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Wortfish wrote:Where you have two diverging populations, isolated from each other, there will come a point when individuals from both populations may not be able to interbreed with each other. However, they can interbreed with members of the same population. That would be called speciation. But if you have just a single evolving population or lineage, then speciation can only happen if parents give birth to offspring who are of a different species and cannot (hypothetically) interbreed with them.
DavidMcC wrote:Shrunk wrote:Wortfish wrote:Thommo wrote:
Ok, so this is very important. You have acknowledged here that it is not true that speciation is impossible under the conditions you specified.
I can understand speciation occurring as a result of two isolated populations diverging apart without violating Dawkin's condition. In that scenario, it is the accumulated differences between the populations that matter, not between parent and offspring.
I see. So differences are accumulating between the two parallel lines, but within the lines (both of which begin with a single common ancestor) nothing is changing.
That isn't what he said. Lines can diverge from each other without becoming internally more divergent.
Shrunk wrote:DavidMcC wrote:Shrunk wrote:Wortfish wrote:
I can understand speciation occurring as a result of two isolated populations diverging apart without violating Dawkin's condition. In that scenario, it is the accumulated differences between the populations that matter, not between parent and offspring.
I see. So differences are accumulating between the two parallel lines, but within the lines (both of which begin with a single common ancestor) nothing is changing.
That isn't what he said. Lines can diverge from each other without becoming internally more divergent.
How so?
Shrunk wrote:No problem. TBH, I don't see how that relates to what Wortfish wrote, ...
Shrunk wrote:
OK, I see the confusion. My statement was a sarcastic rewording of what Wortfish had said. IOW, what I wrote was knowingly and intentionally incorrect. Sorry if that wasn't clear.
NineBerry wrote:You still don't get that the definition of species that is used is not useful when looking at beings that do not live at the same time. The concept of species is a tool used in biology in certain contexts. It is not useful when talking about beings living in different time periods except when for that purpose you use different criteria to decide which beings belong to the same species.
Wortfish wrote:NineBerry wrote:You still don't get that the definition of species that is used is not useful when looking at beings that do not live at the same time. The concept of species is a tool used in biology in certain contexts. It is not useful when talking about beings living in different time periods except when for that purpose you use different criteria to decide which beings belong to the same species.
The basic point is this: If members of a species can only give rise to members of the same species, how can new species emerge?
Thommo wrote:Wortfish wrote:NineBerry wrote:You still don't get that the definition of species that is used is not useful when looking at beings that do not live at the same time. The concept of species is a tool used in biology in certain contexts. It is not useful when talking about beings living in different time periods except when for that purpose you use different criteria to decide which beings belong to the same species.
The basic point is this: If members of a species can only give rise to members of the same species, how can new species emerge?
The exact same way that whole numbers only ever differ by one from their neighbour, yet any number has a number that is a million greater than it.
That's notwithstanding that this isn't how speciation occurs, as was already covered. And it's through speciation that new species emerge.
Wortfish wrote:
To use your analogy, the difference between 1 and -1 is the same as the difference between 10 and 12. But 1 and -1 are different types of signed integers whereas 10 and 12 are of the same positive type. You can't really escape the problem that - for a new species to emerge - offspring must be produced that are of that new species and not of their parents' species.
Wortfish wrote:I get what you mean about different time periods, but the argument only makes sense if - as you say - the definition of a species is just a case of arbitrary biological nomenclature and only reflects the accumulation of change within the same group over a period of time. I thought it meant the distinction made between a group of organisms that can interbreed among themselves only.
Wortfish wrote:Thommo wrote:Wortfish wrote:NineBerry wrote:You still don't get that the definition of species that is used is not useful when looking at beings that do not live at the same time. The concept of species is a tool used in biology in certain contexts. It is not useful when talking about beings living in different time periods except when for that purpose you use different criteria to decide which beings belong to the same species.
The basic point is this: If members of a species can only give rise to members of the same species, how can new species emerge?
The exact same way that whole numbers only ever differ by one from their neighbour, yet any number has a number that is a million greater than it.
That's notwithstanding that this isn't how speciation occurs, as was already covered. And it's through speciation that new species emerge.
To use your analogy, the difference between 1 and -1 is the same as the difference between 10 and 12. But 1 and -1 are different types of signed integers whereas 10 and 12 are of the same positive type. You can't really escape the problem that - for a new species to emerge - offspring must be produced that are of that new species and not of their parents' species.
Wortfish wrote:Thommo wrote:
What remains? We now have two populations which cannot interbreed and are thus different species yet we have not violated the parent-child interfertility condition.
Yep, understood. But looking at our own lineage, we know that we have ancestors that were not members of our species. How did these ancestors produce direct descendants that are not of their own species if Dawkins is to be believed?
Wortfish wrote:Another example would be 99 and 100. The former is two digits and the latter is three. But they are as apart as 6 and 7.
Thommo wrote:
Umm, exactly. You have an arbitrary division (the number of digits in base 10, these numbers have the same number of digits in base 9 and 11) and you can get there with "parent numbers" that are no more different than any other set of "parents".
So to recap:
- You have agreed that there is no barrier to new species emerging (this is known as speciation).
- You have not challenged that large differences of any magnitude can accumulate by incremental small steps.
- You are still puzzled by the arbitrary way species get named and distinguished into categories historically.
The thing you're puzzled over doesn't matter though, it's purely conventional, and that convention is (roughly speaking) that we refer to ancestors of a population as being of a "different species" if their genome would not allow them to directly interbreed with the descendant population if both populations were alive today.
Wortfish wrote:Thommo wrote:
Umm, exactly. You have an arbitrary division (the number of digits in base 10, these numbers have the same number of digits in base 9 and 11) and you can get there with "parent numbers" that are no more different than any other set of "parents".
The point is that, at least in base 10, there is a difference between 99 and 100 which is more than just a small increment.
Wortfish wrote:All I am saying is that a new species can only emerge if the parents of one species give birth to offspring of another species at some point - or else the same species remains.
Wortfish wrote:There may well be a gradual process leading up to this, but the change has to occur if a new species is to arise. If we were to trace your ancestors far back enough, one of them would eventually not meet all of the criteria for being an anatomically modern human and so would have to be categorised as a different species. However, its immediate descendant would (just) be sufficiently modern to be part of our species, thereby refuting Dawkins.
Wortfish wrote:Got that. But it's not just a case of historical retrospection and interpretation.
Wortfish wrote:As I mentioned, the ancestor with the fused chromosomes, 2a and 2b, may have been reproductively isolate from its parents.
Wortfish wrote:... You can't really escape the problem that - for a new species to emerge - offspring must be produced that are of that new species and not of their parents' species.
Another example would be 99 and 100. The former is two digits and the latter is three. But they are as apart as 6 and 7.
Evolution
Further information: Chimpanzee genome project
All members of Hominidae except humans, Neanderthals, and Denisovans have 24 pairs of chromosomes.[10] Humans have only 23 pairs of chromosomes. Human chromosome 2 is a result of an end-to-end fusion of two ancestral chromosomes.[11][12]
The evidence for this includes:
The correspondence of chromosome 2 to two ape chromosomes. The closest human relative, the chimpanzee, has near-identical DNA sequences to human chromosome 2, but they are found in two separate chromosomes. The same is true of the more distant gorilla and orangutan.[13][14]
The presence of a vestigial centromere. Normally a chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere in the q21.3–q22.1 region.[15]
The presence of vestigial telomeres. These are normally found only at the ends of a chromosome, but in chromosome 2 there are additional telomere sequences in the q13 band, far from either end of the chromosome.[16]
According to researcher J. W. IJdo, "We conclude that the locus cloned in cosmids c8.1 and c29B is the relic of an ancient telomere-telomere fusion and marks the point at which two ancestral ape chromosomes fused to give rise to human chromosome 2." [16]
Thommo wrote:
Just because every generation may be able to interbreed with its predecessor and successor that does not mean it must be able to interbreed with its 99th degree predecessor or its 9999th. The intermediate generations have long since passed away, closing down the pathway through which genetic information once flowed in exactly the same way as geographical isolation.
No, because species is a concept relating to multiple populations alive at the same time. Borrowing that concept for a different situation doesn't directly translate like that.
It is. It is purely a matter of convention.
Maybe, maybe not. Wouldn't matter either way in terms of a general principle though. If (as you repeatedly suggested) it's impossible for speciation to occur without such a quantum leap then you're about 4 billion examples short.
Wortfish wrote:Thommo wrote:
Just because every generation may be able to interbreed with its predecessor and successor that does not mean it must be able to interbreed with its 99th degree predecessor or its 9999th. The intermediate generations have long since passed away, closing down the pathway through which genetic information once flowed in exactly the same way as geographical isolation.
Let's play along with this. So your argument is that interbreeding is possible for the immediate predecessors/successors but that if we go back 100 genertions or so, interbreeding might not be possible. OK. But in the gradual model you espouse, there has to be a point at which, if we did have the two generations separated by time living contemporaneously, interbreeding wouldn't work out. Thus, the parents of the offspring of generation Y might just about be able to interbreed with their ancestors of generation X, but the offspring, repesenting generation Z, might not be able to because a threshold has now been crossed and the cumulative number of incremental changes means a new species has now arisen.
Wortfish wrote:... then speciation can only happen if parents give birth to offspring who are of a different species and cannot (hypothetically) interbreed with them...
Wortfish wrote:It is. It is purely a matter of convention.
It is also a matter of definition in terms of reproductive compatibility.
Wortfish wrote:Maybe, maybe not. Wouldn't matter either way in terms of a general principle though. If (as you repeatedly suggested) it's impossible for speciation to occur without such a quantum leap then you're about 4 billion examples short.
Well, at least in plants, whole genome duplications can easily result in a new species suddenly appearing as Dawkins admits. That isn't a quantum leap though.
Thommo wrote:The explanation is the same as it was on the previous occasions.
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