“all mutations are ‘sudden’ in that they occur and appear in a first individual and then spread in a population”.

But they can only spread once a double recessive appears. As you admitted earlier, ‘ AND which eventually get captured in a small population in a chromosomal speciation event’. So we’re back to some level of inbreeding in an isolated population, which leads us back to Ernst Mayer’s position.

“The initial appearance of a new single gene character is always sudden”.

Yes. Because by the time a double recessive appears single recessives can be widespread through the population. In the case of a double recessive breeding with a single recessive half the offspring will be double recessive, and off they go, breeding with more single recessives. Therefore you’re correct in saying that fundamentally I agree with you that ,’there is also a reasonable argument that this type of change could be a single homeotic mutant’. However I still have problems in accepting that individuals with ‘large morphological changes creating new body plans’ would be able to successfully breed with those who don’t.

But I certainly have no problem with your idea that upright walking may be the primitive condition and knuckle-walking is the derived. That would certainly easily explain the connection between the eastern gibbon/orang and the western gorilla/chimp/human groups. We don’t then have to postulate continuous forest between the two regions at some time.

Stop. You are totally misreading what I write. Again and again.

I am not concerned at all with “sudden speciation.” That has no part in my position and I don’t claim it. I don’t state sudden speciation.

You are continuing to confuse mutation with speciation.

Please carefully and slowly re-read my comments. I very carefully said that you would have a first green finch when a green sibling was born. That is not the same as a speciation event. It is just a mutation. The argument is very clear for the 180 degree flip or for the bilaterian mirroring. These are single gene mutations that cause large morphological changes creating new body plans, but they do not form new species. Speciation is entirely a different subject. In my recent comments I have slowly traced this out. Your argument about sudden speciation is irrelevant to my comments and it is irrelevant to the question of what features constitute a human.

Speciation is about reproductive isolation that leads to different sets of alleles in the separated populations. Some types of speciation are related to gradual accumulation of allele changes. I have just tried to point out that this sort of body plan origination can involve a sudden morphological change – remember, all mutations are “sudden” in that they occur and appear in a first individual and then spread in a population. The initial appearance of a new single gene character is always sudden. It is a chemical reaction on one base in a DNA chain of a gamete. You are confusing a variety of different biological processes that occur at various levels.

With regards to mutations, there is a reasonable argument that the vertebral change represents an accumulation of hundreds of small adaptive mutations that gradually repositioned and restructured the vertebra over 500 generations. However, there is also a reasonable argument that this type of change could be a single homeotic mutant. Either way, I believe it happened by 21.6 million years ago and that it defines a new type of hominoid subgroup that then is introduced into a new adaptive zone by the emergence of vertebrally based habitual upright posture and bipedalism.

I’m quite prepared to accept that. Where I have a problem is with your arguments in favour of instantaneous speciation.

“This is why it does not take 15 million years of gradual evolution to change the proconsuliform or cercopithecoid type of lumbar vertebra into the radically different form of the modern human lumbar vertebra”.

I think everyone these days agrees that evolution often is extremely rapid. But one generation?

“The Bilateria (insects, crustaceans, humans) emerged abruptly in a single generation as a single mutation”.

What evidence do you have for that? I doubt it.

“At that point you would have a yellow finch species and a green finch species”.

No. At most you might have two subspecies, or a polymorphic single species. They’d still be capable of forming fertile hybrids unless they remained isolated long enough for incompatible genes to evolve in one or other population.

“The first hominiform hominoid of human aspect is the one offspring 21.6 million years ago that has the septo-neural transposition and is thereby primarily an upright biped”.

Leaving aside arguments over the 21.6 million, the individual with that ‘first hominiform hominoid of human aspect’ cannot be a new ‘species’ if he or she can form fertile offspring with individuals of the old species. Either that, or he or she is a species consisting of just one individual. It’s more likely that the species is polymorphic and the new mutation came to dominate over time.

The term ‘gene’ was used long before we knew how they were carried on the chromosomes. The examples of gross chromosome change you have mentioned are simply one of the ways new genes form. You still need two copies of the change before it’s expressed. So you’re talking about recessive genes.

Further, you’re claiming that once the double recessive appears it is immediately a new species, by definition, unable to breed with the old. So, as well as the ‘very very very occasional circumstances’ in which a beneficial gene evolves you’re also arguing for the even more unlikely scenario that more than one individul of this new species will appear at the same time. I’ll grant some of the individual’s brothers or sisters could also be double recessive, but this would provide a very limited gene pool as the basis for the long-term survival of the new species.

On the other hand a single isolated population of a species could change quite rapidly with the spread of an advantageous recessive gene. Those with the double recessive would still be able to breed with single recessives, and even double dominants. But the double recessive form would replace the previous dominant gene by selection. And the gene could spread rapidly once a double recessive individual appeared if the gene had already spread through the population’s geographic range as a single recessive.

So, ultimately we arrive back at your professor’s comment, ‘This required some sort of geographic barrier to block the flow of alleles before actual speciation took place’. For one species to develop into two we need separation, either geographic or tribal (ecological?).

Ok.
The comment is from TerryT:
“Not likely. If the relevant mutation giving rise to a new species occurred in a single individual (as seems most probable) how on earth would it reproduce to be able to sustain the new species? Asexually?” (apologies to El Paleo Freak on the attribution)

Obviously, any initial mutation will always occur in a single individual. If the change is the 180 degree flip (and this is not my idea, it is a fact – see De Robertis & Sasai (1996) Nature 380: 37-40) then once it has occurred you have the new kind of animal – even before it separates off as a species.

For instance, if finches are yellow, and then one bird is born that is green, then that individual could be the first green finch. The allele could spread and eventually become the most common or the fixed allele in a new species whose individual are always green. At that point you would have a yellow finch species and a green finch species. However, the first green finch would be an individual offspring of a yellow finch. The first vertebrate is one offspring of an invertebrate that has undergone the 180 degree flip during its embryogenesis (even though there is no speciation yet).

The first hominiform hominoid of human aspect is the one offspring 21.6 million years ago that has the septo-neural transposition and is thereby primarily an upright biped. In homeotic change, single base pair changes can result in large morphologic changes.

A similar event is involved in the origin of the Bilateria. Prior to this point, we had non-symmetric or radially symmetric creatures (worms etc). Then a right left mirror occurred (in the initial embryonic read out gradient in the Hox genes) resulting in the first Bilaterian. This would be as distinct from the unlikely scenario of a left half evolving gradually under Darwinian selection over many millions of years from creatures that initially had only a right side with a partial left side sticking out. The Bilateria (insects, crustaceans, humans) emerged abruptly in a single generation as a single mutation. This would be as distinct from the subsequent speciation event that captures the change in a distinct lineage. Starfishes have multiple morphogenetic mirrors.

This is why it does not take 15 million years of gradual evolution to change the proconsuliform or cercopithecoid type of lumbar vertebra into the radically different form of the modern human lumbar vertebra. It is seen 21.6 million years ago – very soon after the emergence of the hominoids from the cercopithecoids and then it stays fairly unchanged on into modern times.

Hence the proposal. The feature that really distinguishes human from ape is not intellect but habitual primary upright bipedal posture (consider the brilliant conversational chimpanzee mentioned above that is still a very smart chimp and not a human) – a matter of body plan. If we can find a lumbar vertebra fossil and see that a given hominiform hominoid has the homeotic mutation that underlies it, then we have a feature you can see in a fossil that reveal the basis of upright posture, hence the basis of the “human” type of animal. This becomes a consistent “primitive” lineage. Other lineages like gorillas, orangs and chimpanzee independently branch off from an upright bipedal ancestor to specialize as diagonograde knuckle walkers or fist walkers. For this reason, we have a creature with the typically human body form (upright primary biped) as the ancestor for various ape lineages as well as being ancestral to our own lineage after the chimpanzee lineage splits off. Our ancestors before the chimp emergence look fairly similar to our ancestors after the chimp emergence with regard to habitual upright bipedal posture.

Again: No, I didn’t :o)

I think you and I are in agreement on all this. A homeotic mutation occurs. This may cause a large of amount of morphologic change, but is just another allele. If it confers an advantage, it’s frequency will increase. A speciation event may occur leading to a daughter species with the allele or it may spread through the population with no speciation event.

For some reason, El Paleo Freak asserted that if there was a large morphologic change (such as the invertebrate to vertebrate change – or even the septo-neural transposition in early hominiform hominoids) then you would need asexual reproduction because it would lead to a species of one individual. I am making the point that the morphological change is entirely independent of speciation. We don’t even need habitat change for geographic separation. This is all not controversial.

I said that. And I still say you don’t get sudden speciation because the new species has to start with a population of reasonable size.

“It is an entirely different phenomenon associated wht the centric fission and fusion of chromosomes”

You pointed out that individuals with ‘centric fission and fusion of chromosomes’ can still interbreed with members of their species who don’t have it. So you do not get a sudden change of species even under the scenario you propose.

“When major morphogenetic changes occur … In very very very occasional circumstances it might even be beneficial”.

But the individual with that beneficial mutation still has to find someone else with that same mutation to have offspring with. An unlikely event, unless the mutation has already spread through the population, behaving in a manner similar to a recessive gene. And ultimately giving the same result.

If you go back through what I’m saying about chromosomal speciation – in fact you can have speciation with no mutation. It is an entirely different phenomenon associated wht the centric fission and fusion of chromosomes but is not directly related to sequence change in the DNA per se.

The point for the scenario here is that they are decoupled. El Paleo Freak objected to sudden morphogenetic change by asserting you would then have to have asexual reproduction. I am just pointing out that with homeotic genes, you can have a great deal of morphologic change with no speciation. In this way, the new morph can begin to appear in many individuals before (quite independently) a speciation event occurs to capture the change.

What’s the difference, apart from the fact that genetic change/mutation doesn’t always lead to speciation. But speciation can only arise through genetic change/mutation.

“reproductive isolation of two populations”

Reproductive isolation would usually lead to subspeciation before it gave rise to speciation.

“What we are concerned with is the homeotic changes that permit survival, do not include a mechanical barrier to interbreeding with other individuals in the species that don’t carry or express the new homeotically altering allele AND which eventually get captured in a small population in a chromosomal speciation event”.

Isn’t that just the same thing as, and more easily explained as, being a recessive gene. An advantageous recessive gene can only be expressed after being ‘captured in a small population in a chromosomal speciation event’: inbreeding. If the recessive has become widely spread through a population the phenotype change could be rapid. By using recessive genes to explain the process you don’t have to postulate ‘In very very very occasional circumstances it might even be beneficial’. Disadvantageous recessive genes are held in check by the individual’s lack of survival. With the expansion of a new recessive gene combination you could well ‘have a new species with a high rate of occurrence of the new morphology. If the allele goes to fixation, it becomes characteristic of the new small founder population. In this case the new daughter species looks very different from the parent species’.