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One problem when discussing species and subspecies is that there is no clear-cut definition between species, subspecies and "just some genetic diversity". The

One problem when discussing species and subspecies is that there is no clear-cut definition between species, subspecies and "just some genetic diversity". The best hint is the genetic distance between any two individuals. In general, that distance is bigger between species than between subspecies. But no clear thresholds can be applied to the whole tree of life. Taxonomy is still mostly defined through differences a researcher can see with his own eyes (or a microscope). Much of taxonomy is in the process of being redefined through molecular methods, though.

JumpCrisscross 7y ago
> there is no clear-cut definition between species It's not clear-cut in all cases. But it's clear cut bilaterally. Two organisms are in the same species if they can successfully reproduce, i.e. produce an offspring that can itself produce offspring. If they can't, they're in different species. (Multilateral fuzziness arises if A can reproduce with B, and B with C, but A not with C.)
- sago 7y ago
  So all animals are in different species to members of their same sex? I'm joking a little of course. But only to make the point that the definition of species taught in high school is very difficult to pin down. You identified one way. Even more common: not all organisms reproduce sexually. But also: some organisms (protists) perform sexual recombination separately to reproduction. And even for well known higher mammals (which is where the 'offspring must also be fertile' clause comes from), 'fertile' is by no means binary! A sexual definition of species is simple, but rather difficult down in the nitty-gritty. It is always worth bearing in mind that species is a human classification concept, not a biological reality. It's not surprising that it has very grey edges.
  - virtuous_signal 7y ago
    At least for sexually reproductive species, your first objection could be addressed by defining the species relation as the transitive closure of the relation A ~ B if A can reproduce with B. (so for instance male1 ~ female0 and male2 ~ female0 by the original definition, and then male1 ~ male2 via the closure)
    - bayesian_horse 7y ago
      You still assume you have animals that breed when you're looking. Lot's of them don't. And it's still about the genetic distance. You can't mate everyone to everyone. You need to cluster by phenotype,genotype or location.
  - Scarblac 7y ago
    It's also just an extremely impractical definition. Say you have two animals and you want to know if they are the same species. How are you going to find out whether they can or cannot mate (and produce fertile offspring, even)? What if they are the same species but just aren't that interested in each other?
- detaro 7y ago
  That definition fails for various birds. I.e. all kinds of ducks are considered separate species, but can successfully produce hybrids, of which some can breed.
- phkahler 7y ago
  So why are different ethnicities of humans not considered sub-species? They developed in different geographic regions with barriers, the can interbreed successfully, and they have different appearances, not just in skin color, but in things like facial features. And yet the article specifically states that humans don't have subspecies.
  - HelloNurse 7y ago
    Because subspecies are a particularly arbitrary taxonomical division. Some of the pragmatic reasons are that human "ethnicities" are very fuzzy because people travel around, that they are only an average of arbitrary large groups of related people, and that people tend to look meaningfully similar only to their closest relatives, and only partially.
  - bayesian_horse 7y ago
    First of all, calling ethnicities subspecies creates a political mess and such arguments have often been used for all kind of injustices. Secondly I'd say that the differences are very fluid and it's hard to pinpoint any hard barriers. The "European" ethnicity only developed in the past 4000 years. Migrations during all that time make the picture even more difficult.
  - K0SM0S 7y ago
    The fundamental reason is that genetic diversity simply doesn't warrant it. You may think we're different based on appearance; but the genetic material says otherwise. I might be genetically much closer to someone of different skin color and gender than to my closest geographical/ethnic/cultural neighbor who looks just like me, in terms of DNA variation. Human beings altogether are not diverse at all genetically compared to most animal species. And if we were to use said differences to classify "races" among the human species, you can be absolutely sure skin color wouldn't correlate, nor would eye or hair color. For better candidates, look at the HLA system for an idea of more relevant classification method (this one defines immuno-regulation profiles among us, and has been shown to drive sexual attraction towards opposites, hypothetically because it favors enhanced, combined immunity in the offspring). But even taking such systems into account, human beings are really far from meeting any common threshold of differenciation worth being called "race" in the animal realm.
- msla 7y ago
  > Two organisms are in the same species if they can successfully reproduce, i.e. produce an offspring that can itself produce offspring. OK, explain European Early Modern Humans and Neanderthals being different species.
  - tabtab 7y ago
    Perhaps they are not. As I mention nearby, successful breeding can be measured on a continuous scale. We don't really know the success rate, other than it being above zero because humans have some of their genes. Maybe 9999 cross-group cave-people boinked for every one cave-baby produced. (Bamm-Bamm?)
  - Sniffnoy 7y ago
    That's not a good choice of counterexample; they're generally not considered separate species these days, largely because of that.
  - bayesian_horse 7y ago
    They diverged a rather long time before they started breeding again. They also seem to have other reproductive rates and behavior. They are anatomically more distant from modern Humans, than any group of modern Humans from each other. In every sense of the word species, it makes more sense to call Neanderthals a different species.
    - msla 7y ago
      > They are anatomically more distant from modern Humans, than any group of modern Humans from each other. This makes them different subspecies, which humanity no longer has.
- bayesian_horse 7y ago
  In some animals it's impossible to observe breeding at all. In other cases, two different species can breed.
- tabtab 7y ago
  Even mating is often a matter of probability. As two genetic groups diverge, the probability of successful mating gradually shrinks over time. We could define the boundary at 0% success, but that's really hard to verify in practice.

JumpCrisscross 7y ago

> there is no clear-cut definition between species It's not clear-cut in all cases. But it's clear cut bilaterally. Two organisms are in the same species if they can successfully reproduce, i.e. produce an offspring that can itself produce offspring. If they can't, they're in different species. (Multilateral fuzziness arises if A can reproduce with B, and B with C, but A not with C.)

sago 7y ago
So all animals are in different species to members of their same sex? I'm joking a little of course. But only to make the point that the definition of species taught in high school is very difficult to pin down. You identified one way. Even more common: not all organisms reproduce sexually. But also: some organisms (protists) perform sexual recombination separately to reproduction. And even for well known higher mammals (which is where the 'offspring must also be fertile' clause comes from), 'fertile' is by no means binary! A sexual definition of species is simple, but rather difficult down in the nitty-gritty. It is always worth bearing in mind that species is a human classification concept, not a biological reality. It's not surprising that it has very grey edges.
- virtuous_signal 7y ago
  At least for sexually reproductive species, your first objection could be addressed by defining the species relation as the transitive closure of the relation A ~ B if A can reproduce with B. (so for instance male1 ~ female0 and male2 ~ female0 by the original definition, and then male1 ~ male2 via the closure)
  - bayesian_horse 7y ago
    You still assume you have animals that breed when you're looking. Lot's of them don't. And it's still about the genetic distance. You can't mate everyone to everyone. You need to cluster by phenotype,genotype or location.
- Scarblac 7y ago
  It's also just an extremely impractical definition. Say you have two animals and you want to know if they are the same species. How are you going to find out whether they can or cannot mate (and produce fertile offspring, even)? What if they are the same species but just aren't that interested in each other?
detaro 7y ago
That definition fails for various birds. I.e. all kinds of ducks are considered separate species, but can successfully produce hybrids, of which some can breed.
phkahler 7y ago
So why are different ethnicities of humans not considered sub-species? They developed in different geographic regions with barriers, the can interbreed successfully, and they have different appearances, not just in skin color, but in things like facial features. And yet the article specifically states that humans don't have subspecies.
- HelloNurse 7y ago
  Because subspecies are a particularly arbitrary taxonomical division. Some of the pragmatic reasons are that human "ethnicities" are very fuzzy because people travel around, that they are only an average of arbitrary large groups of related people, and that people tend to look meaningfully similar only to their closest relatives, and only partially.
- bayesian_horse 7y ago
  First of all, calling ethnicities subspecies creates a political mess and such arguments have often been used for all kind of injustices. Secondly I'd say that the differences are very fluid and it's hard to pinpoint any hard barriers. The "European" ethnicity only developed in the past 4000 years. Migrations during all that time make the picture even more difficult.
- K0SM0S 7y ago
  The fundamental reason is that genetic diversity simply doesn't warrant it. You may think we're different based on appearance; but the genetic material says otherwise. I might be genetically much closer to someone of different skin color and gender than to my closest geographical/ethnic/cultural neighbor who looks just like me, in terms of DNA variation. Human beings altogether are not diverse at all genetically compared to most animal species. And if we were to use said differences to classify "races" among the human species, you can be absolutely sure skin color wouldn't correlate, nor would eye or hair color. For better candidates, look at the HLA system for an idea of more relevant classification method (this one defines immuno-regulation profiles among us, and has been shown to drive sexual attraction towards opposites, hypothetically because it favors enhanced, combined immunity in the offspring). But even taking such systems into account, human beings are really far from meeting any common threshold of differenciation worth being called "race" in the animal realm.
msla 7y ago
> Two organisms are in the same species if they can successfully reproduce, i.e. produce an offspring that can itself produce offspring. OK, explain European Early Modern Humans and Neanderthals being different species.
- tabtab 7y ago
  Perhaps they are not. As I mention nearby, successful breeding can be measured on a continuous scale. We don't really know the success rate, other than it being above zero because humans have some of their genes. Maybe 9999 cross-group cave-people boinked for every one cave-baby produced. (Bamm-Bamm?)
- Sniffnoy 7y ago
  That's not a good choice of counterexample; they're generally not considered separate species these days, largely because of that.
- bayesian_horse 7y ago
  They diverged a rather long time before they started breeding again. They also seem to have other reproductive rates and behavior. They are anatomically more distant from modern Humans, than any group of modern Humans from each other. In every sense of the word species, it makes more sense to call Neanderthals a different species.
  - msla 7y ago
    > They are anatomically more distant from modern Humans, than any group of modern Humans from each other. This makes them different subspecies, which humanity no longer has.
bayesian_horse 7y ago
In some animals it's impossible to observe breeding at all. In other cases, two different species can breed.
tabtab 7y ago
Even mating is often a matter of probability. As two genetic groups diverge, the probability of successful mating gradually shrinks over time. We could define the boundary at 0% success, but that's really hard to verify in practice.