The strength of any scientific theory is measured by its ability to make predictions. It works sort of like this: I might have a guess about what causes (or caused) some phenomenon. I would then say, “If my theory is true, I would expect to find this other thing.” If that other thing is found, it gives weight to the theory that it might be correct. If that other thing is not found, then it could be evidence that the theory is not correct.
I'll give you a hypothetical example of how this works. Before we ever went to the moon, pretend that I insisted that the moon is made of cheese. If my theory were true, I could predict that any rock brought back from the moon would actually be cheese. Technically, theories are never “proven” to be true. If the rocks brought back from the moon did turn out to be cheese, it still doesn't mean the entire moon is made of cheese – it could have been just those few rocks. Even so, it does make my theory seem more likely. However, if all the rocks brought back turned out to be ordinary rocks and none were cheese, it's very strong evidence against my theory.
Some people claim evolution is a strong theory that has made many successful predictions. What they always fail to discuss is how evolution is so plastic a theory that it's virtually impossible to falsify and even some of the things we might predict if the theory were true are epic fails. I thought I'd take a moment and discuss just a few of evolution's failed predictions.
Radiometric dating: Certain, naturally occurring substances are unstable and so will decay over time until it becomes a stable substance. Uranium, for example, decays over time and eventually becomes lead. The rate at which the decay occurs varies from substance to substance. Some decay at an extremely slow rate while others decay more rapidly (relatively speaking). By measuring the ratio of the parent/daughter elements (uranium/lead, for example), scientists can estimate how long the decay has been occurring. Many scientists consider radiometric dating to be the final word in determining the age of any sample and it is from radiometric dating that many people are convinced that the earth is very old. If radiometric dating actually dates things accurately, we could make a few predictions:
Prediction #1: Newly formed rocks should not have any of the daughter element present and should show an age of “zero.”
Results: Rocks formed at the Mt St Helen's eruption were dated using potassium/argon dating, the samples yielding ages up to 2.8 million years even though the known age of the rocks was 10 years old. FAIL.
Prediction #2: Carbon 14 is an unstable element found in all living things. As living things breath and eat, they accumulate C14. Once the thing dies, the C14 begins to decay and becomes C12. The key difference in this radiometric dating method and those discussed in the previous paragraph is that the decay rate of C14 is much quicker than many other types. It has a half-life of only 5,730 years. Due to its short half-life, we can predict that samples more than 100,000 years old should have no detectable C14 remaining in them.
Result: An 8 year long endeavor by creation scientists known as the RATE project, has found it is impossible to find any old samples without detectable levels of carbon. Even diamonds, the hardest natural substance and virtually impossible to contaminate, consistently yield trace C14 even though they are supposed to be a billion years old. FAIL.
Progression in the Fossil Record: According to secular dating, the rock layers represent the accumulation of sediment being laid down over time. The layers further down are older than the layers above them. Where fossils are found in the layers supposedly represents when those creatures lived. Creatures found in fossils in lower strata lived before the creatures found above them.
Prediction #3: If evolution were true, there should be a clear progression of simple to complex in the fossil record where the older creatures are more primitive than the younger creatures.
Result: Dinosaurs allegedly evolved into birds. However, I recently wrote about feathers identified as 78 myo yet are still described as being “nearly identical to those of modern birds.” I also wrote about the supposed human ancestor, Australopithecus afarensis. This very ape-like creature was found in the same age of rocks as the Laetoli Footprints which are described as “almost indistinguishable from modern human footprints,” matching our feet in both the toe pattern and stride. In both of these cases, and many others I could cite, we see evidence of modern creatures living simultaneously as their supposed ancestors. There is no clear progression in the fossils of simple to complex. FAIL.
Transitional Forms: Evolution is a history of descent with modification. A lobed fin becomes a leg which become wing. A fold in the skin becomes a scale which becomes a feather. The structures found on every creature of every species are simply adaptations of more primitive structures found on the creatures' ancestors.
Prediction #4: If evolution were true, we should expect to find volumes of fossil evidence showing creatures in transition from one species to another. In Darwin's own words, “innumerable transitional forms must have existed.... [J]ust in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed on the earth, be truly enormous.” We should not be able to turn over a shovel of dirt without finding another transitional form.
Result: Darwin himself was surprised that we didn't find, “every geological formation and every stratum full of such intermediate links.” After more than a century and a half of looking, and the trillions of fossils that exist, evolutionists have – at most – a few dozen examples of transitional forms and even most of these are suspect. The “innumerable” amount we would expect to find are simply not there. FAIL.
The Appendix: As we've already discussed, evolution is a history of descent with modification. Over time, some structures have supposedly lost their original function and have either become useless or have been adapted for some completely different function. Such structures are called, “vestigial.” The appendix is perhaps the most touted example of a vestigial structure.
Prediction #5: The appendix appears in many different species of mammals. If descent with modification has occurred, we should be able to trace the appendix along the so called, “tree of life” and find that all the creatures who have an appendix also share a common ancestor.
Result: The appendix appears in some species of primates, rodents, and even marsupials but is absent from the intermediate groups linking these species. It appears on the tree of life in no discernible pattern. FAIL.
Tiktaalik: I bring this up because it is often cited by evolutionists as an example of a successful prediction made by their theory. It was even used by Bill Nye in his debate against Ken Ham. The most commonly accepted understanding of history is that life began in the sea and evolved onto land. If this has occurred, scientists would expect to find fossil evidence of creatures with structures in transition from sea-to-land.
Prediction #6: Based on their understanding of when the supposed transition of sea-to-land occurred, researchers began exploring an area of exposed, Devonian deposits in the Canadian Arctic in hopes of finding fossil evidence of a creature in transition from sea-to-land. They found Tiktaalik. According to one website detailing the prediction, “Not only was it exciting to find a new species, but it was made all the better by the fact that scientists had predicted the existence of a creature like this all along.”
Result: A few years after the discovery of Tiktaalik, a track of fossilized footprints belonging to a tetrapod were uncovered in a quarry in Poland. They were dated according to evolutionary dating methods to be 18 million years older than Tiktaalik. This would mean that fully evolved, ambulatory tetrapods were walking around millions of years before their supposed ancestor, Tiktaalik, ever lived (see failed prediction #3). FAIL.
In summary, I'll just say that I'm not sure of any successful predictions the theory of evolution has made. Many of those that endure are what I call, “predictions after the fact,” like “if evolution were true, I would predict that creatures could reproduce.” These types of “predictions” are worthless. I only know that there are many, many failed predictions. It's generous that we still even call it a “theory.”