Religious fundamentalists are quick to claim that micro-evolution (which they equate to adaptation) happens, allowing them to accept small changes whilst simultaneously denying macro-evolution (which they equate to speciation). It only requires a casual investigation to realise that such a distinction between adaptation and speciation is entirely artificial and it is sad to see so many rational people buying into it.
DiscussionScientifically adaptation and speciation (micro and macro-evolution if you're of a creationist mindset) are the same, the only difference being time. Creationists coined the terms micro and macro evolution because they sounded scientific and allowed them to accept one (adaptation) while denying the other (speciation, the evolution of new "kinds"). Adaptation is merely change, it occurs all the time and as anyone (any 5-year-old I imagine) could tell you one plus one equals two, something small plus something small equals something bigger. So, it is with change over time; a small change plus another small change plus yet another small change plus still more small changes over time results in much larger changes. Adaptation happens (creationists accept that, albeit unwillingly) and across sufficient time that results in changes to species. Change at the species level is inevitable, i.e., new species will arise. To give a hypothetical example, imagine a community of animals living in an environment where food is scarce. These animals are small mammals, something like a mouse or a rat, which breeds relatively quickly and has quite large numbers of young, which once they reach the age of three months, need to fend for themselves. Breeding season is early in the year and the animals, while they do not actively hibernate, need to build up stores of food as fat to survive the winter when there is much less food available. The animals are omnivorous and much of their food is living (constantly on the move) so the animals must devote some significant effort to gaining their meals. Naturally enough the young vary in size, in strength, in speed so when they go hunting the fast ones reach their prey first and, when that prey fights back (on account of not wanting to be eaten) the larger, stronger ones have a greater chance of winning. So, slower and less able animals get less food, are more susceptible to death from disease and injury and, lacking the reserves to last the winter, be more likely to die off. Those that are more likely to survive to breeding age will naturally be more likely to pass on these successful characteristics so it is easy to see that at the beginning of each year our community of animals will better "fit" their environment than those of the previous year. Although those changes might be marginal, over numerous annual generations, hundreds of thousands of years, those changes might be significant enough to generate what we would consider an entirely new species. Given the constant change blindly directed by the environment in which a species exists (and there are other forms of selection such as sexual) adaptation will happen and, over large amounts of time, new species will arise. If similar communities existed at other locations, driven by diverse environmental factors, they might adapt differently, developing into species quite distinct from their forbears and from the community described above. This kind of thing (the above is hypothetical) occurs constantly all around the globe at both faster and slower rates, depending on the species under consideration. So, if one is to claim that change (at the species level) does not occur, then one has to postulate a means of preventing it from happening. Science doesn't have to defend the idea of evolution (speciation) because if one claims speciation is not happening, then the claimant has to propose a specific mechanism that stops large numbers of small changes resulting in bigger ones. Complexity exists all around us, we can see simple things becoming more complex purely by natural means and science is open-minded about the relevant explanations (though not so open-minded they'll irrational answers), instead seeking evidence and rational explanation rather than ascribe complexity and change to a supernatural cause. Humans have yet to visit any other life-bearing planets so the only evidence we have of complex organic life is here on this Earth. Of course, there is the oft posited claim that a painting needs a painter, a building needs a builder, therefore the universe needs a, what? A universer? A designer? However, the logic doesn't follow. Yes, a painter creates paintings and a builder creates buildings; the creation linked to the creator's profession but there is validatable evidence for such a link, whereas there is none for a designer. The only evidence we have is that the universe exists and we are rationally seeking explanation for why that is and, apart from the problems the introduction of one causes, until we find otherwise, we must assume there is no designer. As things stand, just about the entire scientific community (and all relevant experts) accept the theory of evolution, meaning that those who argue otherwise must supply reasonable evidence that it doesn't. Speciation is a rare event when considered on a human timescale (no serious evolutionary scientist expects new species to be springing up every week) but it is not unknown and scientists have observed the "birth" of novel species, additional evidence for evolution explaining the Earth's observed biological diversity. Interestingly, creationists act as if species are something magical, something both weird and unique but nothing could be further from the truth. Evolutionary biologists define what species are (and aren't), deciding whether an animal's differences suffice to consider it a species and it does so to allow better cataloguing and identification; speciation is a classification system designed to enhance our ability to study life systems.
ConclusionIn the creationist's mind, something is acting to prevent them accepting clearly demonstrable evidence and one can only assume that is personal belief. They believe that humans are special, exalted above all others, despite there being no validatable evidence to support the idea. Religious beliefs are personal and cannot count as evidence; they do not equate to fact or the reasonable interpretation of the same. The evidence shows that adaptation (small change plus small change plus more small change plus yet more small change and so on) over immense periods of time leads to large changes. Such changes in biological systems can lead to the formation of complex structures, to simple life and to complex life via speciation. Speciation leads to the development of intelligence, of morality and to the biological diversity we see around us today. Given the adverse conditions, the ability to adapt and immense periods of time, speciation will happen and only one who is scientifically illiterate or disingenuous would deny that possibility without first identifying the required mechanism to stop adaptation becoming speciation. Which brings us back to the original claim and the question one has to ask in response. What prevents "micro" evolution from becoming "macro"? Assume, if you will, that we (all of science and its adherents) are simple-minded, that we need to be told exactly why things happen. If the simple concepts outlined above are not true, there must be something that prevents them, so the question creationists have to answer is, what is that mechanism? Perhaps creationists can enlighten us and tell us exactly what the difference is between "micro" and "macro" evolution (without resorting to tiresome clichés like "micro" equals adaption and "macro" equals speciation) and what forever stops one naturally proceeding to the other? It's a simple enough question for an evolutionary biologist and any reasonably well-informed science adherent to answer. The clear implication of the creationist claim is that there are limits to evolution. What limits it? In the meantime, the rest of us can, in principle, simply wait and laugh at those getting themselves wound up about something, despite their personal beliefs, so obviously true. Science adherents will still have to defend against those who either wilfully or through ignorance misunderstand science and evolution but we can sit back, smile, and say, "Show me the evidence!"