Let me first debunk this claim/question in a very clear, very unambiguous answer. Evolutionary Theory is not changing, nor do scientists debate about whether or not there's any truth to it. It's nearly 100% universally agreed within the scientific community that Evolution is fact (the word "Theory" is used in scientific disciplines to mean "fact." Many anti-evolutionists claim that it's "Only a Theory" based on their misunderstanding of this word. The word "Hypothesis" means "an explanation that has not yet been properly proven or disproven.")
So where does this argument come from?
While there are many examples, I'll highlight one just to shed light on the source of the misunderstanding; Neanderthal Man.
Neanderthal Man was found in Europe, and we have many skeletons from various parts of the continent. The bone structures are very similar to humans, but are definitely distinctly different and somewhat more "ape-like." When the fossils were first discovered and studied, it was logically assumed and agreed that these must have been human ancestors. Fast forward to the age of DNA. When the bones were subjected to DNA analysis, it was found conclusively that Neanderthal Man was NOT a human ancestor at all. While we do share many common features, Neanderthal Man was actually a branch of the family tree that completely died out. Modern humans did not descend from them. Cro-Magnon Man on the other hand does prove to be an ancestor. Once again, DNA shows this. We used to know only that humans had predecessors that were distinctly different. We could make logical guesses as to which fossils represented those ancestors. Advances in science helped us to pinpoint which fossils actually did represent our ancestry, and which did not.
Anti-evolutionists point and shout "SEE! They thought Neanderthal Man was a human ancestor and now they've been shown that they were wrong! We can't trust anything these guys say! Evolution MUST be wrong." Let me be 100% clear. Scientists were initially wrong about where Neanderthal Man fit into the tree of life. They then corrected themselves. There was NEVER any disagreement about the Theory of Evolution simply because a specific species was temporarily incorrectly categorized.
The specifics of which fossils represent ancestors of which modern animals is a daunting task to say the least. Scientists continue to study this, put the pieces together, and gradually gain a better understanding. In the process mistakes are made, then they are corrected when new evidence shows them to be incorrect. This is science. If we threw out an entire discipline of science every time a scientist or group of scientists found out that a previous assumption was wrong, we wouldn't have modern medicine. "That pill didn't work like they thought it would! Medicine must all be a sham!"
How it works:
Since Darwin first presented his theory, scientists have searched to understand the specifics. Darwin correctly observed that we can breed dogs (and cattle, vegetables, pigs, cats, etc) in a short period of time to create new varieties that hardly resemble the originals at all. By simple selective breeding we have taken the Asian Grey Wolf and created dachshunds, great danes, pit bull, chihuahuas, yorkshire terriers, dobermans, and labradoodles. These dogs hardly resemble each other at all yet we have concrete proof of their evolution from the original wolves. Darwin wondered whether or not the same process that breeders used could also be the cause of the evolution. Obviously in the case of dogs (or cattle, pigs, vegetables, etc) there is an actual breeder involved that is guiding the process. Could there be a similar "guiding force" in nature?
After watching and studying nature for many years, Darwin realized that there does not need to be an intelligence behind the process. Simple reproductive success and survival success will naturally select for those individuals who best possess the ability to survive and reproduce. Depending on climate and natural territory, this could mean very different things. For example, evolution of humans in the area we know as Europe would have favored lighter and lighter skin. This is because humans need a certain amount of direct sunlight for the processing of vitamin D. Without it, many health problems will occur. In Africa (where the original humans evolved), the sun is far more intense so darker skin with natural protections is favored. Those same protections inhibited the ability to process sufficient vitamin D as early humans migrated north. The result? Those with less protection from the sun (lighter skin) actually gained and advantage through their better ability to survive. They passed these lighter skin genes to their offspring, and the process continued for thousands of years.
Evolution is not a process of perfecting a form, nor is it a process that is striving to reach a pinnacle (thought by many to be humans). It is a process of very slow, very gradual adaptation that results in changing features and forms. The newer features remain only if they convey an advantage in the specific circumstances in which that organism finds itself. We are still evolving, as is every other organism on the planet. It happens VERY SLOWLY, which is why so many people have a hard time with this fact.
If you have a hard time understanding how something works, but all the world's scientists agree on it, there are only two possible conclusions. 1) All the world's scientists are wrong, and you are right or 2) you need to study more in order to better understand the situation. Which makes more sense to you?
