New Definition of 'Species' Could Aid Species Identification

PhysOrg.com ^ | 23 August 2006 | Staff

[PatrickHenry]

Scientists at Texas Tech University argue that defining mammalian species based on genetics will result in the recognition of many more species than previously thought present. This has profound implications for our knowledge of biodiversity and issues based on it, such as conservation, ecology, and understanding evolution. Their study is published in the latest Journal of Mammalogy.

The classical definition of species was proposed by Ernst Mayr in 1942, defining it as reproductively isolated groups of organisms. According to this study, the problem with applying this concept is that it is hard to observe mating and to know whether there is interbreeding between populations and thus creation of hybrid species. Traditionally, species have been recognized based on physical characteristics, although it has been assumed that species differences are inherited and thereby reflect genetic differences.

Study researchers Robert Baker and Robert Bradley define “species” based on genetic data. The new definition distinguishes species that are genetically isolated from one another. Baker and Bradley’s genetic species concept also differs from the phylogenetic species concept proposed by Joel Cracraft in 1989 by emphasizing genetic isolation and protection of the integrity of the gene pool.

New molecular techniques for sequencing genes provide far greater resolution than was previously available. They also allow researchers to quantify problems in understanding the process of speciation. Using genetic data, it is now possible to distinguish species that are morphologically similar — those known as cryptic species. It is also possible to identify species that hybridize but have gene pools that are protected from one another.

The result of using genetic data is that species can be identified that cannot be distinguished using other methods. Baker and Bradley point out that this means there are doubtless many more species than previously thought. They hypothesize that there are 2,000 more mammalian species than are currently recognized.

According to the authors, this means that we will need to rethink the nature of speciation in mammals, barriers that evolve to produce genetic isolation between species, and how diverse mammals are, as well as other species-based issues such as those relating to conservation and zoonoses, communicable diseases from animals to humans.

To read the entire study, click here. SPECIATION IN MAMMALS AND THE GENETIC SPECIES CONCEPT (PDF file, 20 pages long)

[js1138]

Criterion or circumstances	Biological Species Concept	Morphological Species Concept	Phylogenetic Species Concept	Genetic Species Concept
1. Diagnosably distinct populations recognized as a separate species	Sometimes	Most times	Always	Yes, if genetic distances are typical of sister species
2. Species unit often includes diagnosable allopatric populations	Yes, subspecies concept widely accepted	Yes, subspecies concept widely accepted	Never, subspecies concept not relevant	Yes, subspecies relevant but defined genetically
3. Species represent terminal taxa that can be used in phylogenetic and biogeographicalanalysis	Sometimes	Sometimes	Always	Always
4. Inherently provides accurate historical information	No	No	Yes	Yes
5. Gene flow among species	Rarely, if ever	Not directly	Sometimes	Acceptable
6. Reproductively isolated populations recognized as separate species	Always	Always	Always	Always
7. Extensively hybridizing, diagnosably distinct populations important for establishing species status	Rarely	Rarely	Almost always	Yes, if integrities of the 2 gene pools are protected
8. ‘‘Potential’’ interbreeding of allopatric populations important for establishing species status	Yes	Yes, estimated by morphological divergence	Never	Yes, estimated from genetic distance of nuclear and mitochondrial DNA
9. Delimitation of areas of endemism	Coarse, less precise	Coarse, less precise	Fine, more precise	Fine, more precise
10. Inter-and intraspecific genetic biodiversity documented	Sometimes	Rarely	Sometimes	Always, with adequate sampling
11. Estimate of time of origin	No	Sometimes, if fossils available	Sometimes	Always
12. Provides foundation for Barcode Initiative	No	Yes, through voucher specimens/binomial names	Sometimes	Always
13. Resolves geographic limits for cryptic species	No	No	Sometimes, if gene trees are used	Always, with adequate sampling
14. Can be influenced by local environmental conditions	Yes, if morphology database used	Yes	Yes, if morphologic database used	No

[doc30]

Very intersting article. The definition of a species is a contentious matter. Personally, I am very puzzles why we must find a sharp deliniation between types of organisms. Why do they need to be sorted into discrete 'boxes.' With genetic information, variation could be viewed as a continuum and our assignment of a 'species' would be like placing a sharp barrier in the middle of such a continuum. It would be like looking at a rainbow of colors and trying to draw a line where blue ends and green begins. No one would ever agree to any given placement.

Some type of statistical variation based on genetic information would be best. Some form of cluster analysis would be a great way to go, but it would likely entail multidimensional space to do it well and that is not something easily visualized or intuitively comprehensible to someone not familiar with things like Mahalanobis distance measurements. But it would be more quantitative and would allow biologists to determine the classification in which an individual would most probably be a member of and groups that an individual would most probably not be a member of.

Such statistical analysis is used routinely in spectral analysis where samples are mathematically segregated based on spectral response. A spectrum, regardless of type, is a linear array. A genome could also be represented by a linear array so the statisitics would be directly analogous.

[From many - one.]

You would not believe what DNA studies are doing to fungal taxonomy. Looks are turning out to be near irrelevant... even families are being messed up.

[furball4paws]

"I am very puzzles why we must find a sharp deliniation between types of organisms."

Simple - humans love to pigeonhole things and if something doesn't exactly fit, they make it fit.

Humans don't like "messy" things, so we force a sometimes artificial format onto things that don't necessarily work by the same rules.

The microbial world is full of things that "don't fit" - most of us gave up long ago trying to make them fit and have retreated to "this belongs somewhere between X and Y, and until we know more it's not worth the time and effort to try to clearly define where it is".

[furball4paws]

Fungal taxonomy has always been a mess. The very large group of Fungi Imperfecti was nothing but a catch-all for anything that couldn't be more clearly defined. While, periodically, some member of the Fungi Imperfecti would get moved out into the Ascomycetes or Basidiomycetes, more were being added at the same time.

20 years from now we might have a good picture of the whole fungal mess.

[PatrickHenry]

Personally, I am very puzzles why we must find a sharp deliniation between types of organisms.

If it weren't for extinctions, the inter-relatedness of all species would be as immediately obvious as it is among all varieties of dogs.

[doc30]

The microbial world is full of things that "don't fit" - most of us gave up long ago trying to make them fit and have retreated to "this belongs somewhere between X and Y, and until we know more it's not worth the time and effort to try to clearly define where it is".

What you say here is very close to what I was thinking. What I propose is a quantitative measure of how far between X and Y is the thing? But not only how close to X and Y, but to all organisms. Most organisms would be so distant, they would be like the background of stars compared to observed planets from Earth. But there would not only be a 'distance' relationship, but also a 'directional' relationship.

[tongass kid]

morphology is/was the hallmark of comparative vertebrate anatomy. It is still taught at most high schools, even though we know now via genetics that similar morphology may have vastly dissimilar gene bases.

[RightWhale]

Changing definitions seems to be the order of the day. Planets were redefined, twice in one week because of the newly discovered Kuiper belt objects, one larger than Pluto. They will probably redefine race as well.

[PatrickHenry]

Changing definitions seems to be the order of the day.

Darwin was aware of the fuzziness of the definition, and found that it made no sense in trying to decide anything regarding humans:

We might as well attempt without any definition to decide whether a certain number of houses should be called a village, town, or city. We have a practical illustration of the difficulty in the never-ending doubts whether many closely-allied mammals, birds, insects, and plants, which represent each other respectively in North America and Europe, should be ranked as species or geographical races; and the like holds true of the productions of many islands situated at some little distance from the nearest continent.

Source: Darwin, The descent of man: CHAPTER VII. ON THE RACES OF MAN.

[RightWhale]

Philosophers have the approach of trying to understand other philosophers as best as possible first, then moving to other interpretations. Even so, one philospher might call Plato an Idealist and Aristotle a Realist while another might point to Aristotle as the Idealist. What one means by saying Species is probably never going to be exactly what someone else means even if they have an international conference and vote.

[ModelBreaker]

This strikes me as politically driven because this definition will make it far easier to find "isolated" species and stop development than the current one.

It is not really a scientific issue. It is semantic. Currently, the word "species" is generally used to refer to critters who can mate together and produce viable offspring. That is a useful concept that should have a label.

The attempted redefinition refers to a different concept related to genetic diversity. Thus, they would change the meaning of a label instead of just coming up with a new label for the new concept. (One could just as well use something like "genetic diversity segmentation" to refer to that same concept.)

But giving this new concept a new name would not have the desired political results. Therefore, this is politics masquerading as science.

[Alamo-Girl]

Thanks for the ping!

[js1138]

Does race have a definition?

[RightWhale]

It has at least one depending who is talking. The Bureau of the Census has quite a list, but anthropologists tend to identify three depending mostly on hair type.

[js1138]

So there is no objective definition of race, just lines drawn by people with agendas.

[r9etb]

Let's see. Twenty pages of fairly dense science writing. I expect the first troll to denounce it within three minutes. Now back to reading...

Pot ... kettle ... black....

[js1138]

If it weren't for extinctions, the inter-relatedness of all species would be as immediately obvious as it is among all varieties of dogs.

If it weren't for extinctions, the world would be so crowded we'd all be dead.

[RightWhale]

Well, politics and other biases seem to be big on some terms that cause heat just by saying.

[js1138]

Well, damn, I wonder why the word race is politically charged.

Yesterday someone tried to paint Darwin as a racist because the word appears in the subtitle of "Origins."