lundi 21 avril 2008

Update on chromosome numbers

Talkorigins January 99 was the last look I had on Chromosome numbers, my earlier post depends on that info. Talkorigins July 05 adds information, if you scroll down a bit.

Yes, it seems that uneven chromosome numbers from different parents do occur in okapis, and that one chromosome from one may mate with two chromosomes from other: individuals with 22, 23 or 22.5 chromosome pairs (44, 46 or 45 chromosomes). No other mammals than okapis have been sighted so far*. The variations in number would still seem to come about by fission or - though not in this case - polyploidy. And apart frogs, it would seem that rodents too can be polyploid.**

I take the freedom to quote the last talkorigins, after he has answered on chromosome numbers:

Here's your budding tree.

common ancestor Chimp ancestor (single individual)
Human ancestor (single individual)

No, surely the branching involved entire breeding populations -- one band, or a few bands, of apes moving into a new territory far from the lands where other members of their species lived. As noted, at the branch point, both populations would have been apes of the same species; they wouldn't become different species until after the branch point, after geographical separation left them free to evolve in two different directions. Remember that, just as there was no "first French speaker" struggling to make himself understood in a nation of classical Latin speakers, so there was no "first human" or "first chimpanzee," but only a gradual change over many generations from the same ancestral species.***

Can you take it from there? What's the pathway? At this point, can the chimp ancestor still interbreed with either the common ancestor or with the human ancestor? It has to interbreed with something in order to produce more offspring after its own kind, so where does the partner come from?

Most evolutionists hold that most speciation events are "allopatric," meaning that they occur after the ancestral population has split into two groups that could interbreed if they met, but which no longer meet up. Afterwards, mutation, genetic drift, and selection to different environments gradually change the populations into different species. No particular mutation (unless you count polyploidy) is likely to produce a new species. A better (though still oversimplified) approach would be to think of a whole series of mutations, some beneficial, most neutral (but they still made us different from chimps), that each made the bearer a tiny bit more "human" (or, in the other lineage, a tiny bit more "chimp"). No single gene would have made its bearer much different from other members of his species, or unable to interbreed with them.

The scientist seems to have missed the point that "Zoe" had probably read my argument (with God knows how many intermediates) as implied in post 3 about Chromosome numbers: the whole point of her discussion was that there had to be a first individual with a different point of chromosome numbers from its parents and otherwise possible mating partners.

Though the okapi example and that of Przewalski's horse vs domestic horse interbreeding make the impossibility less heavy, it is at least a question of very rare occurrence and in mammals other than rodents polyploidy seems to have no rôle in augmenting chromosome numbers, and the fusions seem to have to have occurred very often if they are to account for all lessenings of chromosome numbers.

Of course I totally agree that single gene mutations do not as a rule interfere with interbreedability. There are lots of single genes where pa and ma were different, same applies to apples (anyone read the book Botany of Desire? It seems apples and men are species where you can count on sexually produced offspring being sensibly different from their origin and indeed unpredictable because of all chromosome differences and recombination possibilities): even in my late guinea pigs, a wavy haired white male, angora guinea pig, I think, a straight haired female golden aguti, a "curly" haired female called "Bianca Croce" with a white cross on darker colours (hence the name) produced varied and fertile offspring. But I am pretty sure they had the same chromosome numbers.

Hans Lundahl

Aix en Provence
Monday of Holy Week
8/21 April 2008

PS: Tried to notify the source of the second statement (the one here quoted), but his e-mail was outdated.

*and mice, sorry, another update will be due, here it is, along with evening primroses.

**Here is that quote:

polyploidy is duplication of the entire genome; plants speciate this way all the time, but it's rarer for animals (though there are strongly supported examples for frogs, rodents, and other vertebrates; presumably, they can't form a new species unless they can either reproduce parthenogenically, or unless polyploidy happens often enough that eventually it produces two members of the same species at the same time and place)
it is left unclear whether polyploid rodents would include only individuals or parthenogenic lines or any new species - note also that in polyploidy it is a question of all chromosomes, terefore same chromosomes doubled, whereas in doubling or halving of chromosomes other than doubling of all, the chance of two independently getting same modification is close to nil.

***But there was a first speaker for every change that differentiates Latin from French! There was a first speaker to drop the final nasalisation in "servum" (-um like -om in Portuguese bom) coming up with roughly servu (as in Sardinian/Corsican) or servo (as in Italian/Spanish), there was a first speaker to drop the vowel altogether (as in French/Occitan), and each first speaker had to make himself understood: and there was a first writer to decide not to write "ser(f)s, serf" as "servus, servum" but as "ser(f)s, serf", and he did it because he wanted it read and pronounced by people who did not have Francogallic Romance as a mother tongue when speaking on a certain occasion to people who did: the Occasion was the Oaths of Strasburg. The other side of the oath was written in passable phonetic approximation of Old High German, like Ripuarian Frankish or something. And passably phonetic approximations were again possible because Alcuin had come from York to teach the French to pronounce written "servus, servum" as, precisely "servus, servum" when speaking in Church. Other difference: we know people who spoke more or less Classic Latin gave rise to speakers of French, Occitan, Catalan, Castillian, Italian, Sardinian, Corsican: because we have the terminus a quo as well as the terminus ad quem (like the Gentle and magnanimous Frenchies that surround me here) under close observation by mainly themselves, including as writers, a capacity in which they sometimes survive their death. We know there was an Ausonius of Bourdeaux, to whom Classic Latin was essentially good grammar, and we know there was a Dante Alighieri of Florence to whom speaking Latin meant using the invention of speaking grammatically, an "invention" he describes in terms reminiscent of Esperanto - all the while using "si" "oc" and "oïl" to distinguish the languages he thinks of as real vernaculars (recognisable as Spanish/Italian, Provençal, French). And between them we know Alcuin and the oaths of Strassburg. Before Alcuin and after Ausonius we know that Gregory of Tours wrote bad Latin and Fredegar worse - as if they spoke Frenchily but wrote nearly Latin. After Alcuin we have the varied literary works in diverse Romance languages coming up, Song of Roland nearly two hundred years before Dante wrote Vita Nuova and Divina Commedia. We know all these individuals as we will never know Lucy - because either Lucy didn't write, or if she did her writing has not survived or if it did it is not put in relation to her sceleton. We know writers of old centuries as well as we know bloggers of foreign continents. At least as well as we know bloggers we have never written to or who have no possibility to write back. There are things we can never know about someone, because he is far away in place or time, because they are not directly there in the text, or because they have not been written down, but there are things we do know, because they have been written down. Lies cannot be totally excluded in all circumstances, but neither can those or other fakes in face-to-face intercourse. And what language someone writes when he writes himself is hard to fake. You may write a foreign language, but you may not write above your level in it, though you may sometimes improve it.