Given a set of facts for which it is claimed that they constitute evidence for some theory, there are four ways to refute the claim.
Creationist objections to evolution have attempted all four. Creationists have also offered many objections to do none of the above and are therefore irrelevant. It is a dismal record.
A note on the first point. If a thousand alleged facts, all of them logically independent, imply the truth of some statement, it accomplishes nothing to prove that 10 of the allegations are false. Whatever a thousand facts imply will usually be implied just as strongly by 990 facts. It is no hyperbole to say that the scientific community infers the factuality of evolution from many thousands of facts.
Scores of the creationists' objections are catalogued and addressed, with appropriate documentation, at Talk.origins. I will not try to duplicate the work done there, but will offer some of my personal observations on a sample of them.
1. Evolution is only a theory. It is not a fact or a scientific law.
The scientific community believes, for the reasons already presented, that descent with modification from a common ancestor is a fact. It is a theory like gravity is a theory.
Gravity can be tested by repeatable experiments. Evolution cannot.
Every discovery that has any bearing on life's history is another experiment testing the theory of evolution. Every time another fossil is found, evolution is tested. Every time something new is learned about genetics or biochemistry, evolution is tested. Every experiment ever done on DNA has been a test of evolution. Every reproductive event anywhere in the biosphere is another experiment testing the theory of evolution.
The definition of "theory" itself seems to be "evolving." What used to be a standard textbook definition was changed to accommodate an emerging Darwinist dogma.
Whatever the motivation, words have no meaning except whatever is in the mind of the person using them. When the scientific community refers to evolutionary theory, the community is not thinking about a bunch of guesswork.
A widely used reference, Norah Rudin's Dictionary of Modern Biology, defines a theory as: "similar to a hypothesis but usually wider in scope."
If I put a Ford label on an automobile and then tell you that it is a Chevrolet, you might be able to prove that it is not a Chevrolet, but in order to do that you will have to do more than just show a video of me putting the Ford label on the car.
Scientists consider evolution both fact and theory. Creationists cannot prove its nonfactuality by fixating on the "theory" label. Nor can they prove that it is not a fact by quoting a few people who say it is something else.
It is inconsistent to call it a fact while admitting that so much is unknown about how it happened.
No one knows anything about how Jimmy Hoffa was murdered, but there is no serious doubt that he was murdered.
2. Natural selection is based on circular reasoning: the fittest are those who survive, and those who survive are deemed fittest.
Natural selection could be thoroughly discredited without any repercussion on descent with modification. However, it is not a fact that fitness is defined merely in terms of actual survival. It is defined in terms of characteristics enhancing the potential for survival in a given environment, relative to any competition. Natural selection would be falsified if organisms with those characteristics failed to outreproduce conspecifics that lacked those characteristics.
Fossils are dated by a circular argument. They are assumed to be of a certain age and then the rock where they are found are assumed to be that age. The next time a fossil in found in the same kind of rock, it is declared to be the same age.
There are plenty of ways to date rocks independently of whatever fossils they might contain, and creationists either know this or ought to learn it before they carp about circular reasoning.
3. Evolution is unscientific, because it is not testable or falsifiable. It makes claims about events that were not observed and can never be re-created.
Theobald's essay at Talk.Origins presents dozens of ways evolution could be falsified.
It can be reasonable to infer the occurrence of unwitnessed and unrepeatable events. Nobody alive today observed the American Civil War, and it can never be re-created. But there is a monumental body of evidence that is simply inexplicable except by supposing that the Civil War happened. I strongly suspect that any creationist would agree with me, unhesitatingly, that it would be intellectually perverse to doubt that there was a war between the northern and southern states between 1861 and 1865.
In the judgment of people not committed to a certain religious dogma, there is a body of evidence equally monumental and equally compelling for the common ancestry of all life on this world.
Some creationists will claim that the Civil War was not unwitnessed, but that assumes the conclusion. There are no witnesses available for anyone to interview. The evidence for the war's occurrence, and the evidence for the existence of the war's witnesses, are essentially the same evidence.
There is admittedly plenty of evidence for microevolution, but none for macroevolution. Indeed, there is evidence against macroevolution. Evolutionists have declared by fiat that macroevolution is the same thing as microevolution, but they are not the same thing.
Creationists seem to think that this becomes true if they say it often enough. The claim of nonexistent evidence for macroevolution is false. Any evidence for microevolution IS evidence for macroevolution UNLESS a fact is discovered establishing a limit to how much an organism can change over an indefinite period of time. No creationist to my knowledge has suggested what it is that will stop changes from occurring cumulatively until a population of organisms is arbitrarily different from its original form.
4. The scientific community itself is coming to reject evolution.
This is another outright falsehood.
How do you account for all the literature attacking evolution, not on scriptural grounds but on scientific grounds?
The claim is about losing adherents within the scientific community. Anti-evolutionary literature is not being generated by any significant number of people with the scientific credentials needed to critique evolution.
If the theory is not in trouble, why did Scientific American recently publish an eight-page article defending it? We have not seen any articles defending Copernicus.
If there were a concerted effort by a religious sect or some other group to have geocentrism taught in the public schools as an alternative to heliocentrism, the scientific community would respond as it is responding now to attacks on evolution.
5. If the science behind evolution were really solid, evolutionary biologists would not be arguing among themselves over such fundamental issues as they do.
The fundamental issue of evolution is common ancestry and descent with modification. There are no arguments within the scientific community over whether that has happened. The debates are about how it happened.
There is some analogy to the death of President Kennedy. You can find plenty of people who will argue about how much if anything Lee Harvey Oswald had to do with it, but nobody doubts that Kennedy was killed by gunfire in Dallas on Nov. 22, 1963.
The disputes are NOT like those found in all other branches of science.
So what? The other branches are working with different kinds of data. Every branch's data are unique to the branch, and so are the debates. Disputes among chemists are not the same as disputes among non-chemists. Disputes among geologists are not the same as disputes among non-geologists.
6. If humans descended from monkeys, why are there still monkeys?
Because ancestors don't have to die before their descendants can live.
It is not fair to cite this as a typical creationist argument. Serious creationists know it is fallacious and they do not use it.
The ones I have heard make it seem pretty serious to me. However, if what is meant is that these creationists are not well educated and perhaps ought to be ignored on this issue, I can accept that.
7. Evolution cannot explain how life first appeared on earth.
It isn't supposed to. It was developed to explain what happened to life after, and only after, it appeared.
But abiogenesis is typically taught alongside evolution. School children are told that primitive nucleic acids, amino acids and other prebiotic chemicals could have become self-replicating.
That happens to be the current scientific consensus with regard to life's origin. A science class should introduce children to current scientific thinking. Should science teachers pretend otherwise? If you're going to talk about the history of anything, it usually makes sense to say a few words about its origin as well.
If the teacher fails to make an appropriate distinction between origin and history, the fault lies with the teacher, not with the subject. If a physics teacher should claim that Einstein's theory of relativity is expressed in Maxwell's equations, that does not refute either Einstein or Maxwell.
8. Mathematically, it is inconceivable that anything as complex as a protein, let alone a living cell or a human, could spring up by chance.
The theory does not claim that a living cell or a human was formed by chance alone. The theory claims that evolution operates under a set of inviolable natural laws, some of which are manifest in probabilistic outcomes. Those laws are known by current observation to be working now, and there is zero evidence for thinking that they have not been working since the origin of the universe.
Mathematical models have been developed, and have been tested on computers, that confirm the possibility of simple structures changing, by replication with occasional random errors, into more complex structures. The process is not mathematically inconceivable. What is inconceivable to some people is that their religious dogma about the history of life on this world could be in error.
Computer simulations would be more convincing if they were confirmed by observation of comparable changes occuring in the real world.
Are the creationists suggesting here that it is irrational for a person to believe something happened unless he or she has actually observed it happening?
The scientific argument is that we reasonably believe, based on what we have observed during historic times, that certain things of the same sort happened during prehistoric times -- and that because they happened for a much longer time, the effects were that much greater than what we have directly witnessed during the few thousand years of recorded human history.
Evolution does not suppose or propose that anything happened in the past other than what is seen to happen in the present. It supposes only that if it happens for a million years, then the result can be very different from the result of its happening for only a thousand years.
Existing structures can change shape only to a limited degree.
And what imposes those limits -- other than time? Creationists never answer this question.
Even disregarding limits to variation of existing structures, mutation and natural selection cannot create new structures.
This is dogma, not fact. Or, depending on interpretation, it is irrelevant. We're not talking about evolution creating anything ex nihilo. The eye (for example) is understood by evolutionary scientists to be only a modification of something that previously existed.
Computer simulations are not real life. You cannot justify the claim that it must have happened just because you can write a program that mimics it in some way.
Simulations can test many assumptions about probabilities and other constraints. If creationists think the test models are unrealistic, they are more than welcome to identify the specific shortcomings if they can. However, to reject the test results solely because of hypothetical problems that we do not even know about is to argue from ignorance.
Simulations are designed and goal-oriented. Therefore they cannot model the undesigned and undirected naturalistic world assumed by most scientists.
All scientific experiments are designed. An undesigned experiment is not an experiment. It is a random and uncontrolled observation.
No scientific experiment can perfectly replicate anything that happens in the absence of an experimenter. We can never know what happens when we're not looking. All we can do is make assumptions and then see what happens when we act on those assumptions.
What the experimenter can do is ensure, insofar as is possible, that differences between the experiment and the real world are either irrelevant or properly accounted for in the analysis.
Evolutionary theory is hopelessly biased. The premise behind it prohibits a designer who has a goal in mind
There is no such prohibitory premise behind the theory of evolution. There is only the absence of a premise that an intelligent designer with a goal in his mind must be assumed to exist in order to account for what we observe about life on this world.
Computer simulations, even though designed by people with goals, can and do disconfirm creationist assumptions that certain things simply cannot happen in the absence of goal-oriented designers.
9. The second law of thermodynamics makes evolution impossible. If disorder must always increase, then living cells could not have evolved from inanimate chemicals, and multicellular life could not have evolved from protozoa.
The second law is about closed systems. Earth's biosphere is not a closed system. If this argument were valid, snowflakes would be impossible.
If the second law really said that snowflakes could not occur naturally, then the first snowfall disproves the second law.
We are not claiming that the second law proves the impossibility of snowflakes. We are saying that IF it prohibits evolution as the creationists claim it does, THEN it also prohibits snowflakes. Since snowflakes exist, we must infer either (A) the second law is false, or (B) the law does not prohibit evolution as creationists say it does.
Many evolutionists claim that the second law does not apply because the Earth is an open system, that adding energy from an outside source fixes the problem.
Evolutionists are not the only people who say this. So do physicists and chemists, particularly those who specialize in thermodynamic theory. If it is a mistake to think that the second law is only about closed systems, creationists seem to be the only people who know that.
If all it takes to reverse entropy is to add energy, why didn't any scientist suggest rebuilding the Twin Towers by detonating a nuclear device over the rubble? The rubble could have grown more complex, completely rebuilding the Twin Towers, because the greater entropy associated with the nuclear blast would have more than offset the decrease in entropy as the Twin Towers reassembled themselves. Why didn't any scientists suggest that?
Because scientists understand what the second law actually says.
There actually was a massive input of energy to the rubble, by the way. Without it, the rubble would have stayed put and there would have been no cleanup.
Creationist caricatures notwithstanding, nobody claims that adding energy necessarily increases order. The claim is simply that an increase of order within a system is possible if energy enters the system. Nothing more is needed to refute the argument about the second law prohibiting evolution. The creationist argument is that the second law makes evolution impossible. But it does not, because the second law as ordinarily formulated does not apply to an open system.
10. Mutations are essential to evolution theory, but mutations can only eliminate traits. They cannot produce new features.
Why not, and according to whom? What are the specific facts about observed mutations from which one infers this conclusion?
Reptiles had to grow breasts to become mammals, didn't they?
Yes, they did. And exactly what, specifically, was supposed to keep that from happening?
Every internal organ of every living creature is a complex structure that had to be produced by a genetic mistake, if the theory of evolution is true.
"Mistake" is a somewhat anthropocentric concept. What we're talking about is something that is observed to happen.
Genes get copied. That is what reproduction is all about. Not even creationists can deny that. Sometimes, after copying occurs, the new gene is not identical to the original gene. Not even creationists can deny that. You can call it a mutation, or you can it a genetic mistake. The label does not matter. It happens, and when it happens, evolution has happened. If it happens often enough and for a long enough time, then structural changes occur in the organisms carrying those genes, and those changes can enhance the organisms' survival. Whether such an enhancement is a mistake probably depends on one's viewpoint. Cheetahs probably don't consider it a mistake that they can run faster than their ancestors could. Gazelles might beg to differ.
11. Natural selection might explain microevolution, but it cannot explain the origin of new species and higher orders of life.
Why not? Where is the wall between species or genuses or other taxa? What puts the wall there? What prevents microevolution from becoming macroevolution if it continues indefinitely? What makes the changes stop happening? What keeps a sequence of small changes from becoming a large change? Creationists might have a theological answer to these questions, but nobody has offered a scientific answer yet.
12. Nobody has ever seen a new species evolve.
That depends partly on how the word "species" is defined. Populations have indeed been observed to change until descendent organisms cannot interbreed with ancestral organisms. The question then becomes whether we may reasonably infer from this that the changes could continue indefinitely until the descendant population is a new species by anybody's definition.
13. Evolutionists cannot point to any transitional fossils -- creatures that are half reptile and half bird, for instance.
When we do point to a transitional fossil, creationists always say, "That fossil is not transitional."
To be transitional, a fossil must be one of a series that are positively dated, in contiguous geographic areas, and showing a definite, gradual, change in shape over time.
Why? By what or whose standard is that a useful definition? The theory of common descent is supported if we have fossils in the right place at the right geological time with characteristics of both ancestral and descendent taxons. The right place is wherever ancestral and descendent organisms both lived. The right time is any time after the earliest ancestral forms. That is a scientifically useful criterion for being transitional.
Creationists don't dismiss fossil studies. They accept the obvious explanations, rather than some contrived explanation of how the fossils could possibly show evolution.
Creationists have special definitions for "obvious" and "contrived." An explanation is obvious if it is consistent with their sectarian religious dogma. Anything else is contrived. That is because their dogma is obviously true, while anything that contradicts it must be contrived in order to justify disbelief.
With exceptions too rare to concern us here, creationists are evangelical Christians. According to their interpretation of certain statements in the Bible, there can be no justification for doubting any evangelical dogma, including the dogma of biblical inerrancy. Anyone who does doubt is ultimately doing so for one reason only: to justify defiance of God's authority.
Of course the fact that creationism is accepted almost exclusively by members of a particular religious sect does not prove it wrong. But if that sect has adopted as an article of faith the notion that anyone who disagrees with them can only have evil motivations for their disagreement, they have little room for accusing other people of preferring contrived explanations to obvious ones.
14. Living things have fantastically intricate features -- at the anatomical, cellular and molecular levels -- that could not function if they were any less complex or sophisticated. The only prudent conclusion is that they are the products of intelligent design, not evolution.
This is either question-begging or an Argument from Personal Incredulity. It assumes that whatever looks designed must have been designed, or else it is just a way of saying, "I cannot believe it happened, therefore it cannot have happened."
Nobody today has any idea how an eye could possibly have evolved.
It is obviously true that creationists have no idea how it could have happened. The rest of the world is not bound by the limitations on creationists' understanding.
The argument that an eye can evolve by chance is just wishful thinking on the part of the evolutionists.
The argument that it is impossible is just dogmatic thinking on the part of creationists.
15. Recent discoveries prove that even at the microscopic level, life has a quality of complexity that could not have come about through evolution.
Irreducible complexity is not a discovery. It is a hypothesis. One discovers facts, but one invents hypotheses to interpret facts. Irreducible complexity is not a fact. It is an interpretation of facts.
It might be argued that any theory is an interpretation of facts and intelligent design is just another theory. So be it. The scientific community has its process for deciding which of two competing theories is the better explanation of the relevant data. So far, the process has ruled against intelligent design. That can still change, but the change will have to be driven by data that are not yet known to exist.