There are two deeply misunderstood and most unfairly maligned terms in all of science: the words "evolution" and "theory". You should not be surprised why this is so about the former; historically, this word has been used (and oftentimes misused) for good, bad and ugly. About the latter, well, let’s talk about it.

A fundamental ability of anyone who really understands science is the capacity to distinguish between the term "theory" in a conversational sense and the word "Theory" as it is used in science. Many people confuse the two and this shows a fundamental misunderstanding about the scientific process. This confusion appears when a clear distinction is lacking between two of the meanings of the word.

A “theory” (all lowercase) is essentially a guess or an opinion, as in “ . . . this is what I think”. On the other hand, a “Theory” (capitalized) in the scientific sense is a model that organizes a wide variety of phenomena, integrating them in a logical way which has little to do with personal opinions. In this way, a Theory is capable of explaining additional observations and most importantly, of making predictions. Over time, these additional observations are used to modify the Theory and in some cases, even change it entirely.

A very dangerous side effect of when someone misunderstands scientific principles is that people confuse "facts" with "Theories", even people who should know better. There is probably no better example than gravity.

Things fall towards the ground. This is an unquestionable fact that is observed millions of times every single day. We (humans) have described this as “gravity”, and we have imagined many different explanations of what makes it happen. In fact, since antiquity, there have been several explanations to account for the fact of gravity, all the way up to Newton and Einstein, yet the proverbial apple does not care about the mechanism; it falls to the ground regardless.

SENSATIONALISM SELLS, BUT WHY?

Therefore, even though we have a current Theory of gravitation, we do not think of gravity as something dependent on our personal opinions. Gravity happens, whether we like it or not. In this sense, gravitation is both a fact and a Theory.  Please don't be like the guy below:

In our age of media sensationalism (or as one of us call it: bite-size science), it's not uncommon to hear someone proudly proclaim, "Newton got it wrong," or "Darwin was wrong." We've even heard, "Einstein got it wrong." Truth is, these claims are spurious and shortsighted; it goes without saying that they are also wrong. We’ll deal with evolution in a moment; let’s finish with gravitation first.

Take for example, Newton and Einstein. Newton didn't get gravity wrong. If anything, given the limited access he had to observational evidence and experimentation, Newton's insights into concepts like the inverse square relationship of the distance between two masses when calculating gravitational attraction is astonishing, and let’s not even talk about how, when the mathematics that he know proved inadequate to explore this aspect of nature he invented calculus (more or less at the same time than Leibnitz).

In turn, Einstein didn't overturn Newton, he improved on the accuracy of Newton's laws. We still use Newton's laws today even though Einstein's theory of general relativity is more accurate.  For example, pure Newtonian mechanics is perfectly adequate to model the motion of the planets around our sun.

APPLYING NEWTONIAN PHYSICS:

If you look at this target at a shooting range from a distance of 200 yards, that large grey-black area encompassing rings 10, 9 & 8 will appear like a smudge, barely more than a dot in the distance. If you're using a handgun or a rifle without a telescopic sight you could proudly claim a bulls-eye if you struck the grey-black area, and yet up close it's apparent there aren't any shots that hit dead-center. In the same way, Newton's laws fall broadly within the bulls-eye. Newton's laws are extremely useful, but Einstein hit dead-center (at least to the best of our current knowledge. If the link between gravity and quantum mechanics is resolved, we may yet increase our accuracy even further).

When talking about theories, the same reasoning that we apply to gravitation can be applied to evolution.

Gasp!

For some, evolution is a scary word, but it shouldn't be.

The term evolution is a catch-all, a summation of more than a hundred and fifty years of scientific research and, unfortunately, religious controversy, compressed into nine letters. As a concept, evolution carries differing weight and different meaning in various people's thinking, but it shouldn't, as evolution has been clearly defined by science.

A vast collection of Theories of evolution, on the other hand, have historically tried to explain this change of biological life over time through a variety of different mechanisms, just like gravitation. The best available evidence points at natural selection as the most likely mechanism that accounts for evolutionary change. More recently, some scientists have suggested symbiosis (the close interaction of organisms belonging to different species) as a source for evolutionary innovation.

ADDITIONAL COMPLICATIONS: 

As much as we like simple, straightforward answers, complex sciences like biology rarely resolves into a single solution. There can be numerous, interrelated causes contributing to the evolution of species. When DNA was first discovered, there were high-hopes that it would provide a single blueprint or instruction set for life. Instead, what we find is that DNA is just part of the answer. The full answer encompasses epigenetics (the impact of the environment on our genes at the molecular level for short) and may even include an emerging field known as hologenetics (where the microbiome of symbiotic bacteria living within animals indirectly contributes to evolution; kind of symbiosis). These are exciting fields of research, teaching us more about the mechanism by which life has adapted, diverged and thrived on Earth for billions of years. Wanna taste of it? Look here.

The important thing is that whether symbiosis, natural selection, a combination of the two, or even a yet undiscovered mechanism explains evolution the best (and trust me, scientists have argued and will keep arguing about this), ***has no bearing on the fact of evolution***. Life happened, and has changed over time, period. The undeniable fact is that life on our planet has changed over time. This has been thoroughly documented by many lines of evidence. This is a fact. Pure and simple. No "buts" or "maybes."

A CONTROVERSIAL MATTER:

Perhaps the most surprising thing about evolution is that it attracts any controversy at all, as it is quite simple and straightforward: the main imperative of life in the biological sense is to reproduce; in turn, to reproduce, you must be alive, therefore the kind of life that survives in a particular environment gets to leave offspring carrying its genetic legacy. Over time, we call this process adaptation.

The challenge species face is that there is fierce competition for limited resources, there’s disease, there’s predators, and so each animal species must produce offspring that survive to mate or they will go extinct. No child is identical to its parents, and so if gradual, generational changes provide an offspring with an advantage over its peers, then that change will be favored and slowly become a dominant characteristic. In this way, birds become lighter, lions become stronger, cheetahs become faster, etc. But there are physical limits, become too light and a bird can’t migrate if it needs to, become too strong and a lion needs more kills to drive its metabolism or risk starvation, become too fast, and a cheetah may be more susceptible to strains or broken bones, etc. And so natural selection finds an equilibrium of maximum efficiency for a given species.

Evolution is a Cold War within the animal kingdom, an arms race not unlike the one between the US and the USSR after World War II, with the stakes being extinction. That life on Earth has changed over time is undeniable, and was a source of considerable interest in Victorian England when the fossils of terrible lizards (dinosaurs) were first uncovered.

And it's not just that dinosaurs are extinct, 99.9% of all the species that have ever lived on Earth, including innumerable mammal species, are also extinct. The species we see around us today represent 0.1% of all the species that have ever existed. The species alive today are not merely survivors, they've inherited their pedigree from prior species. In some cases even really lucky survivors. We have to go back to the dinosaurs, this proud lineage that thrived over millions of years, only to be wiped out in the proverbial blink of an eye by a random event in the form of a big rock that fell on earth.

At its heart, science is about cataloging and categorizing natural phenomena in order to understand cause and effect. As more and more fossils were uncovered in the Victorian era, an interesting pattern emerged. Life could be ordered by strata, or layer. Fossils could be grouped together according to broad families of species, with each successive layer revealing a growing relationship down reaching through time.

Fossils are a time machine, a window into the distant past. They revealed how ancient species diverged and branched out into the species we see around us today.

RABBITS IN PRECAMBRIAN FOSSIL LAYERS:

Biologist J.B.S. Haldane was once asked what it would take to overturn the theory of evolution. His answer? Rabbits in the precambrian fossil layers.

As a humorous as this is, it's an astute observation. In England, rabbits were a pest. Rabbits breed like, well, rabbits. Rabbits were everywhere, except in Precambrian history! And that struck Haldane as profound.

Dig down through layers of rock and stone and you'll uncover fossils with similar skeletal structures to those species alive today, but there are distinct and clear differences. The deeper you dig, the more marked the difference. This sequential progression forms a pattern, revealing clues about how life has evolved on Earth. That life has evolved is a fact beyond controversy. While how life has evolved is a theory that is well-tested and being refined further every day.

One common retort against evolution is to dismiss the concept by saying, "Oh, evolution is just a theory." But such a flippant remark fails to understand that scientific theories are rigorously tested, subject to intense scrutiny, peer-reviewed, held to the highest standards of transparency and consistency, and brutally refined according to the available evidence, just as we said before.

FALLACIES ABOUND: 

A common fallacy prevalent in many circles is that scientific facts can be easily rendered invalid at a moment’s notice. This is not the case. Even if you are talking about the Theory of evolution, the sheer weight of evidence for the mechanism of evolution in terms of the fossil record, genetics, phylogenetics, laboratory experiments that have directly observed Natural Selection, and our own experience with artificial selection revealing the plasticity of species, demands a clear explanation. All the evidence we have, from a variety of different disciplines as far-flung as geology, all converge on the evolution of life on Earth.

In the same way, as Newton hit the bullseye while Einstein improved the accuracy, Darwin hit the bullseye, improving the accuracy of others. Modern scientists like Stephen Jay Gould and Richard Dawkins have reported refinements of the theory behind evolution further and even contributed significant original ideas like punctuated equilibrium and the "selfish gene" idea (take this one with a grain of salt; such is the nature of science), by standing on the shoulders of giants, to borrow Newton’s phrasing.

You see, in Darwin’s time, one of the main difficulties that stood in the way of a more complete acceptance of natural selection as a mechanism of evolutionary change was that nobody really knew how traits were inherited over generations. In the 20th century, the field of genetics (from Mendelian, population and later molecular - Haldane himself, Mendel, Mayr, Dobzhansky, among others, told us how organisms transmitted their genetic endowment and how that endowment changes over time.

Beautiful.

This brings up an interesting point about science, and one that we can only articulate by using the term evolution in a figurative, non-biological sense: Science is about evolution not revolution. We'll explain what we mean.

Fields like string theory may very well lead to a revolution in our scientific understanding, but more than likely they too will be evolutionary, building upon rather than overturning existing notions. There's plenty of gaps to be filled in, but importantly there's no evidence to be thrown out. Our understanding of biology, physics, chemistry, astronomy, etc., is always open to revision so as to be more accurate, but science really is about the evolution of ideas rather than a revolution. And nowhere is that more true than when it comes to evolution itself.

As sincere and well-meaning as creationists and proponents of intelligent design are, they do a disservice to science by cherry-picking concepts that suit their agenda, and this does no one any favors. Science takes no such shortcuts. Good science is critical to our lives. The next time you open a can of soup, drive your car, or open a refrigerator, you’re trusting good science. With the evolution of antibiotic-resistance strains of bacteria, the debate between science and pseudo-science is more than theoretical. Pseudo-rationality can get you killed. Science has and always will be about removing agendas and looking at the evidence with an unbiased, critical mindset.

Evolution's not a scary word. It's an exciting concept, a wonderful glimpse into the mechanism by which life has thrived on Earth for billions of years.

For a more thorough (and much better written) exploration of evolution as fact and theory you can read the words of a true master; Stephen Jay Gould.