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AOU-NACC Proposals 2020 (1 Viewer)

Andy Adcock

Well-known member
England
The data used to support this split, also necessitates the split of Cabot's Tern from Sandwich Tern (accepted several years ago by IOC; still not yet by AOU). It would be odd, and very inconsistent, if AOU accepted this split, while still rejecting the Cabot's Tern split. Surely the two should be considered together?

It's still in their list of 'Proposed splits', not yet on the accepted list, but I'd guess IOC acceptance is a formality particularly if AOU-NACC accept it.

Is it, where, would usuall be in the diary?
 

Kirk Roth

Well-known member
The data used to support this split, also necessitates the split of Cabot's Tern from Sandwich Tern (accepted several years ago by IOC; still not yet by AOU). It would be odd, and very inconsistent, if AOU accepted this split, while still rejecting the Cabot's Tern split. Surely the two should be considered together?

I noticed this too - one of my initial reactions to this proposal is that it seemed a "backdoor" way to get a Cabot's Tern split... or in other words if this gets accepted, then its acceptance could be used as further evidence to revisit that one. Note also that there is a point where the authors use the Cabot's/Sandwich pair as evidence of divergent taxa which look the same. I thought it a bold (but appropriate) stance, given the committee's rejection in 2013.

I was struck by the phylogeny from the Collinson paper that was included in the proposal (Figure 1). Just based on this, one might conclude that Elegant and Cabot's Terns should be lumped into one species.
 

THE_FERN

Well-known member
I was struck by the phylogeny from the Collinson paper that was included in the proposal (Figure 1). Just based on this, one might conclude that Elegant and Cabot's Terns should be lumped into one species.

Yes but elsewhere they note divergence ranges from ~0.5 to ~2% depending on the sequence. This might mean c1M+ years of separate evolution (rough rule of thumb 2% per million years). [edit:] should have noted this is either quite a bit different or not so much depending where you stand. E. g. Some of the canary islands splits for Robin etc are at least as divergent, the iberian green woodpecker ~300 thousand years if memory serves. Passerines presumably evolve faster (shorter generations, smaller body sizes)
 
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Kirk Roth

Well-known member
Yes but elsewhere they note divergence ranges from ~0.5 to ~2% depending on the sequence. This might mean c1M+ years of separate evolution (rough rule of thumb 2% per million years). [edit:] should have noted this is either quite a bit different or not so much depending where you stand. E. g. Some of the canary islands splits for Robin etc are at least as divergent, the iberian green woodpecker ~300 thousand years if memory serves. Passerines presumably evolve faster (shorter generations, smaller body sizes)

So how then does this resolve in relation to species limits? It would seem that there is maybe a half to a million years of divergence, but also rife with crossbreeding? Or something more complicated? The Collinson data have the appearance of either that or orange bills evolving several times over, then magically ending up in the same limited range. Or something weird about the data... Would be curious where the eurygnathus samples were taken from (Collinson does not mention it - nor do they talk about the strange phylogeny, except to say that it was "not well resolved.")
 

THE_FERN

Well-known member
So how then does this resolve in relation to species limits? It would seem that there is maybe a half to a million years of divergence, but also rife with crossbreeding? Or something more complicated? The Collinson data have the appearance of either that or orange bills evolving several times over, then magically ending up in the same limited range. Or something weird about the data... Would be curious where the eurygnathus samples were taken from (Collinson does not mention it - nor do they talk about the strange phylogeny, except to say that it was "not well resolved.")

Not really qualified to say without a fair bit more research. Personally, I don't think half a million years is that long for something as long lived as a seabird. So, perhaps we might expect discordant signals in the phylogeny due to incomplete lineage sorting etc. Also not sure we can say a great deal about ancestral (geographical) ranges based on current distributions. The little I know about this suggests that seabird ranges have often been very different in the past. Morphologically, terns are pretty conserved, but then perhaps half a million years isn't that long.
 

njlarsen

Gallery Moderator
Opus Editor
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Barbados
Not really qualified to say without a fair bit more research. Personally, I don't think half a million years is that long for something as long lived as a seabird. So, perhaps we might expect discordant signals in the phylogeny due to incomplete lineage sorting etc. Also not sure we can say a great deal about ancestral (geographical) ranges based on current distributions. The little I know about this suggests that seabird ranges have often been very different in the past. Morphologically, terns are pretty conserved, but then perhaps half a million years isn't that long.

I think one question here is, does a given percentage difference mean the same thing in different lineages? I believe that some seabirds (though my brain keeps coming forward with tubenoses as examples?) have an on average lower percentage difference between species than often seen in other groups of birds.

Niels
 

THE_FERN

Well-known member
I think one question here is, does a given percentage difference mean the same thing in different lineages? I believe that some seabirds (though my brain keeps coming forward with tubenoses as examples?) have an on average lower percentage difference between species than often seen in other groups of birds.

Niels

>>same thing

No it doesn't. In principle, a single mutation can lead to reproductive isolation/incompatibility. In practice, the number of mutations before this occurs will vary (probably randomly) and hence so will "percentage difference" between species (whatever we mean by that, and always assuming you choose reproductive isolation as your criterion for delimiting species).

Whether something is or is not a separate species may have an "objective truth" in some sense but a) the criteria will probably vary across taxa/populations and b) it's probably unknowable in the broad sense of being able to classify everything into species. To do ecology and other biology we do have to impose some form of taxonomy but we should always realise this is a simplification. Makes "listing" and "listers" seem even more bizarre: the uncertain in search of the unattainable. (But I'm sort-of one of them: I use lists/taxonomies as a shorthand for "diversity" in a general sense)
 

l_raty

laurent raty
but then perhaps half a million years isn't that long.
Beware that half a million years is Cabot's vs. Elegant. Not Cabot's vs. Sandwich -- this would be closer to 3 millions.

So how then does this resolve in relation to species limits? It would seem that there is maybe a half to a million years of divergence, but also rife with crossbreeding? Or something more complicated? The Collinson data have the appearance of either that or orange bills evolving several times over, then magically ending up in the same limited range. Or something weird about the data... Would be curious where the eurygnathus samples were taken from (Collinson does not mention it - nor do they talk about the strange phylogeny, except to say that it was "not well resolved.")
Given how widespread orange/yellow bills are in Thalasseus terns, and the absence of suggestion that they might be limited to a particular subclade in the group, I would expect this to be the ancestral state. If so, what you'd have to explain would rather, arguably, be black bills (with a yellow tip) having apparently evolved twice on different continents -- once in Sandwich, once in northern Cabot's.

There are mtDNA data for Cabot's Tern from USA (North Carolina, Florida, Louisiana), Guyane Française (in the BOLD database, still private, but position assessable from ID trees), Brazil, and Argentina. These all show the same pattern: all these birds are closely inter-related, and form a group which is closer to Elegant than to nominate Sandwich. (These aspects of the phylogeny are certainly not "not well resolved".) (Two slightly divergent haplogroups seem to be present in Cabot's, however; one of them occurring in all populations; the other (so far) limited to acuflavidus, where it occurs in admixture with the first one.)
(The mitochondrial sequences used in Collinson et al were taken from GenBank; the T. a. eurygnathus in Fig. 1 (DQ385180, voucher "G12325" -- same bird as in Given et al 2005) was from Salinas, Brazil; the three birds in Fig. 2 were from Escalvada Is. (FJ356224), Para (AY631375), and Salinas (DQ385095, same bird as in Fig. 1), all in Brazil. Fig. 4 is less straightforward -- Collinson et al cited Dufour et al 2017 as a source for the nuclear sequences they used; but Dufour et al 2017 used sequences from Efe et al 2009 for eurygnathus; Efe et al 2009 had three eurygnathus from Escalvada Is., Brazil, and two from Punta León, Argentina: I would expect the eurygnathus in Fig. 4 of Collinson et al to be among these five birds; the trouble is, there are only three of them, and I can't tell which ones they are.)
 

Nutcracker

Stop Brexit!
Given how widespread orange/yellow bills are in Thalasseus terns, and the absence of suggestion that they might be limited to a particular subclade in the group, I would expect this to be the ancestral state. If so, what you'd have to explain would rather, arguably, be black bills (with a yellow tip) having apparently evolved twice on different continents -- once in Sandwich, once in northern Cabot's.
What about the possibility that black bills evolved just once, in Europe, then a vagrant male (so no mtDNA input) black-billed bird crossed the Atlantic and settled in an orange-billed ancestral Cabot's Tern colony in N America? If black is dominant over orange, this could work? And the new black bill gene still spreading south into ssp eurygnatha?
 

Kirk Roth

Well-known member
Beware that half a million years is Cabot's vs. Elegant. Not Cabot's vs. Sandwich -- this would be closer to 3 millions.


Given how widespread orange/yellow bills are in Thalasseus terns, and the absence of suggestion that they might be limited to a particular subclade in the group, I would expect this to be the ancestral state. If so, what you'd have to explain would rather, arguably, be black bills (with a yellow tip) having apparently evolved twice on different continents -- once in Sandwich, once in northern Cabot's.

There are mtDNA data for Cabot's Tern from USA (North Carolina, Florida, Louisiana), Guyane Française (in the BOLD database, still private, but position assessable from ID trees), Brazil, and Argentina. These all show the same pattern: all these birds are closely inter-related, and form a group which is closer to Elegant than to nominate Sandwich. (These aspects of the phylogeny are certainly not "not well resolved".) (Two slightly divergent haplogroups seem to be present in Cabot's, however; one of them occurring in all populations; the other (so far) limited to acuflavidus, where it occurs in admixture with the first one.)
(The mitochondrial sequences used in Collinson et al were taken from GenBank; the T. a. eurygnathus in Fig. 1 (DQ385180, voucher "G12325" -- same bird as in Given et al 2005) was from Salinas, Brazil; the three birds in Fig. 2 were from Escalvada Is. (FJ356224), Para (AY631375), and Salinas (DQ385095, same bird as in Fig. 1), all in Brazil. Fig. 4 is less straightforward -- Collinson et al cited Dufour et al 2017 as a source for the nuclear sequences they used; but Dufour et al 2017 used sequences from Efe et al 2009 for eurygnathus; Efe et al 2009 had three eurygnathus from Escalvada Is., Brazil, and two from Punta León, Argentina: I would expect the eurygnathus in Fig. 4 of Collinson et al to be among these five birds; the trouble is, there are only three of them, and I can't tell which ones they are.)

Thank you for this, Laurent, and for the interesting conjecture about hybrids below.

I think I was thinking in smaller scale when talking about the bill colors; I was focusing more on just the Sandwich/Cabot's/Elegant node, in which dark bills might be basal and orange/yellow would be derived either one or more times depending on where T.a.acuflavida fits in. But that would indeed be assuming and the sanvicensis backcross idea and potential for recent crossbreeding among the other taxa certainly seems another very plausible way to explain this weird tree.

The point made by others that their distributions have not necessarily been static over the many hundreds of thousands of years is well noted - yet, it is still very interesting that all those Atlantic birds seem nested within the pacific elegans. While it could be that sanvincensis genes made it back to the New World and spread, it also seems (if we believe in the strength of the Collinson tree) that perhaps elegans developed at some point and could have backcrossed with a pre-T.a.acuflavida Cabot's to produce the Cayenne Terns. Or simply that Cabot's Tern itself developed from Sandwich/Elegant hybrids?
 

l_raty

laurent raty
I think I was thinking in smaller scale when talking about the bill colors; I was focusing more on just the Sandwich/Cabot's/Elegant node, in which dark bills might be basal and orange/yellow would be derived either one or more times depending on where T.a.acuflavida fits in.
I understood this -- but if your focus clade includes a mixture of populations with orange- and black-billed birds, and if its sister clade is made of orange billed-birds, parcimony will generally make orange, not black, the ancestral state in this focus clade.

it also seems (if we believe in the strength of the Collinson tree) that perhaps elegans developed at some point and could have backcrossed with a pre-T.a.acuflavida Cabot's to produce the Cayenne Terns. Or simply that Cabot's Tern itself developed from Sandwich/Elegant hybrids?
Under your first hypothesis, hybridization would have to be quite recent, and to have introduced the yellow-billed character into Cayenne while leaving the Cabot's-like mtDNA of Cayenne untouched. (Because Cayenne Terns have mtDNA haplotypes that are identical to haplotypes present in northern Cabot's, and clearly distinct for those of Elegant.)
Under your second hypothesis, hybridization events would have to have occurred 500,000 / 1,000,000 year ago, producing a hybrid population that would have retained pre-Elegant mtDNA only, and that would since have been evolving in isolation of both parent. (Because today's Cabot's mtDNA is distinct from that of both hypothetical parent. Except for the effect of possible mutations, the mtDNA of any bird -- including a hybrid -- is normally fully identical to that of its mother, and unaffected by that of the father. Any recent hybrid population would unavoidably share haplotypes with one of the parent taxa.)
I can't actually fully exclude either. But I'm unclear how I might find evidence for or against it in an mtDNA tree.
 

Kirk Roth

Well-known member
I understood this -- but if your focus clade includes a mixture of populations with orange- and black-billed birds, and if its sister clade is made of orange billed-birds, parcimony will generally make orange, not black, the ancestral state in this focus clade.


Under your first hypothesis, hybridization would have to be quite recent, and to have introduced the yellow-billed character into Cayenne while leaving the Cabot's-like mtDNA of Cayenne untouched. (Because Cayenne Terns have mtDNA haplotypes that are identical to haplotypes present in northern Cabot's, and clearly distinct for those of Elegant.)
Under your second hypothesis, hybridization events would have to have occurred 500,000 / 1,000,000 year ago, producing a hybrid population that would have retained pre-Elegant mtDNA only, and that would since have been evolving in isolation of both parent. (Because today's Cabot's mtDNA is distinct from that of both hypothetical parent. Except for the effect of possible mutations, the mtDNA of any bird -- including a hybrid -- is normally fully identical to that of its mother, and unaffected by that of the father. Any recent hybrid population would unavoidably share haplotypes with one of the parent taxa.)
I can't actually fully exclude either. But I'm unclear how I might find evidence for or against it in an mtDNA tree.

The Collinson data is mtDNA too, however, and it has elegans and eurygnathus all mixed in their node - that's what I can't resolve. If Elegant and Cabot's are so distinct from one another, how did Collinson come up with that tree? I get it that the bootstrap values are very very low, but they felt at least good enough to publish it.

I agree that I don't see how any evidence for or against these hypotheses could be out there - though it would be interesting if the nuclear DNA happened to be well blended between the taxa. For now though, these ideas remain what E.O. Wilson called a "just-so story" of evolution.

Thanks again for looking into this. As always, your talent for investigation is well appreciated.
 

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