(This is the original draft of an essay I was challenged to write for a Creationist website; I'm posting it here in its entirety as I had to significantly modify it to fit the word count requirement for my submission. If the site chooses to upload my abridged essay, I'll update this post and put the link here.
Likewise, if anyone wants to correct me on anything, feel free. I'm studying biology right now for college prep, and so if nothing else this is a good opportunity to practice expressing ideas and to learn things and fix mistakes.)
I was recently challenged to summarize the "best evidence for macro-evolution" that I could find in as few words as possible. However, as it is impossible to comprehensively discuss evolutionary theory in just one small paper, I will simply touch on some of the universals --- common misconceptions, objections and arguments, incorporated into what I hope will be seen as a sort of basic overview. In fact, as many of the objections I encounter tend to be based on a misunderstanding of some aspect of evolutionary theory, I'm hoping that simply explaining it to the best of my understanding and ability will help dispel many of the myths about, and objections to, evolution.
WHAT EVOLUTION IS NOT
Before I get started, let's discuss what evolutionary theory is not; evolutionary theory is not a statement of "morality." Evolution is a statement of what we believe happens. It says nothing about whether what happened was "good" or "bad" or "prescriptive." It is simply a descriptive theory of the development of life on earth. There is no real or necessary moral component to evolution, so this essay will not be addressing moral objections to evolutionary theory. Evolutionary theory says that evolution happens whether we want it to or not.
Also, evolution is not concerned with "random chance" as a driving force. In fact, natural selection, the primary driving component of evolution, is the exact opposite of random chance: it is the principle by which nature favors that which helps something survive and reproduce, and rejects that which makes survival and reproduction more difficult. Nothing "random" about it.
THE ORIGIN OF LIFE
Now, let's start with one of the most common objections: How did the first life arise, according to evolution? This is a simple question; evolution does not have (nor does is require) an answer here. Evolutionary theory can be tested whether life was created by an "intelligent designer" or whether it arose naturally. So I will not go into much detail here, except to say that any life which was formed naturally would have to be simple by definition, thanks to the law of probability (simplicity begets simplicity --- if the first life were complex, it would require an origin as complex as itself, probably moreso, conditions which are exceedingly improbable). However, even if the first life was some kind of miracle that had been zapped into existence rather suddenly, then evolution could still be tested; the only predictions that a naturalistic theory of evolution makes about the origin of life are: (1) it was simple; and (2) that it was the ancestor of every creature that exists on earth today.
How do we know (2) is true, you might ask? We can observe that all life on earth is related from one simple fact, which we will discuss more in a moment: we are all made of DNA. DNA is the basic building block for life as we understand it. There has been no discovery of any other life on earth (or anywhere else) that is not built on DNA. Even bacteria and viruses have DNA. The simplest and the most complex organisms have DNA. If we ever do discover a form of life which does not have DNA as its foundation, then we will have a counterexample to the claim that all life on earth is related; according to most naturalistic abiogenesis theories, the formation of the first life probably only happened once, or during a very short "window" of time --- put that together with the long, long time it would take for evolution through natural selection to produce varation and speciation, and you will see that it was not likely that very many sources of life sprang into being naturally, at separate times. In fact, a naturalistic origin of life almost requires a startling degree of improbability; if a naturalistic origin to life were very probable, we would be seeing a lot more of it, wouldn't we? So it's safe to hypothesize that there was probably just one (or very few) "original ancestor(s)." If the generation of life from nonliving matter is so very improbable, then this ancestor(s) would (naturally) have to be the origin of all other replicating life on earth. This self-replicating organism would eventually give rise to the first "population," or group of organisms of the same species.
All that is required for natural selection, the foundational aspect of evolution, to take effect is a self-replicating (or "reproducing") population, such as the one described above. Once we have a self-replicating population, we have "competition" for resources --- against nature, even against other organisms (organisms sometimes compete even against their own genetic relations or family, without even realizing it, such as siblings who have to share food). With such a simple organism as the first natural life, this competition would of course be very simple, almost poetic --- the organism might not even be aware that it is competing --- but by the simple fact of its existence (and its continued efforts, guided by natural, unthinking chemical processes), it "competes," with nature and with others of its kind (whether or not it "means" to), for the resources which drive these processes.
INCREASING COMPLEXITY: New Information
At this point, some people raise the objection, "how does a simple life-form become more complex?" They have a particularly odd way of phrasing it, by asking, "how can new information enter the genome?"
The best way to answer is through example: take hemoglobin in human blood. All of our globin genes are related to each other (that is, moreso than to any other gene in the body). They appear to have "speciated" from a single "parent" globin gene at some point, within our great great great (times a bajillion) ancestor. At some point, that globin gene duplicated, probably because of a mutation, and made "copies" of itself. Over time, these benign, redundant genes mutated separately on their own, until they adapted to each other's presence within the same chromosomes, isolating into two "clusters," the alpha and beta clusters. This splintering process continued again to give us the zeta cluster and the alphas used in adult humans. Further, this mutation happened far back enough along our descent that evidence of it is visible in many nonhuman species --- species which were descended from the same common ancestor as humans, the ancestor in whom these mutations took place! That is certainly an odd phenomenon --- or it would be, if we did not have evolution to explain the connection.
So the simple answer to the information question is: one gene suffers a benign mutation, creating a copy of itself. Later, that copy mutates further, but in a different way than the original, creating "new" information.
And now, moving on, we come to the crux of the matter: speciation. This essay was written with the assumption that you accept what is called "micro-evolution," so I won't go into detail about that or about artificial selection (dog breeding is a great example of both "micro-evolution" as well as artificial selection), but rather, I'd like to discuss what is normally seen as the logical result of "micro-evolution," called "macro-evolution." Whereas micro-evolution consists mostly of mutation and genetic drift, both of which can be demonstrated easily in just a few generations of selective breeding (such as with dogs), macro-evolution is what happens when this takes place over a much, much longer period of time --- as the title implies, "macro-"evolution is the stacking-up of large amounts of these traits with time, such that they displace the "norm" of a population (or even drive two or more populations apart). Many people have a hard time accepting "macro-evolution," but strangely, accept "micro-evolution." I've been asked to present the "evidence" for macro-evolution, but since I think this rejection of "macro-evolution" is due in part to a significant misunderstanding of what macro-evolution is, I will first diverge and explain that a little bit.
Macro-evolution is otherwise referred to as "speciation" --- the "splintering" of a population into two or more sub-populations of different "species." The common misunderstanding comes from the use of the word "species" --- a "species" in the biological sense is not necessarily a completely different (or even radically different) creature from its parent population; rather, there are a few basic criteria used to determine what sets apart different "species" in similar populations. Given my strict word limit, I'll only focus on one, the simplest --- the "Biological Species Concept," under which a "species" is defined simply as "being reproductively isolated from other species" (which simply means, "they cannot or do not interbreed"). Further muddling the situation is the definition of "reproductive isolation," which, in evolutionary theory, does not necessarily even have to be literal reproductive isolation --- there are many cases of "speciated" birds which fit the technical definition of "reproductive isolation" from one another, and yet in theory are still capable of interbreeding. This also happens with flowers; flowers tend to only reproduce with their own "species," although many are capable of (and sometimes take part in) reproduction with other species (this is called "cross-pollination" or "hybridization"). The fact that they can technically interbreed, though, does not make them the same species. So in order to truly appreciate what is meant by "speciation" in evolutionary theory, one must understand how a "species" is defined. Also important, though, is how speciation occurs.
SPECIATION: Reproductive isolation
Reproductive isolation happens when a population is divided (usually by a natural barrier, like a mountain range or a lake or ocean) into two or more sub-populations, which then adapt separately. An example would be an insect, which latches onto a bird or mountain-dwelling animal near the base of a mountain range; the insect's host travels across the mountain range to the other side, and the insect finds its new home there (or somewhere along the way). That insect goes on to form a new "population" in a separate location, and ultimately, that population adapts (through natural selection) to its location in a way that is different from the way the original population adapts. Over a long period of time, these two populations will drift so far apart that their mating rituals may no longer overlap; across a much, much longer period, interbreeding may even become completely impossible, as with the Oenothera gigas, a form of evening primrose which, although artifically selected, was done so to such an extent that it was no longer capable of interbreeding with its parent species.
It's very important to understand, though, that what happens here is not the spontaneous generation of a brand-new "species;" "speciation" in this sense refers specifically to the branching off of individual populations from the "parent" species, which then become reproductively isolated (whether socially or biologically).
MUTATION AND GENETIC DRIFT
However, speciation is not the only form of "macro-evolution." Over a long period of time, small, accumulated mutations can cause significant changes even within a single population. A couple things to remember when talking about mutations within a single population:
(1) "mutations" simply means errors in gene copying, usually during embryonic development; mutations are a lot more frequent than you might think, given the sheer size of most genomes, although most are benign (have no effect on the creature's survival and reproduction), and so they are inconsequential. But some are harmful (and therefore their carriers are more likely to die, and so natural selection "weeds them out" and they disappear from the population), and others still are helpful, which means that they are "favored" by natural selection because their carriers are more likely to survive and pass on their genes;
(2) positive mutations which occur tend to be accepted into a population with time, through an observed phenomenon called "genetic drift;" over time, successful mutations become more and more frequent in the population until they displace the norm, becoming the new "average."
There is more to this line of thinking than just theory, though; if two organisms are speciated from a common ancestor, then there should be some evidence of this --- some sort of "vestigial" biological trace of the offspring species' relationship to the ancestor (though not usually to each other --- contrary to popular objections, contemporary species (such as humans and apes) do not evolve into or from each other). A prime example of such a vestigial relationship is the connection between the laryngeal and vagus nerves of the giraffe, and the vagus nerve of the shark --- in the shark, the vagus and laryngeal nerves travel along what would be the neck in a mammal, passing along the arterial arches that connect to the shark's heart. It's a very straightforward process with no diversions. However, the giraffe's laryngeal nerve makes a rather large detour along its neck (over 10 feet in the case of a tall giraffe), around and through several other major organs, before connecting to the vagus nerve near the base of its neck, by the heart. This seems odd --- the functionality would be improved manyfold if the nerves connected closer to their point of inception, as they do in the shark. So why the detour? As things stand, the giraffe can barely utter an odd noise because of the odd path of its laryngeal nerve. This relationship makes a bit more sense when we realize that sharks don't have a neck, whereas mammals do. Somewhere along the line of evolution from their common ancestor, the elongation of the giraffe's neck came into conflict with the functionality of its laryngeal nerve, and a trade-off was made, whereas the shark never had this problem because it never evolved a neck. A similar odd case happens with the path of the vas deferens from the testis to the penis in the human male --- the vas deferens drapes noticeably over the ureter from the kidney, a vestige from the descent of the testis from its original position much higher up in the body.
These examples are important for more than just showing evolutionary vestigial relation, though --- they also demonstrate the fallacy of the term "de-evolve." Natural selection is not a "directional" force in the sense that artificial selection is; it does not move "one way" and then "back up." No, natural selection cannot back up; it is self-correcting, but it always works with what it has. This is the binding principle of evolutionary theory which allows us to prove the relationships of organisms through common descent --- because evolution cannot "retrace its steps," it always leaves a trace of evidence where it has taken place. Vestiges exist in all creatures, which help point us towards a common origin.