that Evolution is controversial; scientists disagree on its validity. Creationists have latched onto the fact that evolutionary biologists still have competing theories to explain numerous minor aspects of evolution. Throwing out evolution for this reason would be like dismissing the use of tires on cars because there are competing tread designs. Despite the claim of widespread controversy, no significant number of scientists doubt either the fact of evolution or the validity of the theory as a whole. Creationists often publish lists of scientists whom they say reject evolution. These lists are probably true. In the United States, the majority of the general public are creationists of one flavor or another. But the scientific community has a very different opinion: Most surveys of scientists find that 95 to 98 percent accept evolution just as they do other aspects of the natural world.
Creationists also argue that Evolution is not falsifiable, therefore it's not science. One of the fundamentals of any science is that it's falsifiable. If a test can be derived that, if it were to fail, falsified a proposition, then that proposition meets a basic test of being a science. Something that cannot be tested and falsified, like the existence of gods, is therefore not a science. Creationists accept this to the point that they use it as an argument against evolution's status as a science.
In fact, evolution could be very easily falsified. Evolutionary biologist JBS Haldane famously said that a fossilized rabbit from the Precambrian era would do it. Another way to falsify evolution would be to test any of the innumerable predictions it makes, and see if the observation doesn't match what was predicted. Creationists are invited to go through all the predictions made in the evolutionary literature, and if they can genuinely find that not a single one is testable, then they're right.
The next argument to be prepared for is that Evolution is itself a religion. This argument has become increasingly popular in recent years as creationists have tried to bolster their own position by decorating it with scientific-sounding words like intelligent design. And as they try to convince us that their own position is science based, they correspondingly mock evolution by calling it a religion of those who worship Darwin as a prophet and accept its tenets on faith since there is no evidence supporting evolution. Clearly this is an argument that could only be persuasive to people who know little or nothing about the concept of evolution or Darwin's role in its development. This argument is easily dismissed. A religion is the worship of a supernatural divine superbeing, and there is nothing anywhere in the theory of evolution that makes reference to such a being, and not a single living human considers himself a member of any "evolution church."
Creationists also like to argue that Evolution cannot be observed. Part of what you need to do to validate a theory is to test it and observe the results. Although there are evolutionary phenomena that can be directly observed like dog breeding and lab experiments with fruit flies, most of what evolution explains has happened over millions of years and so, quite obviously, nobody was around to observe most of it. This is true, but it misstates what observation consists of. There's a lot of observation in science where we have to use evidence of an event: certain chemical reactions, subatomic particle physics, theoretical physics; all of these disciplines involve experimentation and observation where the actual events can't be witnessed. The theory of evolution was originally developed to explain the evidence that was observed from the fossil record. So in this respect, every significant aspect of evolution has been exhaustively observed and documented, many times over.
One of the most tiresome creationist arguments against evolution tries to claim that There is an absence of transitional fossils. If the ancestor of the modern horse Miohippus evolved from its predecessor Mesohippus, then surely there must be examples of transitional fossils that would show characteristics of both, or perhaps an intermediate stage. I use the horse example because the fossil record of horses is exceptionally well represented with many finds. If evolution is true, shouldn't there be examples of transitional stages between Miohippus and Mesohippus? The creationists say that there are not. Well, there are, and in abundance. You can tell people that there aren't, but you're either intentionally lying or intentionally refusing to inform yourself on a subject you're claiming to be authoritative on. Kathleen Hunt of the University of Washington writes:
A typical Miohippus was distinctly larger than a typical Mesohippus, with a slightly longer skull. The facial fossa was deeper and more expanded. In addition, the ankle joint had changed subtly. Miohippus also began to show a variable extra crest on its upper cheek teeth. In later horse species, this crest became a characteristic feature of the teeth. This is an excellent example of how new traits originate as variations in the ancestral population.The layperson need look no deeper than Wikipedia to find a long list of transitional fossils. But be aware that many species known only from the fossil record may be known by only one skeleton, often incomplete. The older fossil records are simply too sparse to expect any form of completeness, especially if you're looking for complete transitions. It's not going to happen. However, the theory of punctuated equilibrium predicts that in many cases there will be no transitional fossils, so in a lot of these cases, creationists are pointing to the absence of fossils that evolutionary theory predicts probably never existed.
Here's another creationist argument, and when I first heard it I said "What the heck are they talking about??" It's that Evolution violates the second law of thermodynamics. The second law of thermodynamics states that there is no reverse entropy in any isolated system. The available energy in a closed system will stay the same or decrease over time, and the overall entropy of such a system can only increase or stay the same. This is an immutable physical law, and it's true. Creationists argue that this means a complex system, like a living organism, cannot form on its own, as that would be a decrease of entropy. Order from disorder, they argue, is physically impossible without divine intervention. This argument is easy to make if you oversimplify the law to the point of ignoring its principal qualification: that it only applies to a closed, isolated system. If you attempt to apply it to any system, such as a plant, animal, or deck of cards, you've just proven that photosynthesis, growth, and unshuffling are impossible too. Organisms are open systems (as was the proverbial primordial goo), since they exchange material and energy with their surroundings, and so the second law of thermodynamics is not relevant to them. Innumerable natural and artificial processes produce order from disorder in open systems using external energy and material.
In a related vein, creationists also argue that Evolution cannot create complex structures with irreducible complexity. This argument was made famous by Michael Behe, an evangelical biochemist, who coined the term irreducible complexity. Take a complex structure like an eyeball, and remove any part of it to simulate evolution in reverse, and it will no longer function. Thus, an eyeball cannot have evolved through natural selection, as a non-functioning structure would not be a genetic advantage. It seems like it makes sense at face value, but it's based on a tremendously faulty concept. Evolution in reverse is not accurately simulated by taking a cleaver and hacking an eyeball in half. The animal kingdom is full of examples of simpler eye structures, all of which are functional, all of which are irreducibly complex, and all of which are susceptible to further refinement through evolution. For a dramatic visual example of how irreducible complexity can and does evolve through gradual refinement, and yet remain irreducibly complex, take a look at Lee Graham's applet the Irreducible Complexity Evolver at http://www.stellaralchemy.com/ice/.
Another effort to fight science using logic states that It's too improbable for complex life forms to develop by chance. This is the old "747 in a junkyard" argument. How likely is it that a tornado would go through a junkyard, and by chance, happen to assemble a perfect 747? The same argument was made centuries ago by William Paley, except he referred to the exquisite design of a pocketwatch, and pointed out that such a thing is so complex and delicate that it had to have been designed from the top down by a creator. This argument is simply reflective of ignorance of the extraordinary power of evolution's bottom-up design mechanism. Once you have an understanding of multigenerational mutation and natural selection, and also understand how structures with irreducible complexity evolve, there's nothing unlikely or implausible about evolution at all. In fact, genetic algorithms (the computer software version of evolution), are starting to take over the world of invention with innovative new engineering advances that top-down designers like human beings might have never come up with. Bottom-up design is not only probable, it's inevitable and nearly always produces better designs than any intelligent creator could have.
You should also be prepared to hear that Evolution cannot create new information. Based on a misinterpretation of information theory, this argument states that the new information required to create a new species cannot suddenly spawn into existence spontaneously; new information can only come from an outside source, namely, an intelligent creator. This particular argument doesn't go very far, since any genetic mutation or duplication can only be described as new information. Not all of that information is good. Most of it's useless, but once in a blue moon you get a piece that's beneficial to the organism. New genetic information is observed in evolutionary processes every day.
For a final blow from the logic department, be ready for the argument that Evolution does not explain some aspects of life or culture. This is an argument which is really just a logical fallacy: that since evolution does not explain everything, it is therefore entirely false. Evolutionary biologists are the first ones to stand up and say that there are still plenty of aspects of life we're still learning about. That doesn't make the things we've already learned wrong. It's also increasingly common for creationists to point to things that have nothing to do with the origin of life and speciation, like the Big Bang and the age of the earth, and argue that since the theory of evolution does not explain those things as well, it is therefore false. This is an even greater logical fallacy. Theories explain only those observed phenomena they are designed to explain. They are not intended to have anything to do with stuff they have nothing to do with.
Those are the standard arguments. One thing I can't easily prepare you for are the non-standard arguments you might get from a creationist who doesn't know his business very well. For example, when evangelical actor Kirk Cameron and Christian author Ray Comfort were given a platform by ABC television in April 2007 to express their beliefs to the creators of the Blasphemy Challenge, they didn't even know the standard arguments and just started throwing random stuff out left and right in a way that's much harder to debate intelligently. Phil Plait of Bad Astronomy had a similar experience when debating moon hoax believer Joe Rogan, and he summed it up quite aptly by pointing out that it's easy to know the science better than a believer does, but a believer can easily know the pseudoscience way better than you. Stick with what you know, and don't allow an unpracticed creationist who's all over the place to steer you off the track.
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