Nearly 150 years after its first publication (1859) Charles Darwin's On the Origin of Species is still one of the most hotly contested scientific works of all time. No other groundbreaking scientific theory has experienced such a prolonged backlash as Darwinian evolution. Germ theory revolutionized medicine; Relativity altered physics at its most fundamental level; yet both of these discoveries are almost universally accepted today. So what is it about evolution that ruffles the feathers of so many?
In order to fully understand the controversy surrounding evolution, it is necessary to first understand where it came from. Darwin was well aware of the response his work would engender in the public. As such, he imbued Origin with a multitude of proofs and evidential lines that, when all combined, point inexorably towards evolution by natural selection. Among those lines of evidence are his comparison of natural and artificial selection, a discussion of geographic distribution of organisms, an explanation of the geological record and its imperfections, and several others.
Like any good theory, evolution does more than attempt to explain currently observable phenomena it makes testable predictions, and it has been confirmed by subsequent research time and time again. The two most famous confirmations of Darwinian evolution have to be genetics, which provides a mechanism for inheritance and mutation, and continental drift, which reinforces the line of evolutionary proof regarding geographical distribution as well as providing evidence of the enormous time scales required for evolution to take place.
We will begin where Darwin began, with a topic that may as well be called "Darwin's sales pitch to the masses: Artificial Selection." Darwin opened with artificial selection because, in his time and society, it was a well-known and well understood practice. Animal breeders were learned and skilled in their trade, and were capable of altering animal species to create new breeds with desired characteristics, or to refine the characteristics of already existing breeds.
He opens with a statement of generally acknowledged fact domestic animals tend to vary more than wild animals do. With this established, Darwin moves to explain why such variance exist, saying that "greater variability [in domestic animals] is simply due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent-species have been exposed under nature." (p. 453) In other words, domestic breeds vary more because they're exposed to more varied circumstances or "conditions of life" than wild breeds.
This, again, seems a reasonable assertion to make, and not something that any intellectual of Darwin's time would find overly ambitious. However, consider the ramifications of Darwin's little statement. Different habitat results in different animals. All you need is one more concession to Darwin's viewpoint that animals change as their surroundings change and you've already bought into the bedrock of evolutionary theory: external pressures cause species to adapt to better survive under those pressures.
Rather than leading this logical progression as an exercise for the reader, however, Darwin proceeds to pile on evidence in favor of his theory. After addressing everything from drooping ears in domestic dog breeds to albinism and other strange mutations, he once again slips in an innocuously worded but crucially important hypothesis, "perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly." (p. 457) In other words, since we already accept that albinism and similar anomalies are inherited, might not the correct view be that every trait is inherited, rather than just those few irregularities?
With the precepts of selection pressures and adaptive variation firmly instilled, though not yet openly stated, Darwin moves into more dangerous territory. Reiterating the previously noted variation within species, he takes the plunge. "These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions." (p. 478) That is to say, nature does the same thing that breeders do it selects different beneficial variants within species and promotes the breeding of individuals with those variants.
It is worthwhile to note here that the transformation from one species to another, or within species from one variety to another, is not necessarily a "clean" transition. Beneficial adaptations accumulate in a species over time, while deleterious adaptations are selected against and typically become rarer in a species. There are, however, some traits which are neutral neither beneficial enough to be selected for nor deleterious enough to be selected against. These traits provide some of the strongest evidence for non-directional, random evolution, rather than the creationist viewpoint which had been largely accepted before Darwin's time, and is still present today in significant force. Neutral traits like the human appendix are frequently considered to be leftover adaptations that no longer serve any purpose, but likewise don't harm an individual enough to be evolutionarily removed. If humans were created as-is some six thousand years ago, the presence of an appendix would be inexplicable (along with a whole host of other problems that creationism brings along, but those will be addressed later).
Up to this point, Darwin has argued exclusively for what is sometimes referred to as microevolution, or changes within a species. Having established microevolution in both domestic and wild breeds, he turns his eye towards macroevolution, or the change of one species into another. Some modern evolutionists have criticized the delineation between macro and micro evolution, saying that they are part and parcel of the same whole micro evolution leads to macro evolution. Darwin would likely have agreed since it was his argument that, over many years, as those small microevolutionary changes accumulate, you will eventually produce a variety wholly distinct from its ancestor. It will be so distinct, in fact, as to be considered an entirely new species of animal, rather than just another variety of the original species.
Another common misconception that likely has Darwin spinning in his grave is the statement "humans evolved from apes," or, stated more broadly, currently living species evolved from other currently living species. Though possible, this is almost never the case. For example, humans did not evolve from apes. Humans and apes shared a relatively recent common ancestor, and as such share a great many characteristics such as opposable thumbs and large brains. Apes are not "less evolved" than humans, their ancestors simply experienced different selection pressures that caused them to develop differently from humans.
The reason that an ancestral species and its descendant species rarely exist simultaneously is that the ancestral species typically goes extinct. A descendant species is formed when new selection pressures force a species to mutate, or change, in order to survive. Therefore, the members of the ancestral species that don't adapt die off, while those that do adapt survive and eventually, if the selection pressure is great enough and the time period long enough, form an entirely new descendant species.
There are many possible causes of new selection pressures, but the most frequent cause of new species generation has to be changing geographic and climatic circumstances. Darwin knew that climate is not static, and in fact, "believe[ed] that the world has recently felt one of these great cycles of change." (p. 692) An awareness of this process provided him with an example of a selective pressure that would cause species to evolve. For example, if the American Midwest were to suddenly undergo a rapid climate shift, becoming much warmer and wetter (for whatever reason), organisms better able to take advantage of the new resources would flourish, while those less suited would die off or move elsewhere. Odds are, within a few hundred thousand years, there wouldn't be corn and wheat growing there any more. They would be replaced by more tropical plants better able to utilize the extra water that had been provided.
Darwin was not, however, aware of the theory of continental drift (since it had not yet been discovered). Uncovering the fact that the continents themselves move provided evolutionary theory with several important new bits of evidence. First, if continents move their climates will be in constant flux, providing an impetus for constant adaptation by the organisms inhabiting the continents. If you can't rely on your home to be as it is forever, then you must be able to adapt so that you'll be ready for the next climate to come your way. Second, being able to fit all the continents together allowed scientists to understand the geographic distribution of fossilized animals, some of which appeared on continents separated by oceans a phenomenon which would be inexplicable without continental motion.
The other area of science which has served to bolster the status of evolution in the minds of researchers is called genetics. This branch of science, begun by an Augustinian Abbot named Gregor Mendel in the mid 1860s, turned Darwin's proposed inheritance of traits into a quantifiable science. Similarly, the discovery of the molecular structure of DNA in the 1950s by Watson and Crick revealed the mechanism by which mutation functions. These two developments were certainly fascinating in their own right; however without the evolutionary framework to understand them, they wouldn't have been nearly as useful to biologists. Considering DNA through the lens of evolution has allowed geneticists to pinpoint areas of rapid mutation, which in turn has allowed them to trace exactly where closely related species diverged from one another.
That is the true brilliance behind Darwin's theory greater even than the explanation of current biological diversity is the frame of mind that evolution puts you in. Understanding that everything is changing, and that the biosphere itself is so intricately woven that no aspect can be removed without affecting the whole, enables researchers to analyze their data in ways that they otherwise wouldn't even consider.
Sadly, there are some who think this evolutionary framework, which has enabled such fantastic things in the fields of biology and genetics, is all nonsense. They sometimes move under the banner of "creationist," though more and more often are now calling themselves proponents of "Intelligent Design" or "ID." It is because of the challenges brought forth by these people that an understanding of Darwin's theory is so important today, possibly more so than ever before.
The reason for the danger is not the scientific validity of ID, but rather the tactics by which its proponents attempt to insert it into school biology curricula around the world. Its scientific merit can almost be dismissed out of hand by those with an understanding of evolution, but since most of the public lacks such comprehension, the theory remains a thorn in the side of many evolutionists.
In summary, ID is a theory which posits that certain aspects of biological organisms are too complex to have been created by natural causes. Instead, posits ID, they must have been designed by some "Intelligent Designer." The problems with this are manifold, but the biggest has to be the utter lack of predictive power offered by ID. While evolution provides us with a framework of understanding we can use to interpret and analyze biological discoveries, ID does no such thing. At best, it is a disproof of evolution since it doesn't provide any mechanism to replace it. Simply saying "a designer made it" isn't scientifically rigorous, and introduces many more questions than it answers (mostly because it doesn't answer anything at all).
So, while you yourself may not be interested in the precise morphological shifts underlying particular biological structures, that doesn't mean you should be wholly ignorant of evolution. It is the responsibility of every thinking person to ensure that future generations of children may continue to learn about evolution without being distracted by false or misleading pseudo-theories like Intelligent Design. The best way to go about doing this is to understand evolution for yourself, so that if it is ever challenged in your area you will have the tools you need to fight back and defend it. Darwin's theory painted a picture of nature "red in tooth and claw," and over the years the battle to defend it could very truthfully be described in the same way.
Note: For those who are interested, the page numbers cited are from the following text...
Darwin, C. (2006). On the Origin of Species. In E.O. Wilson (Ed.), From so Simple a Beginning (pp. 437-764). New York: W. W. Norton & Company.