I feel this article was insufficiently integrative across the fields of evolution, ecology, and conservation science. In the first, it largely ignores the research frontier of speciation with gene flow and the speciation continuum. You also note that the phylogenetic (cladistic) species concept is necessary but not sufficent, and yet also make no mention of phylogenetic discordance and/or hemiplasy in macroevolutionary time. Obviously you can’t mention everything, but these are massive holes in your conclusion, ones that contemporary speciation research naturally brings up.
In the second, you say that your speciation concept would improve ecology. Why? Ecologists who can see trait variation that they are interested in are not going to ignore it on the basis of speciation, which trait variation tracks poorly. The fact that still much evidence for hybridisation between pairs of taxa is based on natural history observations makes this situation worse, since it is genuinely hard to circumscribe very plastic species vs a hybrid zone. I suspect for this reason the potential for hybridisation is vastly underestimated. Measures of biodiversity are trying to move away from species richness due to this, something that would not be solved by a more consistent definition that still has the same fundamental issues.
In the third, conservationists are already integrating along these lines. IUCN includes subspecies and populations of distinct conservation relevance. It is not clear to me how the population viability concept connects to the species concept debate, except insofar as it gives us a common language to compare populations that already has consensus. Conservationists and taxonomists definitely understand the definitions—it’s just a bit intractable and tangential to the conservation value.
Having said that, I appreciate you are writing this under your own name and I am writing this under a pseudonym. Compliments to writing good quality conservation content, even if I disagree with it.
Hey CDT, I like the comment! Very substantive critiques.
Speciation with gene flow is exactly what people say is happening with the Hooded Crow and Carrion Crow. I think this is a bad definition, because it can’t be consistently applied (see essay). Between the points of 1) full, spontanous extensive gene flow in a single population (i.e. one species) and 2) literally 0 gene flow bewteen two populations (i.e. two species), one must eventually draw an arbitrary line. My argument is that line should be VERY close to 2. The current research about “gene flow with speciation” is currently extremely confused and hard to integrate PRECISELY because we do not have a consistent definition of species as a field. You will find that many papers talking about ring species (yet to be empirically verified in the wild, though probably theoretically possible), or “speciation with gene flow”, or many related topics either do not define “species” clearly or else use different definitions of “species” from each other. This is exactly why I think a better definition of species would improve basic ecology research. I think now many people see apparent contradiction in the idea that “these are different species AND capable of gene flow!” and so study that system, not understanding that contradiction is really just in their definition of “species.” the study of “speciation with gene flow” is just… the study of gene flow.
Taxonomic inflation is also a huge problem for statistical macroecology!
Similar thoughts on “species continuum” which I think is functionally a meaningless concept as it is usually used, in that it has no standard definition such that two “experts” in the phenomenon can rarely even agree what they’re talking about.
I totally agree that phylogenetic discordance and hemiplasy would be super interesting to fit into this conversation. I chose not to include them because both are fairly technical and require many steps of inference for people who may not even know what a clade is, or the word paraphyletic, or either fairly super concepts. BUT, since you obviously know what these things are, I’ll say this: the complications of reconstructing phylogenies are real and interesting, but regardless there is still clearly a moment at which gene flow between populations stops. This is when I think speciation happens. The fact that this would certainly be super hard to verify for many extict (or extant!) species across evolutionary time is unfortunate, but it does not mean we should change our definition. It just means that we might not be able to responsibly dispel our ignorance using modern tools and understanding. I feel that in a macroevolutionary context, the flexible “species” definition is often used to obfuscate the fact that we don’t know precisely how or when things happened. We should try to overcome that ignorance by actually getting more knowledge, not by changing or definition of “species” and disappear it with semantics.
I don’t think definitions are intractable! We can and should actually define what a species is scientifically. We have an excellent understanding of evolution, despite our remaining ignorance. We could absolutely come up with a good definition. And the species question is SUPER relevant for conservation. This is why people are trying to get the White Rhinos split, keep the Eastern Wolf split, split the House Wren into 12 newly legally protected “species”, etc. The IUCN has some subspecies classifications, but if you start telling them they should lump a bunch of species, it would take like 30 seconds for them to start saying populations will be extirpated without the protection that “species” status affords them. This convo is def relevant for conservation
I feel this article was insufficiently integrative across the fields of evolution, ecology, and conservation science. In the first, it largely ignores the research frontier of speciation with gene flow and the speciation continuum. You also note that the phylogenetic (cladistic) species concept is necessary but not sufficent, and yet also make no mention of phylogenetic discordance and/or hemiplasy in macroevolutionary time. Obviously you can’t mention everything, but these are massive holes in your conclusion, ones that contemporary speciation research naturally brings up.
In the second, you say that your speciation concept would improve ecology. Why? Ecologists who can see trait variation that they are interested in are not going to ignore it on the basis of speciation, which trait variation tracks poorly. The fact that still much evidence for hybridisation between pairs of taxa is based on natural history observations makes this situation worse, since it is genuinely hard to circumscribe very plastic species vs a hybrid zone. I suspect for this reason the potential for hybridisation is vastly underestimated. Measures of biodiversity are trying to move away from species richness due to this, something that would not be solved by a more consistent definition that still has the same fundamental issues.
In the third, conservationists are already integrating along these lines. IUCN includes subspecies and populations of distinct conservation relevance. It is not clear to me how the population viability concept connects to the species concept debate, except insofar as it gives us a common language to compare populations that already has consensus. Conservationists and taxonomists definitely understand the definitions—it’s just a bit intractable and tangential to the conservation value.
Having said that, I appreciate you are writing this under your own name and I am writing this under a pseudonym. Compliments to writing good quality conservation content, even if I disagree with it.
Hey CDT, I like the comment! Very substantive critiques.
Speciation with gene flow is exactly what people say is happening with the Hooded Crow and Carrion Crow. I think this is a bad definition, because it can’t be consistently applied (see essay). Between the points of 1) full, spontanous extensive gene flow in a single population (i.e. one species) and 2) literally 0 gene flow bewteen two populations (i.e. two species), one must eventually draw an arbitrary line. My argument is that line should be VERY close to 2. The current research about “gene flow with speciation” is currently extremely confused and hard to integrate PRECISELY because we do not have a consistent definition of species as a field. You will find that many papers talking about ring species (yet to be empirically verified in the wild, though probably theoretically possible), or “speciation with gene flow”, or many related topics either do not define “species” clearly or else use different definitions of “species” from each other. This is exactly why I think a better definition of species would improve basic ecology research. I think now many people see apparent contradiction in the idea that “these are different species AND capable of gene flow!” and so study that system, not understanding that contradiction is really just in their definition of “species.” the study of “speciation with gene flow” is just… the study of gene flow.
Taxonomic inflation is also a huge problem for statistical macroecology!
Similar thoughts on “species continuum” which I think is functionally a meaningless concept as it is usually used, in that it has no standard definition such that two “experts” in the phenomenon can rarely even agree what they’re talking about.
I totally agree that phylogenetic discordance and hemiplasy would be super interesting to fit into this conversation. I chose not to include them because both are fairly technical and require many steps of inference for people who may not even know what a clade is, or the word paraphyletic, or either fairly super concepts. BUT, since you obviously know what these things are, I’ll say this: the complications of reconstructing phylogenies are real and interesting, but regardless there is still clearly a moment at which gene flow between populations stops. This is when I think speciation happens. The fact that this would certainly be super hard to verify for many extict (or extant!) species across evolutionary time is unfortunate, but it does not mean we should change our definition. It just means that we might not be able to responsibly dispel our ignorance using modern tools and understanding. I feel that in a macroevolutionary context, the flexible “species” definition is often used to obfuscate the fact that we don’t know precisely how or when things happened. We should try to overcome that ignorance by actually getting more knowledge, not by changing or definition of “species” and disappear it with semantics.
I don’t think definitions are intractable! We can and should actually define what a species is scientifically. We have an excellent understanding of evolution, despite our remaining ignorance. We could absolutely come up with a good definition. And the species question is SUPER relevant for conservation. This is why people are trying to get the White Rhinos split, keep the Eastern Wolf split, split the House Wren into 12 newly legally protected “species”, etc. The IUCN has some subspecies classifications, but if you start telling them they should lump a bunch of species, it would take like 30 seconds for them to start saying populations will be extirpated without the protection that “species” status affords them. This convo is def relevant for conservation