What is a species anyway?

[daneeka]

A species concept is basically a hypothisis that's used to demarcate seemingly distinct living populations from one another. In other words it's a definition; and it's purpose is to allow for distinct living populations to be defined objectivily (i.e. free from value judgements) by testing such populations against various opperational criteria. This is obviously of importance to taxonomy but also is a significant tool for ecology and biology.

 

I was just wondering what people's thoughts were on what defines a species. There are over 20 concepts/hypotheses out there at present - as of yet no universally accepted concept has been developed - and each concept can yeild different results. Do species even exist?

[CharonY]

The most accepted definition is probably "an interbreeding population that are reproductively isolated from other populations" as coined by Ernst Mayr I think.

In general this means that the transfer of genetic material is the barrier that distinguishes one species from another. This works reasonably well with complex organisms, but does not apply to asexually proliferating organisms. In addition, horizontal gene transfer can further muddy this concept.

Therefore for bacteria no simple species definition exists. At the moment most have a pragmatic approach based on 16s rRNA comparison. Bacteria are often grouped depending on the similarity in this sequence.

[jeskill]

A species concept is basically a hypothisis that's used to demarcate seemingly distinct living populations from one another. In other words it's a definition; and it's purpose is to allow for distinct living populations to be defined objectivily (i.e. free from value judgements) by testing such populations against various opperational criteria. This is obviously of importance to taxonomy but also is a significant tool for ecology and biology.

 

I was just wondering what people's thoughts were on what defines a species. There are over 20 concepts/hypotheses out there at present - as of yet no universally accepted concept has been developed - and each concept can yeild different results. Do species even exist?

 

 

Taxonomy isn't objective -- there's an underlying philosophy that guides the ranking and naming of taxa. Currently, it's based on geneological relationships, but the original species definitions were based on essentialistic philosophy (unchanging organisms with essential characteristics). To Linnaeus, genera were distinguished by sexual characteristics as well as other "general" traits such as roots and stems, etc. "Species" were simply specific groups of genera that shared specific characteristics.

 

To answer your question, I think that species aren't real entities and they never will be. They are merely groupings that we've defined in order to make scientific research more efficient and effective.

 

I do think that we need a universal concept for species, but I don't think that'll happen in the near future, if at all. There's no really good characteristic that can be used to define all extant and extinct species.

[jeskill]

double post.

[FreeThinker]

I would say, no. Species do not exists and we label animals adapted to different conditions for the sake of convinience.

[daneeka]

Taxonomy isn't objective -- there's an underlying philosophy that guides the ranking and naming of taxa. Currently' date=' it's based on geneological relationships, but the original species definitions were based on essentialistic philosophy (unchanging organisms with essential characteristics). To Linnaeus, genera were distinguished by sexual characteristics as well as other "general" traits such as roots and stems, etc. "Species" were simply specific groups of genera that shared specific characteristics.

 

To answer your question, I think that species aren't real entities and they never will be. They are merely groupings that we've defined in order to make scientific research more efficient and effective.

 

I do think that we need a universal concept for species, but I don't think that'll happen in the near future, if at all. There's no really good characteristic that can be used to define all extant and extinct species.[/quote']

 

 

Hmmm you're right about taxonomy not being objective. However, it's my impression that, in general, it is considered to be objective because the criteria used to form taxanomic inventories is essentailly opperational - the aim to is remove the need to make value judgements. I think the subjecive aspect of naming species, and hence taxonomy, is not really because of the 'underlying philosophy' per se but rather innapropriate application of particular species concepts; and this occurs because we really have no idea on what a species actually is.

 

Yeah I agree that species aren't real - they are basically hypothetical from what I understand

[jeskill]

Hmmm you're right about taxonomy not being objective. However' date=' it's my impression that, in general, it is considered to be objective because the criteria used to form taxanomic inventories is essentailly opperational - the aim to is remove the need to make value judgements. I think the subjecive aspect of naming species, and hence taxonomy, is not really because of the 'underlying philosophy' per se but rather innapropriate application of particular species concepts; and this occurs because we really have no idea on what a species actually is.

 

Yeah I agree that species aren't real - they are basically hypothetical from what I understand[/quote']

 

What exactly do you mean by the statement, "the criteria used to form taxonomic inventories is essentially operational -- the aim is to remove the need to make value judgements"? What is the criteria?

 

I disagree that it's objective -- taxonomists may strive for objectivity but they haven't gotten there yet.

 

I mean, we have no objective method used to group species, and no objective method that could be used to define species in all Kingdoms. We have no objective concept that could be used to define any of the ranks (genus, class, etc). We have this concept of monophyletic groups that I guess you could call objective, but that's not even real -- a ranked classification system is inherently paraphyletic (See Brummitt 2002 in Taxon: How to chop a tree).

[bascule]

Taxonomy isn't objective -- there's an underlying philosophy that guides the ranking and naming of taxa. Currently, it's based on geneological relationships, but the original species definitions were based on essentialistic philosophy (unchanging organisms with essential characteristics).

 

Which fails terribly thanks to the continuist nature of life. Ring species are probably the best example.

[silkworm]

When I say "species" to any biologist or anthropologist I talk to we all know that means members of the same species are is "reproductive isolates."

 

Reproductive isolates can breed with one another, and do so in nature, and produce viable offspring, that can themselves have offspring.

 

The most common way to illustrate this is with a horse and a donkey. The designations of female and male are ommitted in my explaination as it's a given and for brevity.

 

Two horses can have another horse that can also have a horse with another horse. These horses are all the same species. The same is true with donkeys.

 

But also, a horse and donkey can have a wild night and produce a mule. The mule can not have offspring of its own, so horses and donkeys are not the same species.

 

A mule is just a mule.

 

Hope that helps.

[jeskill]

When I say "species" to any biologist or anthropologist I talk to we all know that means members of the same species are is "reproductive isolates."

 

Reproductive isolates can breed with one another' date=' and do so in nature, and produce viable offspring, that can themselves have offspring.

 

The most common way to illustrate this is with a horse and a donkey. The designations of female and male are ommitted in my explaination as it's a given and for brevity.

 

Two horses can have another horse that can also have a horse with another horse. These horses are all the same species. The same is true with donkeys.

 

But also, a horse and donkey can have a wild night and produce a mule. The mule can not have offspring of its own, so horses and donkeys are not the same species.

 

A mule is just a mule.

 

Hope that helps.[/quote']

 

 

Have you tried that with biologists who study protozoa, bacteria or fungi? Bet you'll get a different response.

[jeskill]

Which fails terribly thanks to the continuist nature of life. Ring species are probably the best example.

 

Ring species? I must admit I've never heard that term before.

[Prime-Evil]

I understand a species has to be somewhat arbitrary.

Nature does not know or care what a species is.

[ecoli]

The most accepted definition is probably "an interbreeding population that are reproductively isolated from other populations" as coined by Ernst Mayr I think.

In general this means that the transfer of genetic material is the barrier that distinguishes one species from another. This works reasonably well with complex organisms' date=' but does not apply to asexually proliferating organisms. [/quote']

 

That's not even true. http://en.wikipedia.org/wiki/Liger

 

and http://en.wikipedia.org/wiki/Mule

[daneeka]

What exactly do you mean by the statement, "the criteria used to form taxonomic inventories is essentially operational -- the aim is to remove the need to make value judgements"? What is the criteria?

 

From what I understand a species concept is essentially a testable hypothesis; that is, the concept defines a species by describing certain attributes that, when tested against, provides the worker with an indication as to where it fits within taxonomic rankings. So the aim is to remove, as much as possible, subjective decisions from the worker. The criteria is hence 'operational' because it describes series of actions for achieving a result.

 

I mean, we have no objective method used to group species, and no objective method that could be used to define species in all Kingdoms. We have no objective concept that could be used to define any of the ranks (genus, class, etc). We have this concept of monophyletic groups that I guess you could call objective, but that's not even real -- a ranked classification system is inherently paraphyletic (See Brummitt 2002 in Taxon: How to chop a tree).

 

I agree with you here; there is really no way to make the naming of species objective with a singular species concept. Life is so variable and ambiguous that it is conceptually impossible. I think that it is possible to make it objective though; we simply need to apply species concepts (there are over 20 of them remember) such that the operational criteria can actually be used. The biological species concept (which is what most people have described in responce to this post), for example, is hopeless when used to demarcate asexual or selfing populations. Why not restrict this concept, which is at the end of the day a nice concept, to taxa for which it is appropriate and thus use its operational

criteria. I think the only reason why taxonomy is subjective is because we assume that a universal concept is possible; and because of the ad hoc usage of species concepts.

 

Maybe instead of trying to develop a universal species concept, we should instead try to develop a universal means to apply the myriad concepts we already have - all of which a perfectly valid (although some can be synonymised) when applied to the appropriate life forms.

[jeskill]

From what I understand a species concept is essentially a testable hypothesis; that is' date=' the concept defines a species by describing certain attributes that, when tested against, provides the worker with an indication as to where it fits within taxonomic rankings. So the aim is to remove, as much as possible, subjective decisions from the worker. The criteria is hence 'operational' because it describes series of actions for achieving a result.

 

 

I agree with you here; there is really no way to make the naming of species objective with a singular species concept. Life is so variable and ambiguous that it is conceptually impossible. I think that it is possible to make it objective though; we simply need to apply species concepts (there are over 20 of them remember) such that the operational criteria can actually be used. The biological species concept (which is what most people have described in responce to this post), for example, is hopeless when used to demarcate asexual or selfing populations. Why not restrict this concept, which is at the end of the day a nice concept, to taxa for which it is appropriate and thus use its operational

criteria. I think the only reason why taxonomy is subjective is because we assume that a universal concept is possible; and because of the ad hoc usage of species concepts.

 

Maybe instead of trying to develop a universal species concept, we should instead try to develop a universal means to apply the myriad concepts we already have - all of which a perfectly valid (although some can be synonymised) when applied to the appropriate life forms.[/quote']

 

That's an interesting idea. Can you give an example of a universal method of applying species concepts?

[daneeka]

That's an interesting idea. Can you give an example of a universal method of applying species concepts?

 

Not really, although some relativly recent literature has brushed on the idea. A paper by Mayden (2002) (On biological species, species concepts and individualisation in the natural world. FISH and FISHERIES 3: 171-196) I read a few months ago proposed a pretty good way of approaching the problem. He points out that the evolutionary species concept is perhaps the only species concept that is non-operational; that is, all other species concepts provide the researcher with clearly outlined criteria for when something is or is not a species. The evolutionary species concept is therefore one of the few concepts that can actually encompass all forms of organisms. From memory he essentially explains that species concepts should be used on a hierarchical basis with all species concepts acting a surrogates for the evolutionary species concept. In other words operational species concepts should be used where they are applicable for a certain kind of species but should come secondary, and work alongside, the evolutionary species concept.

 

Don't ask me to clarify this because I read it a while back and still don't really fully understand what he was going on about

 

But I can't really come up with an example of a universal method for applying species concepts actually being put into practice.

[bascule]

Ring species? I must admit I've never heard that term before.

 

They form continuous mating groups spread throughout a region, but wrap around in such a way that when the two groups on the extreme ends of the ring are exposed to each other, they choose not to reproduce with each other because the differences between them are too extreme.

 

For example, Ensatina eschscholtzi:

 

http://www.pbs.org/wgbh/evolution/library/05/2/l_052_05.html

 

[jeskill]

They form continuous mating groups spread throughout a region' date=' but wrap around in such a way that when the two groups on the extreme ends of the ring are exposed to each other, they choose not to reproduce with each other because the differences between them are too extreme.

 

[/quote']

 

Oh yeah. I remember those little suckers from 3rd year Evo. I don't think we called them ring species, although the term does fit.

 

To Daneeka:

So basically, as long as the species is an independent lineage, then it can be defined in any way that's applicable?

 

Sounds OK, but I don't know if I believe that the evolutionary species concept can apply to every single situation. For example, if species are supposed to be distinct evolutionary lineages, how do you deal with horizontal gene transfer or reticulate evolution?

[daneeka]

So basically' date=' as long as the species is an independent lineage, then it can be defined in any way that's applicable?

 

Sounds OK, but I don't know if I believe that the evolutionary species concept can apply to every single situation. For example, if species are supposed to be distinct evolutionary lineages, how do you deal with horizontal gene transfer or reticulate evolution?[/quote']

 

Hmmm good point.....maybe it'd be best to class various forms of bacteria as superspecies and be done with it (man i think I'm gonna get burnt for saying that); like use a species concept suitable for classifying organisms capable of horizontal gene transfere (can't think of one off the top of my head) and develop some sort of phylogeny from that and class this a superspecies; and yep this would be a judgement call.

[CharonY]

ecoli, well hybrids are not considered as species as they usually do not reproduce and are therefore usually ignored. They pose a problem in plant taxonomy, though.

daneeka, actually there was a proposition to remove species tags from bacteria and possibly only refer to ecotypes. But for microbiologists this is of course out of question. Assinging species even though the definition is fuzzy is simply too useful to give up.

[jeskill]

ecoli' date=' well hybrids are not considered as species as they usually do not reproduce and are therefore usually ignored. They pose a problem in plant taxonomy, though.

daneeka, actually there was a proposition to remove species tags from bacteria and possibly only refer to ecotypes. But for microbiologists this is of course out of question. Assinging species even though the definition is fuzzy is simply too useful to give up.[/quote']

 

Hybridization occurs fairly frequently in more than one Kingdom, so it does pose a problem. Especially since speciation can often result from hybridization events. The reason it's important is because we're talking about a universal species concept (that can be applied to all organisms), not one that just applies to animals.

 

But I don't think that ecotype suggestion is a bad idea. I mean, if evolution is the change in the proportion of alleles in a population, and an ecotype is a defined population, then an ecotype may be a good measure of an evolutionary lineage that doesn't involve making up a new species definition for bacteria.

 

Daneeka, what do you mean by superspecies? A rank above species?

[daneeka]

A superspecies essentially involves the grouping of closly related allopatric populations (I'm not sure why it is not considered for sympatric populations - perhaps because some people refute that speciation cannot occur within sympatry); so yeah, a rank above species. I think that for some lifeforms this may be appropriate and may, to a large extent, rule out the problem of species synonymy within taxonomy..especially with bacteria and their ilk.

[ecoli]

ecoli, well hybrids are not considered as species as they usually do not reproduce and are therefore usually ignored. They pose a problem in plant taxonomy, though.

 

There have been a few Mules that have reproduced (one recently in the news) the last one in 1986. As far as I know, Ligers can reproduce.

[CharonY]

Well I suppose it can be argued that a Liger is not a species as tigers and lions are allopatric species. As far as I know that do not have a species name assigned because they of course do not exist in natural habitats.

So in theory they do have an isolation of gene pools.

But in general I do agree. Species concepts are not clear cut functions and especially with regards to protists and especially bacteria more a matter of agreement. With regards to hybridization, if its effects were significant I would argue that with time the gene pools should eventually merge. This is outside of my expertise but to my knowledge there were no reported instance of large-scale mergers of sympatric populations.

With regards to bacteria, this would probably make sense if we got a more thorough view on ecotypes. But our knowledge to bacterial diversity is atm rather sketchy at best.

[Prime-Evil]

All generalizations fail, but are still useful.