It should be noted that “two creatures producing non sterile offspring” is only one possible species definition. As for minute variations resulting in different species, well, sometimes they are, sometimes they aren’t. Similar phenotypes could be the result of minor variations in genotype or environment, but may not prevent interbreeding, even under natural conditions. Or, because of differences in habit or environment, such variations are present in two distinct and isolated populations. If two presumed-species are very closely related, their differences can be expected to be relatively minor until the two lineages have had sufficient time to diverge. So, generally, such a distinction won’t be made based solely on appearance; it often takes some study to be sure that what you have are, indeed, new species.
And then you have cases like the aforementioned Cope-Marsh rivalry. In those cases, there was a certain amount of prestige to be had for naming more dinosaurs than your competitor, so just about every new fossil find was considered a new species, even in the face of minor differences (or, in some cases, just finding a different, but previously unknown, bone from a previously-identified species). So you wind up with an explosion of names; there are something like 1125 named dinosaur specimens, but only about 600 or so are currently considered valid as a result of duplication, misidentification, or poor diagnosis. The same can, and does, still happen today.
There are a couple of misconceptions here. First, as **John Mace ** has mentioned, although interfertility is necessary for two populations to be considered part of the same species, it is not sufficient. There are plenty of good species under the Biological Species Concept that are perfectly capable of producing fully fertile hybrids. (These include wolves and coyotes and also many duck species.)
It is also worth mentioning that there are several different possible definitions of species. The most prevalent today is the Biological Species Concept, which requires that two populations must be reproductively isolated from one another in order to be considered different species. In practice this means that they do not regularly produce fertile hybrids in nature . In the case of species that do not naturally occur together they should show an equivalent degree of divergence to species that do come in contact without hybridizing. The BSC cannot be applied to domestic species or to species that do not reproduce sexually.
A more recent species concept is the Phylogenetic Species Concept (PSC). This merely requires that two populations be (1) distinguishable from one another, (2) have separate evolutionary histories, that is, represent different clades. The PSC does not demand reproductive isolation.
A second misconception is that scientists routinely describe species on the basis of tiny differences. While it is true that many species have only small differences, in order for them to be recognized as such there must be some evidence that these differences are maintained by reproductive isolation.
For example, the Alder and Willow Flycatchers are essentially indistinguishable to experts even in the hand. (They do differ in average measurements but there is overlap.) They occur together over wide areas of North America. However, they have different calls and habitat preferences, and studies have shown that birds of each song type mate only with each other. This reproductive isolation has been confirmed by genetic studies. They certainly represent good species.
In the case of the species I am describing, the different forms differ slightly in plumage. We do not have information on their reproductive behavior where their ranges meet. However, the degree of genetic divergence suggests the different lineages have been separate for millions of years, and thus qualify as species under at least the PSC. They almost certainly do under the BSC too.
In contrast, there are many species that have radically different forms in different areas. Here is the Strawberry Poison-dart Frog . All the color variants shown are classified as belonging to the same species.
While some species of ducks do hybridize in the wild, I am not aware of studies that indicate that this confers any evolutionary advantage to either member of the pair. In general reproductive isolation is thought to evolve specifically because hybrids are selected against. If hybrids are favored, then isolating mechanisms will break down and the two species will merge.
One notable example of interspecific hybridization is that between Mallards and American Black Ducks in the northeastern US. The more widespread Mallard is believed to have invaded the range of the Black Duck due to habitat changes caused by humans, mostly since 1940. The two species hybridize frequently, but because Black Ducks are less numerous they are being genetically overwhelmed by Mallards. From here
If I understand your meaning correctly Jews are an example of this.
Sickle cells, blond hair and blue eyes are all recessive traits requiring reproductive isolation to be common in any given population.
So…there are different species of humans?
Not as much as you might think. But even if it were true, the BSC didn’t exist in the 16th century, so it’s a really a moot point. Whenever some new population of humans has been “discovered”, they have been brought into reproductive contact with other groups.
There certainly was never reproductive isolation between the populations of Africa and Eurasia. Or even Australia, as waves of immigrants came there from Asia at various times. You might be able to make a case for the Americas and Tasmania, but that’s it. Still, even the Americas had multiple waves of immigrants over time (witness: the Eskimo) who would have mixed with local Indian populations in certain regions.
It really doesn’t make sense to define a population as a new species if it only becomes reproductively isolated for a temporary period, especially if that period was in the past.
Jews do not represent a clade:
They may have a creation myth that presents that as an idea, but that myth has no scientific basis. Even the famous “Cohen gene” is not exclusive to Jews, but only more common among some Jewish groups than non-Jewish groups.
But we don’t have any reproductively isolated populations (as you already noted). Also, the sickle cell gene only requires malaria to be present in order for it to be “common”. And recessive traits like blond hair and blue eyes do not disappear when groups with those traits reproduce with other groups unless they confer some reproductive disadvantage to people with those traits. The alleles will still exist within the (mixed) population and be represented in the phenotype of individuals who happen to end up with two copies of those recessive alleles. (Note: skin and eye color are determined by more than one gene, but the principle is the same).
A point of clarification on the wikipedia definition of “clade”. I believe they mean “single common ancestor” not in the sense of a single organism, but in the sense of a single ancestral population. But there is no evidence that Jews of today (or of any other time) were all descended from a single population.
**Colibri **can elaborate on what he meant when he said a clade does not require reproductive isolation, but I think what he means is that a clade can consist of many different population that can’t interbreed, as long as they all descend from the same ancestral population. For example, “primates” represent a clade, even though many different species exist within that clade which cannot interbreed. But they are all descended from an ancestral proto-primate (if you will). If it were suddenly discovered that lemurs just look like primates, but were actually descended from rodents, then they would be ejected from the clade upon that discovery.
I won’t deny I’m somewhat out of my element on the esoteric side of biology, but from an untrained perspective:
I don’t see why the BSC not existing at that time makes it moot.
It was temporary, but long enough to establish distinct traits. How long does it have to be? Long enough that breeding can still possible, but we just don’t want to?
I’m white and no amount of malaria will develop Sickle cells. India has malaria and I don’t believe they have Sickle cells.
Yeah I know, but reproductive isolation allows recessive mutations to appear and become dominant. If they appear, why can’t I make the case that there is then reproductive isolation, and in accordance with your definition, then a different species?
What? You don’t understand the concept. All human populations produce fully fertile hybrids wherever they come into contact. There is essentially no reproductive isolation. Cultural differences may restrict interbreeding in some circumstances, but these have never been great enough to prevent it to any great extent.
Although prior to the 1500s human populations in (for example) South America may have been isolated from those of Australia, they were connected by a chain of interbreeding populations. That’s all that’s required to make them the same species by the BSC. In any case, even if some populations were completely isolated, under the BSC they would be classed as the same species as long as they showed a similar degree of differentiation to interbreeding populations elsewhere.
No. No human populations are distinguishable from all others by having a unique trait restricted to that population alone, which is required for the PSC to hold.
No, you don’t understand my meaning at all. The Jews do not have anything resembling a separate evolutionary history in the biological sense. They have interbred with neighboring populations throughout their history, which can be demonstrated genetically.
Since human populations are not reproductively isolated at all, this is obviously untrue. And whether a trait is recessive or not has absolutely nothing to do with its commoness or rarity in a population. Polydactyly (extra fingers and toes) is a dominant trait but is extremely rare. Blue eyes are recessive but are common in some populations, but these populations are not reproductively isolated in the slightest, unless you think that a Scandinavian and an Italian are incapable of interbreeding.
Greeks and Sicilians are also white and they also get sickle-cell anemia.
They do - especially in eastern India. So do folk in eastern Arabia. It’s not the only genetic defence against malaria - there are others. But sickle cell is a reasonably widespread one, albeit most heavily concentrated in Africa.
Actually, it might and actually it does. Sickle cell gene distribution. IIRC, that mutation seems to have appeared independently in different population.
How does a recessive trait become dominant?
But even if it did, one new dominant trait does not a make a population reproductively isolated.
What would be the point of saying: This population was a separate species for 200 years, but it wasn’t one before that and it isn’t one now?
Again, “distinctive traits” are not sufficient to define a species. As for how long, well that’s going to a subjective response. But it simply makes no sense to call populations that might have once been somewhat isolated different species if they aren’t isolated now. What would be the point? Unless we had some sense that the populations were going to stay separated, then why delineate them? If some European scientist had invented the BSC back in the 13th century, why would he designate populations as separate species if he had no idea they existed? As soon as he was aware of them, they would no longer be isolated.
For example, suppose there are two populations of an animal that are separated from each other during the warm months when a river is flowing, but not separated in the colder months when the river is frozen over. Do we switch their species designation twice every year?
My point has become tangential, and I only revisit it because I’ve spent some time trying to relocate some of my brain cells.
This cite (Caution- PDF) contains some related discussion, including “Today, DNA sequence data and other molecular methods are beginning to show that limited invasions of the genome are widespread, with potentially important consequences in evolutionary biology, speciation, biodiversity, and conservation.”
Further, “It is not generally realized that in spite of rarity on a per individual basis, natural hybridization is usually common on a per species basis. Although hybrids are rare in populations, a few hybrids can provide a bridge to allow a trickle of alleles to pass between species; thus, if species that hybridize are common, even low rates of hybridization per individual can have important evolutionary consequences in a high fraction of species.” The author notes that more than 76% of British duck species (Anatinae) naturally hybridize. These, BTW, are noted to specifically dis-include captive hybrids.
Finally, and in conclusion, “even if genetically isolated species play a role in diversification, today we know that evolutionary progress can continue while species undergo genomic invasions from other species. Not only that, variation introduced via introgression regularly contributes to adaptation and diversification throughout the Tree of Life.” This is what I had in mind when mentioning an evolutionary advantage of hybridization to the hybridizing species.
None of this changes the example Colibri offers where a species (Black Duck) is being overwhelmed by another (Mallard). Mallards are also genetically overwhelming Mottled Ducks here in Florida. Certainly hybridization can have this effect too.
My only point in all of this is to suggest that the OP cannot be simply and clearly answered at a lay level. A species isn’t just a morphologically defined group, as it may have been for Linnaeus. The Biological Species Concept (no, or virtually no, fertile natural hybridization) also fails to provide a neat, simple definition as it must ignore the role and frequency of hybridization noted in the cite. And a cladistic approach becomes even more esoteric and problematic from an outside perspective.
You have all done an excellent job of explaining the usage of “species” in today’s biology. But if anything, you have demonstrated that not only is there no single, simple and all inclusive definition of species— rather, there are several definitions (ways in which scientists “know when they discover a new species”) and ALL of them are often necessary to make such a decision.
Some earlier species concepts, such as Morphological or Typological, may have led to some early taxonomists deciding there were several different species of humans. However, even in this case these taxonomists did not appreciate how much human populations intergrade where they come in contact. At any rate, these species concepts are out of date and no longer employed.
As I said above, no human populations have any distinct traits that reliably distinguish them, as populations, from all other populations. Although blue eyes and blond hair may be more common among Scandinavians, there are lots of Scandinavians that have dark hair and brown eyes. Although West Africans have relatively dark skin, so do lots of other human populations in other parts of the world.
Completely irrelevant to the discussion.
You are completely misunderstanding what is meant by reproductive isolation. Reproductive isolation means that there are specific genetically-based behavioral or physiological characteristics that prevent two populations from interbreeding. Humans lack any such traits.
It seems that you are confusing physical isolation by distance with reproductive isolation. These are completely different things.
To be fair, I started posting saying this isn’t my field so forgive my ignorance…but that’s what we’re here to fight right?
I’m not sure how you define ‘isolation’? Do they have to be on the moon? 'cause to a 13th century mariner they may as well be.
Was there some ancient intercontinental transit system I’m unaware from Africa to S. America or N. America to Europe? I’m not sure the Bering Strait was the genetic superhighway your making it to be.
How does evolution maintain a similar degree of differentiation between isolated populations?
I didn’t say it did, what I said was for a recessive gene to ‘appear’ it must be in both parents. Since all it takes is a few brown eyed travelers to gum up the works it doesn’t seem to me an incredible stretch to call a population with an overwhelming majority of ‘active’ recessive genes isolated.
But if Scandinavians and Italians interbred there’d be few blue eyes. And you said that’s not sufficient anyway:
I’m not arguing with you, I’m asking you. If two creatures can mate and produce fertile offspring, but still be different species depending on a nonspecific isolation interval and showing a similar degree (ever open to interpretation) of differentiation to interbreeding populations elsewhere it seems rather subjective. And if it is, maybe the science of biology should find a more solid foundation. Tamerlane, the incredible things you learn on this site…why I keep coming back.
As I said, you are confusing physical isolation in space with reproductive isolation.
It doesn’t need to be a “genetic superhighway.” Speciation as a process generally takes tens of thousands to hundreds of thousands of years. Even a relatively small amount of gene flow would maintain species cohesion.
As far as we know, there has been a continuously interbreeding chain of human populations throughout the world since modern humans left Africa. No human population has been completely isolated for long enough to develop anything like species status.
Gene flow is not instantaneous. Distance may be sufficient for differences in gene frequencies to develop between distant populations. But such populations are not isolated - gene flow is still possible, so species-level distinctions do not develop.
Again, you are misunderstanding what “isolation” means. For speciation to occur, in most cases (excluding cases like polploidy) it generally requires complete or nearly complete absence of any gene flow, even indirectly through intermediate populations, over a period of thousands of generations. Differences in the frequencies of a few alleles does not constitute evidence for such isolation.
Speciation is a continuous process. Populations gradually become distinct when isolated. When they re-unite, they may either merge, or if behavioral or physiological traits have developed that cause reproductive isolation, they may differentiate further. There is no single point on this continuum where you can say that speciation has irrevocably occurred, except after it becomes completely impossible for them to produce fertile offspring. And it is unrealistic to use the latter as a dividing line, since most people would agree that lions and tigers should be regarded as species, even though they can produce some fertile offspring.
The category of “species” to a degree is a human construct. Where exactly we place the dividing line between full species and incipient species is somewhat arbitrary. But there certainly is an underlying reality there. Should we not recognize the a distinction between ice and water just because slush exists?
Africa has always been genetically connected to the Middle East (via modern day Israel, Yemen or Saudi Arabia). The M.E. has always been connected to the rest of Asia, which has always been connected to the Americas via the Bering Strait. Now, there may have been times with the connection between Alaska and Siberia was cut off, but there was never just one group of people who came to the Americas, and then the continent was cut off forever. Are you aware that modern day Eskimos are much more recent arrivals in Alaska and other northern regions that are American Indians?
It might be better to think of it as if you were a Martian biologist equipped with the same notion of the BSC. You fly around the world and look at the various populations, and look at how they interact as well as their physical variation. You see that there are some populations that are relatively isolated, but you also notice that these relatively isolated populations do not have any unique physical characteristics that cannot be found in some other population. You are then pretty much forced to consider all the populations to be one species.
Which populations are you referring to that have an “overwhelming majority” of active (by which you actually mean expressed) recessive genes (by which you actually mean alleles)? Blond hair and blue eyes are just one trait among thousands that human possess and that differentiate from other species.
If everyone in Norway were dispersed throughout the Italian peninsula, there might end up being fewer blue eyed individuals, but that trait would not disappear. (Firstly, note that there are already plenty of blue-eyed Italians.) Otherwise it would have disappeared in the US. The only way it could disappear is for everyone carrying that allele to die without leaving offspring. While that is possible, it is highly improbable-- at least not in a species as well established as humans.
The way that might happen is if some terrible disease wiped out everyone in Italy except for a few families in a remote village somewhere who just happened to be all brown-eyed individuals with no recessive blue-eyed genes. Then, over time, those few families repopulated the Italian peninsula, leaving only brown-eyed individuals. That type of event may have happened to various human populations tens of thousands of years ago (when we were much less numerous), but is almost impossible to happen today.
However, even if that event did happen, it wouldn’t create a new species. There is nothing inherently unique in a population with only brown-eyed individuals. That eye color is the most common one found throughout the world. So, forget about blue-eyed individuals. That one trait is insignificant in defining a reproductively isolated population.
Hear, hear! And I’ll suggest that its “somewhat arbitrary” nature is only due to our lack of comprehensive knowledge. We are forced to infer history, gene flow, sterility barriers, isolating mchanisms, gene frequencies, et cetera. No matter how much we investigate any group or any species, there is always so much more to find out. From this we attempt to abstract that “underlying reality”.
I don’t think so. It’s a human construct that we try to map onto the world, but isn’t inherent in the world itself. Even if we had perfect knowledge, we would still have to come up with some arbitrary delineations. There really is a continuum of life-- it’s a mistake to think that our natural propensity to categorize things implies that categories have some fundamental meaning.
