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? That’s why geographical isolation is used to define species; because they (mostly) don’t overlap, we don’t have evidence if they can or cannot interbreed.
Do you think that we should categorize e.g. the Sentinelese people as a separate species due to lack of evidence of hybridization? That seems like a strange thing to do given that (I assume) we’re all pretty confident they can still interbreed.
The Sentinelese has been reproductively isolated for probably less than twenty generations, which can be sufficient for speciation (given strong selection pressures), but relatively unlikely. So I don’t think it’s necessary to assume that speciation has occurred.
(aside, estimates of Sentinelese isolation are full of wild speculation and misinformation. We have no genetic samples from North Sentinel Island, which means we have no evidence. All estimates are speculation, with an upper bound at a few thousand years due to shared ethnographic artifacts. Missing artifacts (metalworking, etc.) may or may not help establish a lower bound, but is confounded by natural resources available on the island. Also, going back more than tens of generations would imply a high amount of inbreeding given the limited carrying capacity of the island, which could theoretically be sustained with strong selection.)
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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?
(under most species frameworks) No! That’s preposterous. The two populations have been isolated for less than a day. Sure, they may at some point in the future diverge (and now natural selection is acting on the two groups differently), but at this point the two species are genetically identical.
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But now imagine you stumble across on species of frog in one forest, and a similar-looking frog in a different forest. The two population are reproductively isolated. Are they the same or different species?
Well, you might ask how long they’ve been reproductively isolated for, since if the forests were connected yesterday then we converge on the same scenario as above. A week, a month, a year, a decade, a century, a millennium, a million years. At some time of separation we cannot reject [no speciation] out of hand, or, rather, number of generations, since that’s what matters in (genetic) speciation. And in nature, we can generally assume that the groups have been isolated from each other for thousands of generations (unless the barriers are human-made, or with recorded history, and because our genetic divergence estimation tools don’t work well for times less than that).
Can they produce fertile offspring? Well, no, not in nature, since that’s what reproductive isolation means. Maybe you can put two of them in a box and test it out manually, but you don’t have the funding to do that. And genetics don’t help too much in determining hybridization potential.
(an aside, what counts as producing fertile offspring? If they can produce fertile offspring via IVF, but never crossbreed if you cage them together because they don’t have the same mating calls or compatible mating apparatus or look funny to each other or sleep at different times of the day, does that count as producing fertile offspring? this is why “natural populations that are reproductively isolated” is added to the definition)
So you have two groups of frogs, isolated from each other for thousands of generations. Evolution is acting upon them as two different groups. But they look suspiciously similar. Do you call them different species?
I could go either way, and lean towards labelling them as different species until we have evidence to the contrary. Otherwise one of the forests might be bulldozed before we can gather the evidence.
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.”
Do you think that we should categorize e.g. the Sentinelese people as a separate species due to lack of evidence of hybridization? That seems like a strange thing to do given that (I assume) we’re all pretty confident they can still interbreed.
The Sentinelese has been reproductively isolated for probably less than twenty generations, which can be sufficient for speciation (given strong selection pressures), but relatively unlikely. So I don’t think it’s necessary to assume that speciation has occurred.
(aside, estimates of Sentinelese isolation are full of wild speculation and misinformation. We have no genetic samples from North Sentinel Island, which means we have no evidence. All estimates are speculation, with an upper bound at a few thousand years due to shared ethnographic artifacts. Missing artifacts (metalworking, etc.) may or may not help establish a lower bound, but is confounded by natural resources available on the island. Also, going back more than tens of generations would imply a high amount of inbreeding given the limited carrying capacity of the island, which could theoretically be sustained with strong selection.)
--
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?
(under most species frameworks) No! That’s preposterous. The two populations have been isolated for less than a day. Sure, they may at some point in the future diverge (and now natural selection is acting on the two groups differently), but at this point the two species are genetically identical.
--
But now imagine you stumble across on species of frog in one forest, and a similar-looking frog in a different forest. The two population are reproductively isolated. Are they the same or different species?
Well, you might ask how long they’ve been reproductively isolated for, since if the forests were connected yesterday then we converge on the same scenario as above. A week, a month, a year, a decade, a century, a millennium, a million years. At some time of separation we cannot reject [no speciation] out of hand, or, rather, number of generations, since that’s what matters in (genetic) speciation. And in nature, we can generally assume that the groups have been isolated from each other for thousands of generations (unless the barriers are human-made, or with recorded history, and because our genetic divergence estimation tools don’t work well for times less than that).
Can they produce fertile offspring? Well, no, not in nature, since that’s what reproductive isolation means. Maybe you can put two of them in a box and test it out manually, but you don’t have the funding to do that. And genetics don’t help too much in determining hybridization potential.
(an aside, what counts as producing fertile offspring? If they can produce fertile offspring via IVF, but never crossbreed if you cage them together because they don’t have the same mating calls or compatible mating apparatus or look funny to each other or sleep at different times of the day, does that count as producing fertile offspring? this is why “natural populations that are reproductively isolated” is added to the definition)
So you have two groups of frogs, isolated from each other for thousands of generations. Evolution is acting upon them as two different groups. But they look suspiciously similar. Do you call them different species?
I could go either way, and lean towards labelling them as different species until we have evidence to the contrary. Otherwise one of the forests might be bulldozed before we can gather the evidence.
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.”