Of Course There Are Monkeys
The denialist loves to ask, "If we evolved from monkeys, why are there still monkeys?" Part of this is to deliberately set up the notion that evolution declares an equivalence between humans and monkeys to drive a wedge between the audience and evolution. However, this myth at its core misunderstands speciation.
Some species lives in a region A. It spreads, it grows in number, it explores the limits of its environment, then spills over into a second region B. Over time, geology happens and regions A and B are separated, isolating the two groups. Geology continues to happen, climate slowly changes, the environment changes. Perhaps region A becomes wetter and turns into a marshland while region B becomes more arid and turns into a grassy area with some stands of trees. The population of our species lives and adapts to each of these new climates. They mutate, they change. Their physical forms change and less obvious biological changes occur within. Eventually the two have changed enough that they can no longer interbreed. You now have two species where before you had one. At no point could you look at either and say, "Ah, this is clearly different to what came before." The change is always very slow, very gradual, and there will never be a clear dividing line.
Regions A and B don't even need to change for speciation1 to occur. Simple separation will be enough. Random changes will still accumulate in both groups and eventually genetic drift (also known as allelic drift) will build up to the point where the two groups will be chemically infertile. The geographical separation doesn't need to be drastic; it doesn't have to be something like a mountain chain springing up or an island separating from the mainland; simple distance will suffice. A ring species is one that has a long enough range that divergence has occurred between the individuals at either end. Subspecies A can interbreed with B, which can mate with C, which can make babies with D, which can totally start a family with E, but A and E are far enough apart that they're they're no longer interfertile. That's speciation in progress.
Another key item to note is that speciation doesn't mean that you start with species A and all of a sudden you have species B branching off of it. You begin with species A, then species A will split into subspecies B and subspecies C, still similar enough to interbreed but separated by geography2. They will continue to diverge until you have two separate species. They will still be obviously related, like the many species of cats alive today, and they'll also be clearly related to the ancestor species (with which they may or may not be able to interbreed; like ring species, but with time instead of space).
As for humans, we share a common ancestor with the two species of chimp, the common chimp and the bonobo. We share a more distant ancestor with the other great apes, the gorillas, and even more distant with the orangutan. None of those ancestors were the same species as any extant species.
The question above "why are there still monkeys" is like asking, "If you came from your grandparents, why are your grandparents still alive?", and what they're really asking is "If you came from your grandparents, why do you have cousins?"
Evolution Is Not a Tautology
Denialists like to claim that evolution doesn't contain any information. Natural selection chooses the fittest to survive, right? But how do we know which are the fittest? They're the ones who survive! It's circular! Like a donut!
Fitness has an actual definition. It's the relative propagation of a trait in a population. It's actually a mathematical definition. If the relative abundance of a trait increases from one generation to the next, then fitness is greater than one. If it decreases, it's less than one. For a single organism, it's defined as the relative contribution by the individual to the gene pool for that particular trait. Fitness is a multigenerational consideration and has to do not merely with how well an organism survives, but how it thrives and contributes to the survival and alteration of its species.
Evolution Does Not Predict Chimeras
Behold, the mighty crocoduck! That particular mythical creature is the favorite of Kirk Cameron, erstwhile child actor cum fundamentalist rabble-rouser. The denialist believes that speciation means mermaids and catdogs. That between species A and its successor species B, you'll find something that is literally half of species A stitched to species B. Evolution does NOT predict this. It definitely doesn't predict a transitional form that's two halves of modern species. This is a gross misunderstanding of the nature of transitional forms.
Look at red and yellow on the rainbow. In between you'll find orange. Zoom in some more and you'll find a color between red and orange. Zoom in further and you'll find another color, neither red nor orange, nor red-orange, but red-red-orange. Zoom in further and you'll find something that's redredorange-redorange in between the two. You'll never find a discrete line dividing any two colors. you'll also never find a color that's a bit of red stapled to a bit of orange.
As it is with the colors, so it is with evolution. When A changes to B, it does so very slowly, very gradually. Tiny, incremental changes accumulate. A lizard gives birth to a dizard that gives birth to a bizard that gives birth to a bzard that gives birth to a bzrd that gives birth to a burd that gives birth to a bird, and it all takes millions of years. You never see something with the head of lizard and the tail of a bird. A transitional form is something like this:
That's an archaeopteryx. Discovered just two years after Darwin published On the Origin of Species it was a stunning confirmation of exactly what his theory called for: a transitional form between reptiles and birds. You can see that it has feathers, wings, and scaled hindlimbs like both bird and lizard. However, it still has a toothy snout like a reptile, along with a long, reptilian tail among those feathers. You can also see differentiated fingers on its fore-limbs, which aren't yet wings; birds will lose those fingers to create a fully functional wing. The archaeopteryx is an actual example of a transitional form. That's why evolutionary theory took the world by storm; it accounted for all the facts and made incredible and accurate predictions like this. That's what a theory does.
The chimera is not the prediction of evolution. It's the prediction of religion and myth, a magically created creature with no antecedents in the fossil record and no natural way of appearing.
Humans Are Still Evolving
The last myth I'll cover here, and one I haven't put on the TVTropes page yet, is that humans are somehow above evolution.
Lots of people, even people who are fully willing to accept that evolution occurs, that the earth is 4.5 billion years old, that the universe is 13.7 billion years old, and that humans actually aren't the be-all and end-all purpose of creation in a universe so vast that we don't even count as lint... they just don't accept that we're evolving. I believe it's related to the notion that we're rational animals (not really), smart enough to avoid stuff like that (we're really not), and that we've seized control of our destiny thanks to our intelligence (absolutely false. It is to laugh).
We're totally evolving. As I said last time, each of us is born with dozens of mutations. We can't know where we'll end up. Hopefully we'll end up without some of awful crap I mentioned last time. Who knows, maybe we're turning into this guy.
Well, that's it for now. There are tons of myths out there about evolution, but these have been some of the most popular. Maybe later I'll talk about global warming denialism.
1 - The term for the formation of a new species.
2 - Geography is too specific a term. I should be saying that their habitats become separated. If one group spends all its time on the ground and the other spends all its time in the tree tops, then they can still be isolated enough to speciate.
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