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
David Goodman
Hey, Ninety-Three,
I totally agree that invoking humans here would have been useful! If the post hadn’t already been pushing 8000 words (and I didn’t have work to do on my master’s thesis lol) I probably would have done so!
In terms of why I think crows are grayer than humans, I think it’s because the assortive mating in crows is probably significantly more stable (and probably genetically determined) than assortive mating in humans. It’s true that humans do assortive mating, but it does not tend to be stable over anything approaching the geological time that is (usually) required for speciation. The Casta system in South America is a good example -- 350 years of legally enforced, super racist assortive mating that lasted for many generations, but eventually collapsed and now South America is one big intermingled, intermarried population. Interracial marriage statistics in the US are another example—they were insanely low before the 1950s, but have risen substantially since then as cultural mores changed, which is over only like 80 years. Social divisions from only like 1,000 or 2,000 years ago seem absolutely absurd to us now. Spartans being forbidden to. marry helots, for example. Nobody in modern day Greece has such a distinction. Because they all ended up interbreeding, despite assortive mating preferences and laws etc.
that whole time, the hooded crows and the carrion crows have been assortive mating. They’ve probably been doing so for a very long time, tens of thousands of years. This indicates that either 1) it’s very genetic, and relatively stably so, or 2) that crow “culture” (which does exist, can be seen in mating structure differences around Europe, though it’s obviously not comparable to human culture in extent) is also extremely stable. My guess is that it’s genetic, but I suppose both are possible.
In both these ways, this assortive mating looks different to evolution (and thus speciation) than human assortive mating. Since it’s so stable with these crows, we can say with (relatively) high certainty that these two populations will speciate along these precise lines, and probably very soon. We cannot say such a thing about any two populations of human with any certainty, because human culture and migration and history is so contingent and unstable.
I still don’t think this is a reason to split the crows. I stand by them being different species. This is just why I empathize with people who say they should be different species. They’re probably about to become them.
Hi eniteris! I appreciate your long and detailed responses. Clearly you’ve thought a lot about this topic!
To respond to this point:
”Imagine a forest with one species of frog. Then a road is built through the forest, separating the forest in two. The day after the road is built, I discover that there are now two reproductively isolated groups of frogs! Are these now different species? … No! That’s preposterous.”
If you think this is preposterous because the two populations are not genetically distinct enough, then you are not using reproductive isolation as your definition of species, you are using genetic distinctness. Genetic distinctness COMES from reproductive isolation, but you need to find a definition that you can apply consistently. If you separate two populations so they are reproductively isolated, but you don’t split them into separate species because you actually have a different, more important criteria that they don’t yet satisfy, then reproductive isolation is upstream of your true definition.
If you want a scientific definition, it needs to be consistently applied. If you consistently apply the BSC, then those two frogs would be considered different species. Since I agree that is preposterous, we need to frame our definition more precisely than “potentially interbreeding populations that are reproductively isolated.”
Hey Eniteris!
I have many thoughts here. First I would say that I totally agree that the species concept stops making any sense as a strict, discrete category the second horizontal gene transfer / true asexual reproduction enter the picture. I should probably have said this at the beginning of the post so that people who work in viruses and bacteria (as you seem to!) knew that my argument didn’t apply to organisms capable of that.
In my experience, however, microbiologists are too quick to generalize from their part of the field to all parts of it. It is 100% true and defensible that “everything is a bag of genes, and genes constantly flow between them; species aren’t real and are just useful lines to draw in a continuum of gene flow, which constantly changes over time” if you’re talking about bacteria. But genes do not constantly flow between humans and chimpanzees via HGT. When you’re talking about most of the animals that the IUCN cares about (for better or worse), you’re talking about sexually reproducing organisms that (imo, once they SPECIATE) stop sharing genes. You cannot treat them the same way you would a virus.I also totally agree with this statement: “We form these categories not because they carve reality at the phylogenetic joints but because these categories are useful. Fish are fish. Trees are trees.” I don’t mean to propose that everyone stop using the words fish, or tree, even though they’re not monophyletic. But there is (and should be!) a different between technical scientific vocabulary and common usage. I don’t insist people call bananas “berries” because, in common usage, they’re not. But botanically speaking, we have a definition for “berry” that we apply consistently, no matter what the common usage is, and this definition makes bananas a berry. I believe that the technical definition of “species” should be similarly transparent and consistent. I agree that “there are multiple joints at which you can carve reality, and which joints you choose to carve depends on why you are carving reality in the first place.” But as I hope the essay above illustrates, the Biological Species Concept as it is usually formulated cannot hope to be consistently applied to relevant organisms (ie. sexually reproducing macroorganisms incapable of HGT etc)
In terms of this question asking about all human populations being able to reproduce:
”But can you verify it? Do you have experimental evidence that a human from location A can form fertile offspring with a human from remote location B?”
There have been many many recorded instances of human populations that have been long isolated being perfectly capable of being able to reproduce with other populations. To this day, there has NEVER been a recorded instance of the opposite. We know that Homo Sapiens could produce fertile offspring with Neenderthals, MUCH more different from us than any modern humans.
It would be very strange to lend much credence to the idea that not all human populations could produce fertile offspring. The burden of proof would clearly be on the person trying to say otherwise.
In terms of your philosophical problems with conservation: “(I also have philosophical issues with “conservation”—what, exactly, are you conserving and why is that time more worth conserving than any other time?)” These are all worthy questions that bear consideration. But noting that a practitioner in any given field will be required to make contestable value judgements is not really an argument against that field. The same could be said of doctors, politicians, lawyers, journalists, etc.
Hi Tandena,
I pretty much just totally agree with this actually. I think that evolutionary distinctness should be a MUCH larger part of conservation conversations about biodiversity than it is now. There are monotypic families going extinct in preventable ways while we pour tremendous resources into things which are not that evolutionarily important (though still morally relevant, of course!) like the Eastern Wolf. Obviously human favoritism will come back in (otherwise, as you say, Coleoptera would dominate, which is hard for human aesthetic sensibilities), but at least evolutionary lineage distinctness is relatively objective. I’m literally just repeating you. Totally agree. In conservation, the “species” conversation is an unhelpful tangent, and in an ideal world we would just cut the Gordian knot of splitters and lumpers and come up with better metrics.
However, I think the species conversation is still important to have in basic research into ecology, macroecology, statistical evolution, etc. The ring species conversation, for example, is totally handicapped by the fact that we don’t have a consistent understanding of what we mean when we say “species.” It is, depending on who you talk to and their personal definition, either totally explicable and unsurprising, just a spatial replication of how speciation usually happens in time, or it’s a total refutation of the entire species concept, period. These kinds of disagreements resemble the “does a tree falling in an empty forest make a sound” debate, in that people essentially never disagree about any facts-of-the-matter, and are instead arguing unproductively about definitions, often without realizing it. This is bad for science. We should stop doing it. The best way to do that is to actually define the “species” concept consistently and scientifically.
So in conservation, ideally we would stop having the conversation by getting better metrics for our goals. In basic science, the project of just accurately describing and talking about the world, we would also stop having the conversation by just defining the term, applying it consistently, and moving on.