Siberian Chiffchaff (4 Viewers)

[njlarsen]

Well I can't argue against any of these points! However, regarding human v bird perception of distinctiveness of traits, isn't that applicable to the entire discipline of taxonomy?! We judge differences in morphology, plumage and vocalisations through human eyes and ears.

I'm not sure how to interpret introgression from a species-level taxonomic perspective. Presumably it represents two distinct and previously disjunct species lineages that have expanded in range and are in the process of merging into a single species? If so, how does one treat the birds outside the contact zone? Is this comparable to Pine Bunting v Yellowhammer?

Or does this represent an early stage in selection for traits that will make the separation stronger? Or is it the early stages of producing a third species through hybridization? Possibilities are multiple.

Niels

 

[Jacana]

Well I can't argue against any of these points! However, regarding human v bird perception of distinctiveness of traits, isn't that applicable to the entire discipline of taxonomy?! We judge differences in morphology, plumage and vocalisations through human eyes and ears.

This is exactly the case, and one of the reasons why adhering too hard to the human construct of "species" can be fraught sometimes.

I'm not sure how to interpret introgression from a species-level taxonomic perspective. Presumably it represents two distinct and previously disjunct species lineages that have expanded in range and are in the process of merging into a single species? If so, how does one treat the birds outside the contact zone? Is this comparable to Pine Bunting v Yellowhammer?

Introgression is essentially the process by which genes move from one species/subspecies/lineage to another initially via hybridisation and then by backcrossing. Again, the phenomenon of introgression can play merry havoc with many of the species concepts. The Pine Bunting/Yellowhammer example is a classic problem for this. But introgression need not lead to complete merging. That would only be the case if hybrid and backcrossed individuals had a selective advantage over the parent species, and, crucially, that advantage would remain across the entire ranges of the parental species.

Speciation is as messy as it is interesting!

 

[MJB]

This is exactly the case, and one of the reasons why adhering too hard to the human construct of "species" can be fraught sometimes.


Introgression is essentially the process by which genes move from one species/subspecies/lineage to another initially via hybridisation and then by backcrossing. Again, the phenomenon of introgression can play merry havoc with many of the species concepts. The Pine Bunting/Yellowhammer example is a classic problem for this. But introgression need not lead to complete merging. That would only be the case if hybrid and backcrossed individuals had a selective advantage over the parent species, and, crucially, that advantage would remain across the entire ranges of the parental species.

Speciation is as messy as it is interesting!

Hence my mottos...
MJB

 

[THE_FERN]

This is exactly the case, and one of the reasons why adhering too hard to the human construct of "species" can be fraught sometimes.


Introgression is essentially the process by which genes move from one species/subspecies/lineage to another initially via hybridisation and then by backcrossing. Again, the phenomenon of introgression can play merry havoc with many of the species concepts. The Pine Bunting/Yellowhammer example is a classic problem for this. But introgression need not lead to complete merging. That would only be the case if hybrid and backcrossed individuals had a selective advantage over the parent species, and, crucially, that advantage would remain across the entire ranges of the parental species.

Speciation is as messy as it is interesting!

Probably more common is introgression of one or a few genes which replace those of another species entirely. This without any obvious impact on the phenotype and hence little or no impact on the species assignments.

 

[Jacana]

Probably more common is introgression of one or a few genes which replace those of another species entirely. This without any obvious impact on the phenotype and hence little or no impact on the species assignments.

I would say that is actually quite a bit rarer, although, as people dig deeper into the genomes of different species, these signals will become more common. Still, they will remain comparatively rare.

 

[THE_FERN]

I would say that is actually quite a bit rarer, although, as people dig deeper into the genomes of different species, these signals will become more common. Still, they will remain comparatively rare.

I would say that is actually quite a bit rarer, although, as people dig deeper into the genomes of different species, these signals will become more common. Still, they will remain comparatively rare.

I'm not sure why this would be rarer; where there's replacement of genes which have clear phenotypic effects then we tend to call things "hybrids" so to an extent it's a definitional thing. IIRC there are quite a few examples of "silent" displacement of (e.g.) mitochondrial genes in Drosophila, red wolves/coyote, other genes in humans/neanderthals etc.

 

[Jacana]

For a couple of genes to cross from one species to another, leading to complete replacement? I would say that is very rare.

Certainly there are some well known examples, but they are well known, precisely because they are rare. I'm not sure about the Drosophila examples, but I think the wolf example is correct. The human example is incorrect though- not all human populations have signals of Neanderthal genes (although I could imagine that the last Neanderthal populations had at least some modern human genes in them).

There's a great newt example (Lissotriton montandoni has had its mitochondria completely replaced by L. vulgaris mitochondria).

 

[Farnboro John]

This is exactly the case, and one of the reasons why adhering too hard to the human construct of "species" can be fraught sometimes.


Introgression is essentially the process by which genes move from one species/subspecies/lineage to another initially via hybridisation and then by backcrossing. Again, the phenomenon of introgression can play merry havoc with many of the species concepts. The Pine Bunting/Yellowhammer example is a classic problem for this. But introgression need not lead to complete merging. That would only be the case if hybrid and backcrossed individuals had a selective advantage over the parent species, and, crucially, that advantage would remain across the entire ranges of the parental species.

Speciation is as messy as it is interesting!

I think you've just argued yourself out of using introgression as evidence for or against prior speciation.

John

 

[Jacana]

I think you've just argued yourself out of using introgression as evidence for or against prior speciation.

John

I don't understand? Speciation is a process, not an end point. A process that does not have, nor need to have, a consistent direction.

 

[Farnboro John]

I don't understand? Speciation is a process, not an end point. A process that does not have, nor need to have, a consistent direction.

Indeed. However, what you were engaged in doing at #59 was suggesting that the ongoing introgression between tristis and collybita was a factor implying the two were in fact one species (and against a split) whereas at #62 you argue that it need not lead to complete merging, thus simultaneously reducing the impression of impact of the introgression and introducing a subsidiary implication that up to a certain point in the progress of the introgression the two were/are in fact separate.

I therefore conclude that introgression while in progress cannot be adduced in favour of one hypothesis (Siberian and Common Chiffchaffs are one species) or the other (Siberian and Common Chiffchaffs are two species). One would need to see how it plays out.

John

 

[Jacana]

Indeed. However, what you were engaged in doing at #59 was suggesting that the ongoing introgression between tristis and collybita was a factor implying the two were in fact one species (and against a split) whereas at #62 you argue that it need not lead to complete merging, thus simultaneously reducing the impression of impact of the introgression and introducing a subsidiary implication that up to a certain point in the progress of the introgression the two were/are in fact separate.

I therefore conclude that introgression while in progress cannot be adduced in favour of one hypothesis (Siberian and Common Chiffchaffs are one species) or the other (Siberian and Common Chiffchaffs are two species). One would need to see how it plays out.

John

Ah, I see what you mean.

On this point in particular:

... #62 you argue that it need not lead to complete merging, thus simultaneously reducing the impression of impact of the introgression and introducing a subsidiary implication that up to a certain point in the progress of the introgression the two were/are in fact separate.

What I meant by this, is that there are far more factors in play than genetic incompatibilities that might prevent complete breakdowns in species barriers. There could be ecological reasons why certain introgressed genotypes might not persist in different parts of the parental species' ranges, due to local (mal)adaptation, thereby limiting the breadth of a hybrid zone.

All this goes to the wider point that I was making that species concepts are messy (and often contradictory) and not every taxon will fit neatly into the boxes that we want to make for them.

We would certainly need to wait and see how it plays out, although none of us here will be there to see it.

 

[njlarsen]

For a couple of genes to cross from one species to another, leading to complete replacement? I would say that is very rare.

Certainly there are some well known examples, but they are well known, precisely because they are rare. I'm not sure about the Drosophila examples, but I think the wolf example is correct. The human example is incorrect though- not all human populations have signals of Neanderthal genes (although I could imagine that the last Neanderthal populations had at least some modern human genes in them).

There's a great newt example (Lissotriton montandoni has had its mitochondria completely replaced by L. vulgaris mitochondria).

I am not sure I have seen proof of any human population not having any Neanderthal genes. There are some that have a percentage of Denisovian genes in them, possibly on top of a little Neanderthal.

In birds, there are several examples of mitochondrial introgression as far as I recall? This is of course also the group of genes where a selection sweep with limited impact on the rest of the genome is easiest to understand.

Niels

 

[jts1882]

Iirc, Neanderthal genes are not in sub-Saharan Africans and Denisovan is mainly in east and southeast Asia populations on top of the Neantherthal component.

 

[Farnboro John]

I am not sure I have seen proof of any human population not having any Neanderthal genes. There are some that have a percentage of Denisovian genes in them, possibly on top of a little Neanderthal.

In birds, there are several examples of mitochondrial introgression as far as I recall? This is of course also the group of genes where a selection sweep with limited impact on the rest of the genome is easiest to understand.

Niels

Last I heard, and I may be out of date on this, was that the only pure-bred humans are those who never left Africa, so essentially limited to sub-Saharan populations. Everyone else, descended from settlers of Europe and Asia, has either Neanderthal genes or Denisovan, or both.

Phenotypical characters associated with Neanderthal genes are said to include short arms, though I haven't heard whether a tendency to long pockets also results.

John