I know @gbrooks9 will be interested in this example since we spent so long discussing whether “anti-ring-species” was a concept relevant to dogs—I think we’ve now found a much better example of the concept he wanted to discuss: ravens! It’s also relevant to recent discussions of human ancestry being more like a braided stream than a strictly branching tree.
I look forward to the day when it will be common knowledge how two distinctive species, sharing a recent common ancestor, lost the ability to interbreed.
I can only suppose that there are dozens and dozens of factors that can interfere with the successful production of fertile offspring. And like the straw that breaks the camel’s back, there must be a final change (or group of changes) that makes one population incompatible with another in terms of genetic compatibility!
Thank you very much for providing yet another treasure trove of journal articles!
This paragraph is quite the mouth-full !!! In just a few sentences, relevant distinctions and scenarios representing genetic high-drama are drawn out and tease the scientist with lots of possible outcomes!
Ecol Evol. 2017 Aug; 7(15): 5808–5820.
Published online 2017 Jun 20. doi: 10.1002/ece3.3093
"Factors contributing to the accumulation of reproductive isolation: A mixed model approach"
by Dean M. Castillo
“The divergence of lineages, in the process known as speciation, is only complete after gene flow is reduced following the evolution of reproductive isolation. The two general modes of reproductive isolation are prezygotic and postzygotic barriers to reproduction. Patterns of reproductive isolation inferred from comparative analyses of species willingness or ability to mate (i.e., prezygotic isolation) and produce viable and fertile offspring (i.e., postzygotic isolation) have generated important and influential observations about the evolution of reproductive isolation.”
As I have stated many times before, our natural human bias wants to put things in groups with with no crossover, but biology just isn’t like that. Speciation is a process with a continuum of divergence, and speciation can even stop if interbreeding resumes after incipient speciation begins. Some people want to call this the “Species Problem”, but it is really a problem of human bias. This is just how biology is.
And “speciation” is a “process” that is only discernible in retrospect. At any given moment, it isn’t “happening” in the same way that reproduction and survival and migration and mutation are happening. Our minds want “species” to exist as separate entities, and that’s bad enough, but then because we want them to be real, we think of them as things that are “formed” by processes that we imagine to be as straightforward as reproduction and survival and migration and mutation. In fact, “species” are just populations, and if you exert effort to think of them that way, a lot of the simplistic confusion melts away. Along with most of the easy answers to clumsy simplistic questions.
@T_aquaticus and others -
The species problem is answered in Genesis. In English translations of Genesis, “kind” doesn’t mean General Category (such as All Primates, or All Monkeys with Tails vs. All Monkeys without Tails). The text is quite clear.
If two living creatures have offspring, the two are “a Kind”. Period. Full Stop.
The human need to pigeon hole immediately intervenes when someone asks … but what if the two that have offspring are outliers ? And don’t really represent the average member of the “kind”? If that is the case, it is really only relevant to human conceptual frameworks. It doesn’t matter to God whether any given two individuals are “adequately similar to the central tendencies of a population”. It only matters to humans and what they try to do to define “this thing” as distinct from “that thing.”
So how does that work for horses and donkeys, given that mules are sterile? Separate species or not?
How does that work if two species produce offspring in captivity but not in the wild?
Let’s say that there are two populations on separate continents that are genetically and phenotypically divergent, but are capable of producing offspring in captivity but only after many tries?
While many might be looking for definitions or some sort of hard rule, biologists are more interested in cause and effect. Biologists are more interested in what is actually happening in populations than having a definition for a word that has no exceptions. What if a group of songbirds changes their song slightly so that they no longer attract mates from other populations of birds in that species? What if this allows new mutations to start building up in the bird population with the new song? How does this affect changes in that group over time? These are the questions that biologists are interested in.
I was expecting your question.
Are you asking from the viewpoint of God? Or are you asking from the viewpoint of Humans?
Some translators of Hebrew would suggest that part of being this “replicating Kind” is that it be adequate to producing generation after generation. So, with that proviso, Horses and Donkeys would not satisfy.
Conversely, Tigers and Lions seem to be able to produce equally fertile offspring. Which, from God’s view, would tell us that actually Tigers and Lions are of the same “kind”.
From a human viewpoint (or, shall we say, pre-Darwin), Lions and Tigers will still be considered separate species, simply because of the pragmatic fact that Lions and Tigers don’t interact in nature. But I’m not too impressed with that argument, nor do I think would Meyer who first developed the reproductive compatibility criteria back in the 1940s or 1950s!
This would be consistent with your statement that Biologists are more interested in what is actually happening.
But I’m not sure whether you mean “actually happening” in the natural world? … or if you mean what is “actually happening” in the laboratory, zoo or the natural world?
The Biblical definition for Kind becomes, then a Fluid Definition, which is what I think you were trying to describe in your prior posting, yes?
Genesis says a “kind” is what keeps reproducing itself. It doesn’t attempt to disqualify a Kind if a present day member of an alligator population would be UNABLE to reproduce with one of the earliest alligators. But biologists understand that there is this possibility - - phenotype is not genotype. So, just because something looks the same is not a guarantee that reproductive compatibility exists for the entire time the population is living. I don’t think God cares much about this nuance.
Only biologists would care.
Since God does not post in this forum (at least to my knowledge), my question is directed at my fellow human beings. Also, I think it would be dangerous to equate the views described in Genesis with God’s actual views, given the fact of human authorship and the irrelevant nature of speciation to the message the Bible seems to be communicating.
From the biological view, lions and tigers are separate species because there is no meaningful gene flow between the populations. Biologists are focused on effect, and that effect is the accumulation of population specific mutations.
I would say that the Biblical definition lacks the nuance needed to describe biology as we know it, not to mention the fact that the Bible was not meant as a biology textbook.
All you are doing is signing off on the pre-genetic designations… it’s part of the contrived matrix of phenotypes.
Technically speaking, they are quite closely related… and probably should be considered part of the same species…
Just as Neandertalensis is a sub-species designation.
I am going off of heritable characteristics which is post-genetics. I am citing gene flow as the criteria.
At what point are populations too close, and at which point is gene flow low enough? It would seem that these are arbitrary lines that we feel we need to draw.
@T_aquaticus, as I have been saying, in agreement with you, these lines (regarding gene flow or too close), are arbitrary from a human viewpoint.
But from the viewpoint of the “Genesis Test of Kinds” - - aka, from the viewpoint of an all-knowing being - - if two individuals can produce a fertile offspring, the two represent a “kind”.
So, from the viewpoint of the Genesis Test, Lions and Tigers are members of a single kind.
From the viewpoint of the Genesis Test, there is no “arbitrary line” - - they either can or cannot demonstrate they belong to the same kind.
Conversely, if two individuals should be able to produce fertile offspring, but for whatever reason do not, then the question cannot be answered from the viewpoint of human knowledge.
If we take a pair of non-productive individuals and apply genetic testing and can actually determine the reason for the failure to produce fertile offspring, then we can use human knowledge to decide whether one or both of the pair are defective (while still appearing to be long to the same kind) - - or if one or both of the pair does in fact belong to a new or different kind.
Just some thoughts . . .
There are humans who have incompatible genes that prevent them from conceiving. Are they different species?
Are chimps and humans interfertile? Wouldn’t that blow some minds (not that we should ever test the idea).
I also find it interesting when people argue that anatomically modern humans and Neanderthals were the same species because you can find small amounts of Neanderthal DNA in some modern human populations. The first question that pops into my head is how they know which parts are Neanderthal DNA. If they were truly the same species then it should be just human DNA and you wouldn’t be able to pick out the parts of Neanderthal DNA due to the amount of genetic divergence between the populations. The fact that anatomically modern humans and Neanderthals did occasionally have offspring does not do away with the fact that very little interbreeding occurred, so little as to allow the two ancestral populations to diverge to the point where you can pick out bits of one species in the genome of another.
I think you are putting more on the word “kind” here than it was meant to bear. I prefer a looser definition: animals reproduce according to kind, meaning that an animal will be the same sort of animal its parents were: no more, no less. I would say ‘same kind of animal’ but that might have readers using the hyperstrict YEC sense of ‘kind’ instead of the more natural normal English use.
So a pair of black bears with blondish fur that pass on their genes would be reproducing according to kind, and a horse and mule would also be reproducing according to kind. It’s tempting to overthink it but that’s all it says. It’s perfectly compatible with evolution and common descent. Just not with “baraminology.”
I anticipated your comment. I will repeat it here.
Doesn’t it, again, reinforce your very thoughts on the matter? That Humans are always trying to categorize things, and come up with neat pigeon holes?
But maybe the solution is not to talk about a “kind” being represented by just “two individuals that can produce offspring”. Maybe I was being too narrow in my earlier discussion - - and thus just making it harder on myself.
Instead, we could say that a “kind” is represented by a group, rather than by specific individuals. And this would help us avoid the pot holes of 2 individuals that cannot conceive offspring… while at the same time recognizing that virtually all Lions and all Tigers can still interbreed.
What do you think?
I agree. This only makes sense at the population level. Again, here are the questions that pop into my head when faced with the kind designation for populations:
Does the kind definition also require fertile offspring, or just offspring? Are horses and donkeys in separate kinds because mules are sterile? Does the kind definition also extend forever into history where all ancestors of any two modern kinds also lacked fertility between kinds? How do you determine which kind a fossil species belongs to? How do you categorize asexual organisms?
Okay, now that we are on the same page about evaluating at the population level, rather than based on single pairings (which may or may not reflect exceptional situations)… let’s look at your questions.
 Does the kind definition also require fertile offspring, or just offspring?
Mr. T., I think the Genesis Test that I have proposed is built around the context of producing a new
generation of a given population. This means “fertile pairs” producing offspring for future “fertile pairs”.
 Are horses and donkeys in separate kinds because mules are sterile?
Based on  above, Horses and Donkeys represent separate kinds, because at the population level,
infertile mules are not able to replace their generation with a new generation.
 Does the kind definition also extend forever into history where all ancestors of any two modern kinds also lacked fertility between kinds?
I really like this question. It asks for coherence where YECs have always claimed no clarity. But if we follow the logic of the Genesis Test (creatures begetting creatures by “kind”), you can see that the Genesis Test, by definition, has the same open-ended nature that Evolutionists have also come to recognize. If we follow a specific branch of dinosaurs, there is an organic unity of this branch which runs from conventional dinosaur, to feathered dinosaurs, to feathered dinosaurs with air-filled bones - - and so forth (please forgive me if I got some of the steps in the wrong sequence) - - until we arrive at Birds !
Are we saying that the Bible would insist that feathered dinosaurs and birds are the same species? No, I don’t think anyone thinks that. But I do think the Bible and Evolution both require a pragmatic way of embracing this long-term reality of a continuous, uninterrupted population over centuries, eons and millions of years!
One might say that the Bible would be consistent if we found a verse that specifically discussed how an uninterrupted “Kind” could conceivably be divided into “Cohorts” … based on “time brackets”. Do we know for a fact that a pig population used to feed the Pharaoh of the earliest Egyptian dynasties could reproduce with a pig population used to feed visiting Roman dignitaries to Jerusalem? No. We certainly don’t. This uninterrupted population, a kind, could have some dramatic shifts from time to time, creating unique sets or cohorts that reflect the genetic (and/or phenotypical) for just a limited period of time. This is no different from the situation faced by Evolutionists when categorizing various time frames of an uninterrupted population.
 How do you determine which kind a fossil species belongs to?
So what would someone like Ernst Meyer say to answer this question? He was the one who pioneered the definition of species as a reproductively compatible population of individuals.
Just because we have lots of bones doesn’t mean we really know what the population for any given fossil was like. We can only estimate what the limits of the population were. I think you will agree that sometimes the only blessing we have is that there is frequently enough time between fossils that when we see a significant difference in phenotype, it will generally be treated as a different species - - for convention’s sake!
 How do you categorize asexual organisms?
I guess we can only wonder what a Bible scribe might have had to say about this. Since we don’t need “pairings” to validate a “kind” - - we end up starting with the old conventions of similarities in appearance, behavior and eco-niche. And if you were an Evangelical geneticist, you could extend this analysis into the genetic realm, looking for convenient markers in the genetic content of multiple individuals that seem to reflect a population aggregate - - even though we are no longer able to use fertility of pairings as a huge flashing sign !
I was thinking more of the tree-like structure found in biology. Instead of starting at the nodes and working towards the tips of the branch, I was starting at the tips (i.e. living species) and moving backwards to where the branches meet. What justification can one make about past interfertility between kinds? Is there any justification for extending infertility between modern populations into the past? If we go back far enough will we end up with a population of horse ancestors and donkey ancestors that were able to produce fertile offspring?
I was thinking more along the lines of YEC claims about hominid fossils being either ape-kind or human-kind. It is interesting to note that different YEC advocates draw the line at different places when we look at the spectrum of hominid fossils.
Obviously, we can’t use fertility as our guide for fossil species. Paleontologists use multivariate analyses of different diagnostic features to group fossils, but such a test would obviously fail for species such as dogs. You are right in saying that it is a convention, but it fails to follow the convention you previously pointed out for Genesis kinds.
The phrase “Evangelical geneticist” gave me a chuckle. I would imagine that they would sing, “The A T C G’s, yes that’s the molecule for me” in children’s Sunday school.
How to categorize asexual organisms is as arbitrary as anything else, and even more so. I think we can forgive the authors of Genesis for not fully understanding the nuances of biology when it comes to organisms that they never even imagined could exist.
A little surprising. It’s not clear to me from the NG summary whether it is a situation where they simply cannot interbreed or they do not interbreed. Maybe the California side smells bad to the eastern crow.
I recall in looking at a mouse genome and the rat genome, I was very surprised at how the genomes are so vastly rearranged. The introns are also really different, yet the coding sequences are not really all that different. I guess maybe mitosis might not be such a problem, in principle, but meiosis or the exchange of genetic material when genetic material from the sperm and egg fuse could be a big problem.
Does anyone know what the happens in the fusing of the horse-donkey genetic material? You have half the material from one and half from the other and fused in together, but is it one direction, is it that all the sexual parts become locked up, … ? If one had some simple-minded Mendelian picture, then if mules reproduced, it would be some AA, Aa, aA and aa outcome, but I guess there is no female mule … or something like that?