This is for everyone here who doubts that evolution is scientific and based on evidence. Whether the evidences presented here are convincing to you is irrelevant -- that fact is that these are real evidences, and they do provide a means to falsify the theory.
You don't have to accept the theory as being true, but it is indisputable, in my opinion, that it is a scientific theory.
If you want to criticize these evidences, that's fine. But after you do that, I want someone to start a separate thread for their particular theory, whether it is creationism or intelligent design or whatever, and provide the following things:
1) evidence
2) testable predictions
3) a means to falsify the theory
All three are required for a theory to be considered scientific.
Before I go any further, I'd like to clarify what the scientific community considers as "evolution". Simply put, it is the idea that all living creatures (animal and plant) on Earth share a common ancestor. There is potential for variation within each species, and selective pressures can cause species to change over time as they adapt to changing conditions. Usually, it is a part of a species that adapts as it takes advantage of changing conditions.
One powerful technique for organizing life is a method called "cladistics", which groups organisms according to a common ancestor. All members of a "clade" must descend from a single ancestral species. Recently-formed clades, like hominids, (human-like apes) are relatively small. Ancient clades, like vertebrates, consist of huge groups of animals that descended from the first vertebrates.
For a theory to be considered "scientific", it must:
1. be based on verifiable and reproduceable evidence
2. make predictions that allow the theory to be falsified.
The two most important qualities of any theory are its explanatory power and its predictive capability. Evolution has both of these in spades. If it can't explain anything, then a theory is meaningless. If it can't predict anything, then it is not testable and cannot be falsified -- therefore it is not scientific.
Some evidence will be more convincing than others, and some falsifications are more testable than others --- but I am not a biologist. When predictions are tested, they either falsify the theory or not -- nothing can ever prove that any scientific theory is 100% true. However, when a theory is tested continuously and is never falsified, it becomes generally accepted by the scientific community as a fact. That is where evolution is at today, in my opinion.
Evidence 1: The fossil record
Far from being a weakness to evolution, the fossil record is one of its greatest strengths. The nature of common descent is that species have a distinct BEGINNING in time, and did not exist before their ancestral species. This is exactly what we see in the fossil record. The oldest rocks have evidence of single-celled life, followed by invertebrates, fish, amphibians and the reptiles. From reptiles onward, land-based life diversified greatly.
Prediction: no fossils of a species will be found in rocks preceding its ancestors.
Falsification: although there is always a 'gray area' as scientists try to pin down exactly when a species came into being, there are many, many ways for the fossil record to falsify evolution. For example, finding humans or large mammals in Mesozoic rocks or earlier would cause problems. Finding reptiles, mammals or birds in rocks from the Devonian or earlier would falsify evolution. Finding any vertebrates in pre-Cambrian rocks would be an earth-shaking scientific discovery. However, more and more fossils are found, and none falsify the theory of evolution.
Evidence 2: Variability within species
For evolution to work, there HAS to be variability within a species so that it can adapt to new surroundings. Obviously, natural selection is an unintelligent process, so it can take a long time for a species to drift to a new form. A "long time" might be 10,000 years -- long for a human, but a blink of an eye for geology. Humans are able to provide intelligent selective pressure on species to create change at a much faster rate.
Prediction: any species will change when subjected to selective pressure from humans
Falsification: any species that could not be "bred" for particular qualities would cause a problem for humans. But look at the animals that civilization has currently bred: dogs, cats, cattle, horses, pigs, chickens, etc. These were all "wild" animals that were domesticated by humans. But despite each being derived from a single species in the wild, humans have created countless varieties due to selective breeding. So while it is easy to say that "all dog varieties are still just dogs", the truth is that a fossil of a chihuahua and a fossil of a St. Bernard would most certainly be considered separate species in the fossil record.
Also, scientists breed mice, rats and bacteria in different ways for whatever scientific testing is needed. This would not be possible unless each species had inherent variability. And every time a new species is bred successfully for specific traits, it is another missed opportunity to falsify evolution.
Evidence 3: Vestigial structures in animals
Just as evolution can, over time, create new structures in animals, it can also eliminate structures that are no longer necessary. For example, the fossil record shows that whales evolved from mammals that returned to an aquatic lifestyle. However, the whale still grows "legs" in a way, but they are stunted and remained internal in the animals. Snakes are the same way. Both snakes and whales have vestigial pelvises, which cannot be easily explained without the idea that the animals evolved from ancestors that had legs.
Prediction: no species will have a vestigial structure that was not present in its ancestral species
Falsification: since birds (and feathers) evolved on a separate ancestral line than mammals, finding vestigial feathers on a mammal would falsify common descent. We should never find any invertebrates with vestigial backbones, and we should never find reptiles or birds with vestigial mammary glands. We do, however, find vestigial structures in many types of animals, but none that would falsify evolution.
Evidence 4: Shared characteristics among all living creatures
The idea of common descent implies that ALL living creatures descended from a common ancestor. One powerful evidence in favor of this is that all living creatures, from single-celled organisms to human beings, have similar structures on the cellular level that make life possible. The basic functions of replication and metabolism are accomplished with polymers on the cellular level. Of the hundreds of polymers known to exist, all life on Earth uses the same three types, regardless of species. Of the 250+ known amino acids, all proteins in living organisms are composed from a set of just 22. Note that there is overlap in the function of polymers and amino acids, and there is no reason why other polymers and amino acids cannot be used by different species. But every time DNA is sequenced for a new species, the same polymers and amino acids are used. The most logical conclusion is common descent.
Prediction: all forms of life will use the same, small subset of polymers and amino acids for their basic cellular functions.
Falsification: each time the DNA for a species is sequenced for the first time, evolution risks falsification on this point. If species were created spontaneously, there is no compelling reason why commonality like this would exist on the cellular level. That it does is an indication that our most ancient ancestors solved the problems of cellular activity in this way, and it have passed it down to their descendants.
This is stuff I know because I am familiar with the theory. I believe that the explanatory power of evolution far exceeds any of the alternatives, and it makes falsifiable predictions. I am not a biologist, but I hope that I have done the theory justice.
