The point is that no, it's not "close enough". And there's nothing "precious" about insisting upon an accurate statement instead of a misleading inaccuracy, especially in a scientific context.
Let's say that your name is Joe Smith. Is it acceptable to say that you are descended from your cousin Frank Smith, because that's a "close enough" description of a non-Joe Smith? Your cousin may have family similarities to both yourself and your grandfather, sure, but that doesn't mean that he's interchangable with your grandfather.
To point out another flaw in your "close enough" argument, the same (poor) reasoning would allow the claim that chimps are "descended from humans", since "'human' is a close enough describe [sic] a non-chimp primate, without getting precious about it."
Accuracy is important.
And while your more general implication is a somewhat valid point -- the the common ancestors of chimps and humans could fairly be described as "chimplike", the problem with just calling them "chimps" is that it misleadingly hides the point that they were also "humanlike".
Over the course of 20 years, I wrote about evolution as describing the origin of man for for mass-market magazines.
It worries me that you wrote about evolution for the public for twenty years, yet neither understood the more critical details, nor cared much if you didn't get them right.
Unfortunately, a strong contributing factor to many people's impression that evolutionary biology is "ridiculous" and unbelievable can be laid squarely at the feet of pop-science writers who present it in an inaccurate or cartoonishly oversimplified manner. Some of the pop-science presentations of evolution *are* ridiculous. But the science itself isn't. The public often mistakes the former for the latter.
But the origin of species is something completely different, and much thornier.
No, it isn't. If you thought your following "problem" was an example of a "thorny" issue in speciation, you're mistaken.
The definition of a species is that it can't reproduce with anything outside the species. This gets really tough when you're talking about sexually reproducing species—such as chimps and us. Let's say you get a freak that has a different number of chromosomes from its parents, a new species. Where do you get two of them (male and female), to reproduce? A brother and sister? The offspring of such unions have a low survival rate—and we're asked to believe they flourish and multiply better than the competition? And yet this is assumed to have happened, not just once, but for all of the billions of sexually reproducing species on, pardon the expression, God's green earth.
Excuse me, but AAARRRGGGHHH!!!
How on *Earth* did you manage to write about evolution for 20 years, and interview "scores" of biologists, and manage to maintain such an elementary misunderstanding of it? Didn't you bother to *ask* any of those biologists about your "oh my god this looks like a real problem for evolution" notion? Or did you just keep it to yourself, thinking, "ha ha, these poor deluded fools, going about their research, without realizing that I hold the secret which makes all their theories unworkable..."
And did you ever let this bit of misguided ignorance get into your articles about evolution?
I know that you either never asked the question, or if you did you didn't understand the answer, because there's no way your consultants wouldn't have known the answer -- this is such a basic, elementary aspect of population dynamics that finding a biologist who couldn't rattle off the answer would be like finding a car mechanic who couldn't tell you what a spark plug was for.
Oh my, where to start... At the top, I suppose. You start with, "The definition of a species is that it can't reproduce with anything outside the species." No, this is incorrect. While it's true that if two groups *can't* interbreed, they are necessarily separate species, the converse is not true. Groups that can interbreed to some degree can still be separate species. Consider lions and tigers, for example. A better definition is that species are groups that *don't* interbreed to any large degree. A more technical way to put it is that they are independent breeding populations. But there are exceptions and gray areas -- this is because nature itself does not recognize the "species" concept. It's a manmade label applied for convenience and utility to certain groups. If Darwin was right, there should not be clear-cut distinctions between groups as they are in the process of diverging evolutionarily. And indeed, this is exactly what we find, which is why there's no "one definition fits all situations" meaning for "species". Groups like "ring species" throw a monkeywrench into any "nice and neat" definition of "species" that humans might care to try to formulate, for example. Nature is nowhere near that tidy.
But even leaving that aside, your idea about how a population can split into two distinct species (even by your definition) is a wildly incorrect misconception about how it actually works.
You have two major misconceptions and wrapped them around each other.
The first is that species formation involves a sudden "freak" with a massive mutation that occurs in a single individual in one generation. Nope, wrong. This is widely snickered at in the biological community as the "hopeful monster" scenario. But it's not how evolution proceeds.
Your second misconception is that having a different number of chromosomes would prevent successful mating. It doesn't. Or at least it needn't, depending on the nature of the difference, and there are many known cases where it doesn't. For example, the Przewalski horse, which has 33 chromosomes, and the domestic horse, with 32 chromosomes (due to a fusion), are able to mate and produce fertile offspring.
A third misconception, a combination of your first two, is that speciation requires anything like an "extra" chromosome. It doesn't.
What actually happens (or at least in most cases -- as in my earlier discussion of the definition of "species", nature is flexible and abounds with variations, and refuses to follow any one "script" in every single case) is that accumulated small changes in a population diverge if from a parent population.
Note for example that there is no one "big mutation" separating humans from our nearest extant cousins, the chimps. There are *thousands* of genetic differences, as one would expect after five million years of divergent evolution between the two groups. Heck, there are hundreds of genetic differences between *human* groups, and we share common ancestors a lot more recently.
[Sidebar: However, the nature of any one specific difference considered by itself is minor and of the type one would expect to be produced by evolution. There are no portions of the human -- or chimp -- genome which are so different that they seem "completely rewritten", or "written fresh on the drawing table" when compared with the other group. Both the human genome and the chimp genome have been completely sequenced and are available on several online databases. I challenge any creationist to compare any portions of the two and look for any difference between them which are "unique", or are major minor variations from the other to be of the sort -- in both amount and kind -- which one would not statistically expect to result merely from five million years of evolutionary "drift". Good luck! None have been found so far by anyone, but hey, maybe you could be the first.]
One genetic mutation does not a new species make (again, usually). Often *hundreds* are not enough, as proven by the many genetic differences occurring even within human populations.
Instead, it takes *many*, *many* accumulated mutational differences to separate one population from another to a degree large enough to warrant describing the two as different species, and/or to interfere significantly with their ability/willingness to reliably interbreed.
So the answer to your question is simple: Speciation does not occur in a single generation by one mother suddently giving birth, *poof*, to an offspring so mutated that it's a "new species" from its mother, and unable to interbreed with the rest of its (sort of) kind. Instead, subpopulations of a larger population (often separated by distance, geography, or other barriers) each accumulate genetic differences apart from each other as new mutations accumulate separately in each subpopulation, each mutationoccurring originally in a single individual then spreading through the subpopulation in succeeding generations (while detrimental mutations get constantly weeded out by natural select, and beneficial mutations get "amplifed" by it), until eventually the two populations are different enough from each other in their overall genetic makeup so that morphologically they are obviously different "subtypes" of creatures even to the unaided eye, and no longer reliably interbreed with each other.
And yes, there are countless field studies and genetic studies and all sorts of other studies which have established the reality of this, it's not just a hypothetical scenario.
Now, was that so hard?
But since you brought up differences in chromosome number, here's a somewhat relevant prior post of mine which covers some associated issues, and provides further evidence for the common ancestry of humans and the other apes:
Humans have 23 pairs of chromosomes ---chimps and gorillas have 24 pairs. How many pairs of chromosomes did the "common ancestor" have? Was it 23 or 24 pairs? How do you "evolve" missing or added chromosomes ---that would happen all at one time.The common ancestor had 24 chromosomes.
If you look at the gene sequences, you'll find that Chromosome 2 in humans is pretty much just 2 shorter chimpanzee chromosomes pasted end-to-end, with perhaps a slight bit of lost overlap:
(H=Human, C=Chimpanzee, G=Gorilla, O=Orangutan)
Somewhere along the line, after humans split off from the other great apes, or during the split itself, there was an accidental fusion of two chromosomes, end-to-end. Where there used to be 24 chromosomes, now there were 23, but containing the same total genes, so other than a "repackaging", the DNA "instructions" remained the same.
If a chimpanzee gives birth to a creature with 23 chromosomes, that offspring isn't going to be a well-formed chimpanzee able to survive well.
It is if the same genes are present, which they would be in the case of a chromosome fusion.
Evolve would imply the genetic material changes little by little --not some big loss of two chromosomes at once but I don't see how they'd go away gene by gene.
Tacking two chromosomes together end-to-end is not a "big loss" of genes, and it really is a "little by little" change in the total genetic code. It's just been "regrouped" a bit. Instead of coming in 24 "packages", it's now contained in 23, but the contents are the same.
So how, you might ask, would the chromosomes from the first 23-chromosome "fused" individual match up with the 24 chromosomes from its mate when it tried to produce offspring? Very well, thanks for asking. The "top half" of the new extra-long Chromosome 2 would adhere to the original chromosome (call it "2p") from which it was formed, and likewise for the "bottom half" which would adhere to the other original shorter chromosome (call it "2q"). In the picture above, imagine the two chimp chromosomes sliding over to "match up" against the human chromosome. The chimp chromosomes would end up butting ends with each other, or slightly overlapping in a "kink", but chromosomes have overcome worse mismatches (just consider the XY pair in every human male -- the X and the Y chromosome are *very* different in shape, length, and structure, but they still pair up).
In fact, the "rubbing ends" of the matched-up chimp chromosomes, adhering to the double-long human-type chromosome, would be more likely to become fused together themselves.
For studies in which recent chromosome fusions have been discovered and found not to cause infertility, see:
Chromosomal heterozygosity and fertility in house mice (Mus musculus domesticus) from Northern Italy. Hauffe HC, Searle JB Department of Zoology, University of Oxford, Oxford OX1 3PS, United Kingdom. hauffe@novanet.itIn that last reference, the Przewalski horse, which has 33 chromosomes, and the domestic horse, with 32 chromosomes (due to a fusion), are able to mate and produce fertile offspring.An observed chromosome fusion: Hereditas 1998;129(2):177-80 A new centric fusion translocation in cattle: rob (13;19). Molteni L, De Giovanni-Macchi A, Succi G, Cremonesi F, Stacchezzini S, Di Meo GP, Iannuzzi L Institute of Animal Husbandry, Faculty of Agricultural Science, Milan, Italy.
J Reprod Fertil 1979 Nov;57(2):363-75 Cytogenetics and reproduction of sheep with multiple centric fusions (Robertsonian translocations). Bruere AN, Ellis PM
J Reprod Fertil Suppl 1975 Oct;(23):356-70 Cytogenetic studies of three equine hybrids. Chandley AC, Short RV, Allen WR.
Meanwhile, the question may be asked, how do we know that the human Chromosome 2 is actually the result of a chromsome fusion at/since a common ancestor, and not simply a matter of "different design"?
Well, if two chromsomes accidentally merged, there should be molecular remnants of the original chromosomal structures (while a chromosome designed from scratch would have no need for such leftover "train-wreck" pieces).
Ends of chromosomes have characteristic DNA base-pair sequences called "telomeres". And there are indeed remnants of telomeres at the point of presumed fusion on human Chromosome 2 (i.e., where the two ancestral ape chromosomes merged end-to-end). If I may crib from a web page:
Telomeres in humans have been shown to consist of head to tail repeats of the bases 5'TTAGGG running toward the end of the chromosome. Furthermore, there is a characteristic pattern of the base pairs in what is called the pre-telomeric region, the region just before the telomere. When the vicinity of chromosome 2 where the fusion is expected to occur (based on comparison to chimp chromosomes 2p and 2q) is examined, we see first sequences that are characteristic of the pre-telomeric region, then a section of telomeric sequences, and then another section of pre-telomeric sequences. Furthermore, in the telomeric section, it is observed that there is a point where instead of being arranged head to tail, the telomeric repeats suddenly reverse direction - becoming (CCCTAA)3' instead of 5'(TTAGGG), and the second pre-telomeric section is also the reverse of the first telomeric section. This pattern is precisely as predicted by a telomere to telomere fusion of the chimpanzee (ancestor) 2p and 2q chromosomes, and in precisely the expected location. Note that the CCCTAA sequence is the reversed complement of TTAGGG (C pairs with G, and T pairs with A).Another piece of evidence is that if human Chromosome 2 had formed by chromosome fusion in an ancestor instead of being designed "as is", it should have evidence of 2 centromeres (the "pinched waist" in the picture above -- chromosomes have centromeres to aid in cell division). A "designed" chromosome would need only 1 centromere. An accidentally "merged" chromosome would show evidence of the 2 centromeres from the two chromosomes it merged from (one from each). And indeed, as documented in (Avarello R, Pedicini A, Caiulo A, Zuffardi O, Fraccaro M, Evidence for an ancestral alphoid domain on the long arm of human chromosome 2. Hum Genet 1992 May;89(2):247-9), the functional centromere found on human Chromosome 2 lines up with the centromere of the chimp 2p chromosome, while there are non-functional remnants of the chimp 2q centromere at the expected location on the human chromosome.As an aside, the next time some creationist claims that there is "no evidence" for common ancestry or evolution, keep in mind that the sort of detailed "detective story" discussed above is repeated literally COUNTLESS times in the ordinary pursuit of scientific research and examination of biological and other types of evidence. Common ancestry and evolution is confirmed in bit and little ways over and over and over again. It's not just something that a couple of whacky anti-religionists dream up out of thin air and promulgate for no reason, as the creationists would have you believe.
