Speciaion in humans

[Orbital]

Ok, I love the subject of speciation, particularly in the realm of zoology, so I can't help but think that, if the world had not been so cross populated with different 'races' of people, these 'races' would have eventially developed into different species. So I guess I was just wondering if people would agree that human 'races' were/are kind of a starting point to speciation in humans, which may or may not have been stopped by cross breading.

[Paralith]

any divergence between populations of a species has the potential to lead to eventual speciation given that gene flow between them is blocked long enough. in humans, even a cultural trend could serve to block gene flow, if for example it was taboo to have children with anyone outside of the group.

[Sisyphus]

Isolated populations could be considered starting points. "Races" are probably a bad example of that, since what we culturally consider to be races (white, black, asian, etc.) have never actually been isolated from one another.

[insane_alien]

i'd go with sisyphus here. more likely to happen with isolated populations than anything as broad and ill fitting as 'races' as it is possible for individuals in the same 'race' to have more genetic differences than with some one in another 'race'

[Orbital]

Isolated populations could be considered starting points. "Races" are probably a bad example of that, since what we culturally consider to be races (white, black, asian, etc.) have never actually been isolated from one another.

 

Really? I would have thought there was a time before humans really had the ability to travel the world, when there certainly was very little interbreeding of people from different geological areas. Surely these races appeared for a reason, what is the biological reason if not as a result of speciation? Do you consider humans to be somehow different to every other animal in the animal kingdom?

[Mr Skeptic]

At least some of the characteristics of races are an adaptation to the environment. In theory, if groups of people remained separate enough for long enough, they could speciate. In the modern world, that's not going to happen anymore because travel has become so much easier. Races may have been a starting point for speciation, as they are genetic differences between groups. I don't know how far that got, but I read somewhere that the children of parents from two different races are a little worse off than children of either race, which might be the beginning of speciation. If as someone else said there was some interbreeding even across continents, that might be enough to prevent speciation.

 

In any case, such discussions are not politically correct, and I would expect to hear from plenty of angry people shortly.

[Sisyphus]

Really? I would have thought there was a time before humans really had the ability to travel the world, when there certainly was very little interbreeding of people from different geological areas. Surely these races appeared for a reason, what is the biological reason if not as a result of speciation? Do you consider humans to be somehow different to every other animal in the animal kingdom?

 

That's not different from every other animal, it's the same as any animal with a very large, continuous range. Until recently, it wasn't plausible that a south African would mate with an east Asian, but there still would have been a link between them via the continuous presence of human beings. "Very little interbreeding" is a world of difference from "no interbreeding." You would get some genetic tendencies depending on the environment and degree of isolation, but it would reach equilibrium very quickly due to the genetic exchange that does take place. As for populations that actually were isolated, they wouldn't speciate for a different reason: not nearly enough time. "Native" Americans, for example, can only trace their ancestry there a few thousand years, during which time there were several crossovers from the Old World!

[Dr.Evil]

Good points guys. I think the main reason for the dissaperance of Neanderthals if due to interbreading. If anything I think we will eventually become one race but without interbreading we would definitely become seperate species. There is a tribe in afria with only 2 toes for example and Aborigines are very differnt from all the other races.

[SkepticLance]

An interesting extension to the original question is :

"Do you think humans will speciate in the future?"

 

With the extension of the human species off this planet, I think it could happen very easily. Imagine a generation starship going to the nearest alternative stellar system - alpha Centauri -about 4.3 light years away.

 

Forgetting impossibilities such as warp drive, and bearing in mind basic physical limitations, we might get such a ship with a cruise speed of 10% of the speed of light, and a need to take ten years to accelerate to that speed, and another ten years to decelerate back down to stop.

 

A quick calculation reveals that the journey will take 53 years.

 

While this number comes from a number of assumptions that may well prove wrong, we can assume that interstellar travel will take a hell of a long time. This means that physical travel to established colonies will be rare. Thus we get the genetic isolation required for speciation. Add sufficient time, and guess what will happen?

[Orbital]

Regarding political correctness, I'm not so sure why this topic should anger people. I am not suggesting in any way, shape, or form that one race is better or worse than another. We are all animals though and I thought it interesting to examine things from a biological point of view, given that we are all living. I understand that speciation of humans may be non existant in modern times, but I'm not sure how else you would explain race without it being some form of speciation, if only a very early one.

[Paralith]

I understand that speciation of humans may be non existant in modern times, but I'm not sure how else you would explain race without it being some form of speciation, if only a very early one.

 

I don't think this is politically incorrect, but I do think it is incorrect to call what is merely a slight genetic divergence actual speciation. If it were correct, then we'd have to say that speciation is occurring in many species all over the world. Slight genetic variations in subpopulations have the potential to lead to speciation, but are not events of speciation themselves. As has been pointed out several times, what is thought of as human "races" are nowhere near reproductively isolated enough at this point for any "race" to seriously begin to speciate away from other humans.

 

Long story short: human races are not incipiently speciating sub-populations. They could be one day, but are not right now.

[Orbital]

I don't think this is politically incorrect, but I do think it is incorrect to call what is merely a slight genetic divergence actual speciation. If it were correct, then we'd have to say that speciation is occurring in many species all over the world. Slight genetic variations in subpopulations have the potential to lead to speciation, but are not events of speciation themselves. As has been pointed out several times, what is thought of as human "races" are nowhere near reproductively isolated enough at this point for any "race" to seriously begin to speciate away from other humans.

 

Long story short: human races are not incipiently speciating sub-populations. They could be one day, but are not right now.

 

 

This is exactly my point. I'm NOT saying it is full blown speciation but meerly that it would have been the start.

[lucaspa]

This is exactly my point. I'm NOT saying it is full blown speciation but meerly that it would have been the start.

 

Orbital, the problem is that "race" is a meaningless term in biology. What most people, including you, seem to think of as "race" is the 3 "races": Caucasian, Mongoloid, Negroid. However, in the history of science, race has had several meanings. In Darwin's time "race" meant much smaller groups. People talked of the Irish "race" and the German "race". Basically, "race" equaled nationality in Darwin's day.

 

The better term in biology is population. They are much smaller than "races" and each of the 3 "races" is made up of dozens/hundreds of populations. Scientific American article on race http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=00055DC8-3BAA-1FA8-BBAA83414B7F0000

 

In terms of skin color -- what people often denote as the major hallmark of a "race" -- the different amounts of melanin content did arise by natural selection. I refer you to the following article: 1 G Kirchwager, Black and white: the biology of skin color. Discover 22: 32-33, Feb. 2001. Nina Jablonski and George Chaplin have first comprehensive theory of skin color. Need sweat glands to cool skin and brain; then need less hair for sweat glands to work better. Then need dark skin to protect from sun on hairless skin. An hour of intense sunlight cuts folate (vitamin B) in half. This results in neural tube defects in embryogenesis. Folate also necessary for sperm production. But you need UV to make vitamin D. Not enough UV getting to the skin, not enough vitamin and the result is rickets. Skin color correlated to sunlight: the weaker the UV light, the lighter the skin.

 

In species with large numbers of individuals with an extensive range, natural selection acts as disruptive selection. The populations face slightly different environments and natural selection acts on them to diverge the population -- eventually producing new species by allopatric speciation. BUT, gene flow between the populations acts to oppose this divergence. As human technology became better and travel became more common, gene flow increased.

 

Now, remember that, in sexually reproducing species, speciation = reproductive isolation. There is evidence today that isolated populations are undergoing disruptive selection and restricted gene flow necessary for speciation.

 

1. People who live in the higher altitudes of the Andean and Himalayan mountains show genetic physiological adaptations to living at the higher altitudes. Each population has different adaptations.

Hum Biol 2000 Feb;72(1):201-28 Tibetan and Andean patterns of adaptation to high-altitude hypoxia. Beall CM

Comp Biochem Physiol A Mol Integr Physiol 1999 Sep;124(1):1-17 Adaptation and conservation of physiological systems in the evolution of human hypoxia tolerance. Hochachka PW, Rupert JL, Monge C

Am J Phys Anthropol 1998;Suppl 27:25-64 Human adaptation to high altitude: regional and life-cycle perspectives. Moore LG, Niermeyer S, Zamudio S

 

2. There is evidence that the !Kung people living in the Kalahari desert in Africa have a) adaptations to the dry climate, b) alleles unique to them and to no other population and c) do not have gene flow into the !Kung. When a !Kung marries, that person must go live with the spouse's people. If you ever saw the movie The Gods Must Be Crazy, one part is that a blond Englishwoman views a !Kung male as extremely ugly and not as a potential mate. He views the Englishwoman as ugly and not as a potential mate. This is one mechanism of reproductive isolation: individuals of one population simply don't view members of another population as mates.

 

Am J Trop Med Hyg 1994 Oct;51(4):460-5 Low prevalence of human T lymphotropic virus type I in !Kung San in Bushmanland, Namibia. Steele AD, Bos P, Joubert JJ, Evans AC, Joseph S, Tucker L, Aspinall S, Lecatsas G

Ann Hum Genet 1979 May;42(4):425-33 Red cell adenosine deaminase (ADA) polymorphism in Southern Africa, with special reference to ADA deficiency among the !Kung. Jenkins T, Lane AB, Nurse GT, Hopkinson DA

Am J Phys Anthropol 1988 Nov;77(3):303-19 Fitness and fertility among Kalahari !Kung. Pennington R, Harpending H

 

It's an open question whether this process will result in new species of Homo. It depends on how isolated the populations remain. As I said, with increased transportation and intermixing of populations, gene flow is easier and this would prevent speciation.

[bombus]

Human races are sub-species of human. The characteristics attributed to subspecies are generally derived from changes that evolved as a result of geographical distribution. There are three or four sub-species of human (depending on how you look at it):

 

For want of better terms they are essentially:

 

1. Caucausoid: includes Europeans, North Africans, Jews, Arabs, Persians, Indians, etc.

2. Archaic White (sometimes categorized as a subclass of the above): Australian aboriginies, Veddans (South India), Anui (Japan), Uralic peoples (Russia).

3. Negroid: Sub-Saharan Africans

4. Mongaloid: Chinese, Japanese, Korean, 'Eskimo', North and South American 'Indians'.

 

Mixing has occured on the fringes of these peoples such as the 'Indo-chinese' (e.g. Vietnamese, Burmese, Nepalese), Ethiopians, some North Africans, Polynesians/Micronesians etc. Some suggest that certain North American 'Indians' are mixed with North Europeans who crossed to America from Europe during the last glaciation.

 

All interesting stuff, and a great argument against racism IMO!

 

Anyway, all are capable of developing into different species if isolated, but remember the concept of species is quite fuzzy anyway. Herring Gulls and Black Backed Gulls are the same species in Canada, but do not breed with each other in Europe. They are known as a 'ring species'.

 

Good points guys. I think the main reason for the dissaperance of Neanderthals if due to interbreading. If anything I think we will eventually become one race but without interbreading we would definitely become seperate species. There is a tribe in afria with only 2 toes for example and Aborigines are very differnt from all the other races.

 

Australian aborigines are a very old race, but are not that unique. They are closely related to the Veddans of South India, the Anui of Japan and Uralic peoples. They show characteristics of the earliest Homo sapiens. And the tribe in Africa with two toes is just a genetic trait that occurs in a small group. It's not linked to a race.

[SkepticLance]

Sorry bombus

 

On this matter, you have taken a small difference and stretched it too far.

 

Races are not sub-species. There is a massive difference in terms of genetic change. Human races differ from each other to a degree that is not more than the genetic difference between myself and my neighbour - both of us European stock.

 

Your classification of races is an obsolete one that is not followed any more by respected scientists. There are just too many points of overlap, which make the classification totally arbitrary and not very useful.

 

The difference in terms of melanin is caused by just a few genes, and it often happens that a person from a 'white' culture will be born much darker than others, or the reverse - a 'black' set of parents give birth to someone much paler than they. This happens because the number of genes involved are small and a slight genetic difference can create a large difference in skin colour.

 

Ditto for other racial traits. They are caused by a very few genes, and cannot constitute the degree of genetic difference to call those different populations human sub-species.

[bombus]

Sorry bombus

 

On this matter, you have taken a small difference and stretched it too far.

 

Races are not sub-species. There is a massive difference in terms of genetic change. Human races differ from each other to a degree that is not more than the genetic difference between myself and my neighbour - both of us European stock.

 

Sorry SkepticLance, you're wrong there. Also, I suggest you read more about the human genome project. There is far more variation in humans than previously thought.

 

Your classification of races is an obsolete one that is not followed any more by respected scientists. There are just too many points of overlap, which make the classification totally arbitrary and not very useful.

 

The difference in terms of melanin is caused by just a few genes, and it often happens that a person from a 'white' culture will be born much darker than others, or the reverse - a 'black' set of parents give birth to someone much paler than they. This happens because the number of genes involved are small and a slight genetic difference can create a large difference in skin colour.

 

Ditto for other racial traits. They are caused by a very few genes, and cannot constitute the degree of genetic difference to call those different populations human sub-species.

 

I repeat, Human races can quite correctly be called sub-species. If humans cannot be so sub-divided then neither can Tigers/Wolves/Cats etc! The differences are not that small, but for obvious reasons, small enough to allow fully successful interbreeding.

 

There is no "degree of genetic difference" accepted by science that separates one sub-species from another. All that is required is that one sub-species is recognizable and linked to a geographic area. I suggest you read about Carrion and Hooded Crows, for example.

 

The following criteria is for Species, so is even less stringent for sub-species. My highlighting:

 

1. Members of the group are reliably distinguishable from members of other groups. The distinction can be made in any of a wide number of ways, such as: differently shaped leaves, a different number of primary wing feathers, a particular ritual breeding behaviour, relative size of certain bones, different DNA sequences, and so on. There is no set minimum 'amount of difference': the only criterion is that the difference be reliably discernable. In practice, however, very small differences tend to be ignored.

 

2. The flow of genetic material between the group and other groups is small and can be expected to remain so because even if the two groups were to be placed together they would not interbreed to any great extent.

[SkepticLance]

To bombus

You are correct in that these classifications can be rather arbitrary, and there is no clear cut definition. This makes for lots of argument.

 

Originally, the term species meant a group of organisms incapable of interbreeding successfully across many generations with any other group. However, some strange exceptions have been seen across groups originally seen as distinct species. Hence the rather nebulous and silly definition you quoted.

 

Subspecies and race are even more nebulous. However, sub-species is supposed to represent a genetic difference from other sub species that is significantly greater than the genetic difference within its own group. By this distinction, human races are not sub species.

[bombus]

To bombus

You are correct in that these classifications can be rather arbitrary, and there is no clear cut definition. This makes for lots of argument.

 

Originally, the term species meant a group of organisms incapable of interbreeding successfully across many generations with any other group. However, some strange exceptions have been seen across groups originally seen as distinct species. Hence the rather nebulous and silly definition you quoted.

 

Subspecies and race are even more nebulous. However, sub-species is supposed to represent a genetic difference from other sub species that is significantly greater than the genetic difference within its own group. By this distinction, human races are not sub species.

 

Well, I'm sure one can come up with any number of definitions that exclude human races as sub-species if one wants. However, your definition is not scientifically accepted anywhere as far as I have been able to research. Also, by your definition the very act of two sub-species interbreeding in an overlap area would cancel both sub-species out as such.

 

Considering that the hooded crow and the carrion crow breed with each other where their ranges overlap does not cancel them out as sub-species.

[SkepticLance]

To bombus

 

Of course interbreeding between two sub species will cancel out the sub species if that interbreeding is total. However, if a hybrid population forms only in one place, then the two sub species remain distinct elsewhere, and thus still exist.

[bombus]

To bombus

 

Of course interbreeding between two sub species will cancel out the sub species if that interbreeding is total. However, if a hybrid population forms only in one place, then the two sub species remain distinct elsewhere, and thus still exist.

 

Maybe I misunderstood you.

 

However, I maintain that human races as described above can be considered sub-species, although I accept that it may depend on exactly what definition is used. As there is no universally accepted definition of species (see here http://en.wikipedia.org/wiki/Species_concept#Definitions_of_species)

there can't really be one for sub-species...

[SkepticLance]

To bombus

 

As you imply, the problem is semantics - what is the exact definition of sub-species? However, most of the reading I have done on the subject of human races emphasizes how little the various races differ genetically. In fact, I recall one article written by a geneticist who argued that even the term 'race' was an over statement. Human groups differ too little, according to him, to even be called different races.

[foodchain]

To bombus

 

As you imply, the problem is semantics - what is the exact definition of sub-species? However, most of the reading I have done on the subject of human races emphasizes how little the various races differ genetically. In fact, I recall one article written by a geneticist who argued that even the term 'race' was an over statement. Human groups differ too little, according to him, to even be called different races.

 

I think a lot of this has to do with the modern paradigm I would say lots of people hold of life. I am talking about in particular the genotype centered view. I would just like to make a simple example in that we don’t classify cars of course on number and placement of windows and or bolts typically. With that said I am not trying to reduce of course the role of genetics in biology just that life comes in organismal units typically. In which the whole is what makes up the living thing and its various functions. Also as far as I know science cant really support the concept of race, either from a hard science or soft science point of view. Different cultures, now that seems to exist, but human cultures themselves don’t seem to be static, homogeneous, or immutable.

[lucaspa]

Human races are sub-species of human.

 

They don't fit the definition of sub-species.

 

Futuyma's Evolutionary Biology:

"Subspecies: A taxonomic term for populations of a species that are distinguishable by one or more characteristics, and are given a subspecific name (e.g. the spuspecies of the rat snake Elaphe obsoleta; se Figure 21 in Chapter 9). In zoology, subpecies have different (allopatric or parapatric) geographical distributions, so are equivalent to "geographic races;" in botany, they may be sympatric forms. No criteria specify how different populations should be to warrent designation as subspecies, so some systematists have argued that the practice of naming subspecies should be abandoned." pg 450

 

"Race: A vague, meaningless term, sometimes equivalent to subspecies and sometimes to polymorphic genetic forms within a population."

 

None of the "races" you mention have been given sub-species names.

 

There are three or four sub-species of human (depending on how you look at it):

 

These are the general "races" listed, but none of them are sub-species.

 

all are capable of developing into different species if isolated, but remember the concept of species is quite fuzzy anyway. Herring Gulls and Black Backed Gulls are the same species in Canada, but do not breed with each other in Europe. They are known as a 'ring species'.

 

The "races" you named are too large to develop into different species. And, of course, with human transportation NONE of them are isolated.

 

There is no precise definition of species, but that doesn't mean the term is fuzzy. It simply means that evolution is true: since species change from one to another gradually over generations, any definition of species is always going to be able to find a population in that transition.

 

"Races" is a meaningless term, as Futuyma (and the Scientific American article) note and demonstrate. "Races" are not the same as biological populations.

 

Also, I suggest you read more about the human genome project. There is far more variation in humans than previously thought.

 

But the variation is still very low compared to other species. The whole variation within the entire human species is less than within one population of chimps in western Africa!

 

If humans cannot be so sub-divided then neither can Tigers/Wolves/Cats etc!

 

Humans can be divided into populations. See my post. But "races" are not those populations! Each "race" as you defined it consists of dozens/hundreds of populations.

 

There is no "degree of genetic difference" accepted by science that separates one sub-species from another. All that is required is that one sub-species is recognizable and linked to a geographic area.

 

It's more than that. Again, see the definition of sub-species by Futuyma.

 

1. Members of the group are reliably distinguishable from members of other groups. The distinction can be made in any of a wide number of ways, such as: differently shaped leaves, a different number of primary wing feathers, a particular ritual breeding behaviour, relative size of certain bones, different DNA sequences, and so on. There is no set minimum 'amount of difference': the only criterion is that the difference be reliably discernable. In practice, however, very small differences tend to be ignored.

 

What's your source, Bombus? This might be a way to define populations. It is not a way to define "species". It is closest to the phenetic species concept: which is based on morphological/physiological differences. However, "a particular ritual breeding behavior" nor only "number or primary wing feathers" can be criteria, because number of primary wing feathers can vary from individual to individual.

 

The phenetic species concept was the one used in Darwin's time and is still applied to fossils. It is NOT the one used between contemporary sexually reproducing species -- such as humans.

 

2. The flow of genetic material between the group and other groups is small and can be expected to remain so because even if the two groups were to be placed together they would not interbreed to any great extent.

 

This one doesn't exist. You are misrepresenting the biological species concept. That concept states:

 

Biological species are defined as "different species represent different gene pools, which are goups of interbreeding or potentially interbreeding individuals that do not exchange genes with other such groups." D Futuyma Evolutionary Biology pg 27. Notice how you have changed "do not exchange genes" to "flow of genetic material ... is small". You have warped the terminology to suit your own ends.

 

Bombus, the history in the last 400 years have conclusively demonstrated that humans from all races can mate and produce viable offspring. There are none of "even if the two groups were to be placed together they would not interbreed to any great extent". Humans of different the different "races" interbreed quite freely to a great extent. You only have to look at the statistics on how many "Negroids" in the US have "Caucasian" genes. The Caucasian slaveholders interbred with their Negroid slaves "to a great extent".

[bombus]

They don't fit the definition of sub-species.

 

Futuyma's Evolutionary Biology:

"Subspecies: A taxonomic term for populations of a species that are distinguishable by one or more characteristics, and are given a subspecific name (e.g. the spuspecies of the rat snake Elaphe obsoleta; se Figure 21 in Chapter 9). In zoology, subpecies have different (allopatric or parapatric) geographical distributions, so are equivalent to "geographic races;" in botany, they may be sympatric forms. No criteria specify how different populations should be to warrent designation as subspecies, so some systematists have argued that the practice of naming subspecies should be abandoned." pg 450

 

According to the above, I'd say they do - or at least, the 'races' we have today developed from three (four?) sub-species.

 

"Race: A vague, meaningless term, sometimes equivalent to subspecies and sometimes to polymorphic genetic forms within a population."

 

Agreed.

 

None of the "races" you mention have been given sub-species names.

Agreed, but that doesn't mean a thing. E.g., Humans belong the Genus Homo, but Apes are classed as Pongo, for no specific reason apart from the fact that they are not human!

 

The "races" you named are too large to develop into different species. And, of course, with human transportation NONE of them are isolated.

 

Agreed, but not really on topic. The question was whether different races could develop into different species if isolated. It only takes some members each race (however one defines them) to be isolated for new species to emerge.

 

There is no precise definition of species, but that doesn't mean the term is fuzzy. It simply means that evolution is true: since species change from one to another gradually over generations, any definition of species is always going to be able to find a population in that transition.

 

Agreed.

 

"

Races" is a meaningless term, as Futuyma (and the Scientific American article) note and demonstrate. "Races" are not the same as biological populations.

 

Agreed.

 

But the variation is still very low compared to other species. The whole variation within the entire human species is less than within one population of chimps in western Africa!

 

Not according to the Radio 4 program I recently listened to with the guy who mapped the human genome. Anyway, genetic difference is not necessarily relevant.

 

Humans can be divided into populations. See my post. But "races" are not those populations! Each "race" as you defined it consists of dozens/hundreds of populations.

 

Yes, but that doesn't affect the argument.

 

What's your source, Bombus? This might be a way to define populations. It is not a way to define "species". It is closest to the phenetic species concept: which is based on morphological/physiological differences. However, "a particular ritual breeding behavior" nor only "number or primary wing feathers" can be criteria, because number of primary wing feathers can vary from individual to individual.

 

The quoted source was Wiki, but I was taught it in University. Basically, although human races are not divided into subspecies, if the 'rules' were applied to humans one could quite correctly divide us up into three or four subspecies.

 

The phenetic species concept was the one used in Darwin's time and is still applied to fossils. It is NOT the one used between contemporary sexually reproducing species -- such as humans.

 

Maybe, but again, doesn't really affect my argument.

 

This one doesn't exist. You are misrepresenting the biological species concept. That concept states:

 

Biological species are defined as "different species represent different gene pools, which are goups of interbreeding or potentially interbreeding individuals that do not exchange genes with other such groups." D Futuyma Evolutionary Biology pg 27. Notice how you have changed "do not exchange genes" to "flow of genetic material ... is small". You have warped the terminology to suit your own ends.

 

No idea what your on about there?? Are you on drugs?

 

Bombus, the history in the last 400 years have conclusively demonstrated that humans from all races can mate and produce viable offspring.

 

And?

 

There are none of "even if the two groups were to be placed together they would not interbreed to any great extent".

 

Well, now we are in the 21st Century obviously not! But humans are probably starting to merge as a single 'race'/sub-species, from three/four 'historical' ones.

 

Humans of different the different "races" interbreed quite freely to a great extent. You only have to look at the statistics on how many "Negroids" in the US have "Caucasian" genes. The Caucasian slaveholders interbred with their Negroid slaves "to a great extent".

 

Sub-species can freely breed with each other and produce fertile offspring.

[CharonY]

Let's put it like this, the species is the strongest possible taxonomic unit. However its universality is being questioned (especially in prokaryotes, though even in higher eukaryotes it is not that straight-forward anymore). As such it is unlikely that races are anything more than arbitrary distinctions with hardly any biological basis.