What Is The Evidence For Evolution?
Hypothesis and Theories
Science does not use certain terms in the same colloquial sense that most of us them. For instance a scientific theory is a factual explanation that explains what we observe occurring in the natural universe. So the feeble protest that evolution is, "just a theory," is a misuse of the very word.
Theories derive from a number of tentative hypothesis that are checked by experimentation and observation. A hypothesis is the result of theoretical links in the minds of scientists and it is what independent studies hope to establish. Notice I did not say a hypothesis is what a study expects to prove; that's not the way science works. Every physical and social study expects to prove what is known as the, "null hypothesis." This is the hypothesis that any statistical correlation or conclusion draw from a study is merely the result of chance or flawed methodology. It is only when results reach a certain level of probability that they did not derive from chance that the default position of the null hypothesis is rejected. This level of statistical probability is called an alpha level and is usually set at 95% and often at 99% or higher.
When the results are convincing enough to confidently say that within this one single study there is only a 5% or a 1% chance that the results were a product of chance a hypothesis becomes provisionally validated. But if this result can not be replicated with the same degree of probability over and over again as other peers within a discipline replicate the study then the hypothesis is abandoned. So let's say a study with an alpha level of 95% finds significant evidence for a hypothesis. At this point the study is published and attempted to be replicated by other independent scientists. let's say 10 different labs with the same 95% alpha level replicate the same experiment 10 times. (This is a modest number many more independant labs than this will verify the results of a study) Our .05 (5%) acceptance that the result was simply chance is now decreased to 0.00000000000009765 or a 000000000009765% likely hood that the results were due to chance. Numbers like these are very convincing and as I said in reality the experiment would be replicated many more times than 10.
There seems to be a pernicious meme alive and well on the internet that scientists (for some reason I fail to comprehend) collude to be sure that some a priori result is ensured to come from their collective work. This is simply false. The way to make a name for one's self in science is to prove your peers wrong (you even gather respect for proving yourself wrong). Scientists conduct their research independently all over the world often without ever having met the people orchestrating the other laboratories that will either confirm or deny a hypothesis. So this idea of scientific conspiracy must be dismissed as the propaganda that it is.
Once a hypothesis is confirmed through independent testing enough times it is combined with other relevant hypothesis to explain vast amounts of observational data and testing. These hypothesis unify into a overarching theory that makes sense of the world around us, explains our observations, and perhaps most importantly can make predictions about future phenomena.
Evolution is one such theory, as fastidiously verified as the theory of gravity, the atomic theory, or the theory of heliocentricity (the earth orbits the sun). I want to talk about just a few of the many converging hypothesis, coming from different disciplines being practiced in different laboratories by different scientists, and some of the predictions that have cemented evolution as fact.
Fossils Are Just a Bonus
You hear ad neaseum from Evolution deniers that scientists cannot produce even one fossil that is an intermediate between species. In fact since Darwin wrote, "The Origin of Species," we have found 10's of millions of fossils that show slow, elegant changes from any one species to any other species. But this is simply icing on the cake, the theory of evolution would be just as well verified without a single fossil.
The comparative anatomy of living species shows a similarity of vestigial organs and appendages that suggest common ancestry between variegated species. These similarities suggest a top down improvisation on pre-existing structures that allowed species with common ancestors to split off into separate species as needed to survive in changing environments.
Further, Embryology shows traces of the evolutionary changes undergone over millions of years during the gestational period of many animals. Human embryos for example have gills at an early point in their development, they develop a reptilian brain and nervous system before the higher cerebral cortex is differentiated and they grow a thick coat of down fur on their bodies that is shed before birth.
While most evolution is too slow to observe within a life time or even many lifetimes this is not always the case. Many experiments with fish placed into new environments have shown how they change to better survive in a new environment in a matter of decades. The resulting populations change so much that they are no longer able to inter-breed and produce fertile offspring. (The ability to do so is what defines a species and differentiates it from other species.) Even more drastic changes have been shown with experimentation involving bacteria because they reproduce so quickly.
Remember it was said that a good theory can make useful predictions. Well, by employing the tenets of evolution drugs are able to be developed to combat new bacterial and viral pathogens before they have even evolved. All of modern medicine is predicated on the assumption that evolution is true.
Further, if evolution is indeed true we would expect to see similar species along the coasts of the continents which are slowly drifting apart at about the rate that your finger nails grow. They were once joined as one land mass, called Pangaea. We would also expect to find similar species along the coasts of present day continents with species that inhabit near by islands. This prediction is validated in every studied case.
DNA; The Nail in The Coffin
It is only within the last couple decades that spectrometers and other technological advances have allowed us to map the genomes of any organism we wish. We can calculate the amount of genetic similarity between any and every set of living species, from bananas to humans, and the amount of genetic similarity is exactly what one would expect. Species that look closely related have more genetic material in common than species that look further apart. The result of these percentages of similarity is a beautiful tree (sometimes described as a bush because of the denseness and multi-directional shape it takes) of life showing that all living species arose from a single common ancestor.
So in closing, to reject evolution is to reject the independent findings of tens of thousands of studies in Biology, Chemistry, Physics, Geology, Paleontology, Comparative Anatomy, Embryology, Plate Tectonics, Archaeology, Immunology, and Radio-metrics. If you are prepared to do that then I would posit that you have an underlying reason for doing so. Incredulity of that type can only be derived from the succor that people derive from mythologies.
I would in no way concede that because I am a member of a highly evolved species of primates that this is denigrating to the human species. I find quite the opposite; the fact that all organisms share a universal origin supplies a deep sense of personal compassion for every living creature. It also insists upon the somewhat narcissistic conclusion that we are all the descendants of the survivors of what can be a very cruel natural environment and I find this to be emboldening.