I've been fairly active on Khan Academy answering various questions in chemistry, biology and medicine-related topics. The questions I find the most appealing are the ones about evolution, because despite evoking a strong sense of "I don't want to live on this planet anymore"-esque disappointment, the innocent simplicity of the misconceptions I see in many questionable objections (or objecting questions) gives me some consolation. Maybe there really isn't a large wall of information (or disinformation) separating creationists from accepting proper scientific theories?
In this series of rants I am going to go through some common misconceptions about the theory of evolution by natural selection, although I can't promise any consistent activity given that the exams are ominously creeping up on me. I'll do my best though.
This particular question is one that pops up wherever you turn your head, and is notoriously mocked by anyone who hasn't already fallen for the crocoduck hypothesis or other straw man misrepresentations of the theory of evolution by natural selection.
The question is of course "If humans evolved from apes, why are there still apes?".
So why is that? Let's first of all try to dissect the question to reveal the assumptions that are firmly lodged inside its loaded shell.
Q1. Did humans evolve from apes?
Q2. Are humans and apes separate species?
Q3. Does speciation necessarily imply the extinction of the original species?
Q4. What is a species?
Let's try to answer the above questions in attempt to clear up some misconceptions that can be found already in the underlying premises of the question. I will deal with Q3 and Q4 in unison in a much longer response that I will post later on in a second part (for those tenacious enough to bear with me 'til the end of this series of rants).
A1. Did humans evolve from apes? This is a malformed question, but let's assume they mean "the other apes that we see walking around today" (notice the word "other"). No, we speciated departing from a common ancestor, with which both we (homo sapiens) and other apes share genes with. If by "apes" we mean the correct meaning (which we shall discuss in a moment), then it follows by definition that we evolved from apes, although this doesn't actually relate to the issue most people have.
A2. Are humans and apes separate species? When we talk about the evolution of species (speciation), we trace one species' descent leading up to another species. For the previous question to make sense, both "humans" and "apes" have to be two separate species, but is this really the case? No (this is a link). Apes (hominoidea) are a "huge" (pardon my esoteric jargon) superfamily (order) of species, encompassing both the hylobatidae and the hominidae families, which both include many species. Only once you zoom in, going from superfamily to family to subfamily (tribe) to genus to species do you actually get to the level where you can compare species directly to each other. Using the word "ape" is in other words a massive misconstruction of what speciation entails. What I'm trying to say is "humans ARE apes by definition".
A3. Does speciation necessarily imply the extinction of the original species? Well does it? I mean coincidentally the answer to the main question is always "we did not evolve from apes, we evolved from a common ancestor", but given the information in the previous answer, such an answer is clearly inaccurate, although it does get the common ancestor part right. What such an answer does, it seems to me, is to shift away from the notion of speciation without the extinction of the original species. Just because it didn't happen to us in relation with the other apes, this doesn't imply that there is a problem with such a process. What if we evolved directly from the same species we see walking around today? What if we evolved from orangutans, and not a common ancestor? What if? Does this pose any problem for evolution? Of course not. A species will only change its phenotypic traits (the physical result of your genes that comes in contact with the environment and is acted on by natural selection) if the environment requires it. There are many animals that have remained the same for millions and millions of years without having speciated in any way, although there may have been members of such a population of species that decided to migrate to another environment, where other selection pressures would of course favour other sets of genes, in which case you would ultimately get speciation over time - this does not mean that the first species died out though. Maybe the title of this specific topic makes a bit more sense now?
A4. What is a species? A species is classified as any group of animals that are able to reproduce to produce fertile offspring. That is all, there is nothing more to it. However, even if things are very black and white when it comes to species in our day and age, it isn't in principle like that. If I were to rape a female rabbit (that's right, I just went there) the probability of getting a son or daughter Rablexander or Rablexandra would be 0. However, if I were to rape a female human (rape is justified if used for scientific analogies), the probability of getting a son or daughter would be extremely close to 1 (let's not exclude the possibility that the child for some reason becomes infertile). Is there anything in between these two extremities (0 and 1), or are we really dealing with absolute borders in nature? Obviously there is anything and everything between those two extremities. Let's travel back in time to a point where the common ancestor between me and the female rabbit existed. We are now both part of the same species, and we know that an extremely long time away, we are going to end up as different species (humans and rabbits) - and by "we" I mean the population of species. At this point, the probability of getting fertile offspring by means of rape (still justified it seems) is 1. As populations diverge (as described in A3) the genetic similarity between the two population pools (total set of genes in that population, including all variations of genes etc.) will become smaller and smaller over time. The smaller the genetic similarity becomes, the lower the odds of producing fertile offspring, giving us a smooth gradient ranging from 1 to 0. At which point do we become separate species? I HAVE NO IDEA. The only thing we can witness right now is the end product, once we have reached 0, although we know with unquestionable certainty that the probability was once 1.