Defining Paleospecies

[CDarwin]

If we're going to go by Mayr's notion (which I like), there are only two valid species concepts: The Biological Species Concept, and a typological species concept. I'm going to assume everyone has a good conception of the BSG, so let me go into depth on the typological concept.

 

A typological species concept uses the phenotypic difference between two animals to define them as separate species. Obviously, all paleospecies are therefore described under a typological concept. We can't get a bunch of hominids together and see if they'll interbreed. So the question is, should paleontologists look at some specific 'diagnostic' features (known as essentialism, or sometimes confusingly enough typologism) or should they consider the weight of features by simple enumeration ('populationism') to define different species?

 

This also ties into to cladistic vs. classical taxonomy, with cladists preferring the former tack and classicists the latter.

 

I realize my analysis of the competing philosophies is probably pretty incomplete and perhaps even flawed, but I think the question is salient regardless of any philosophical or historical quibbles. Feel free to correct me of course. That's how I learn.

[foodchain]

If we're going to go by Mayr's notion (which I like), there are only two valid species concepts: The Biological Species Concept, and a typological species concept. I'm going to assume everyone has a good conception of the BSG, so let me go into depth on the typological concept.

 

A typological species concept uses the phenotypic difference between two animals to define them as separate species. Obviously, all paleospecies are therefore described under a typological concept. We can't get a bunch of hominids together and see if they'll interbreed. So the question is, should paleontologists look at some specific 'diagnostic' features (known as essentialism, or sometimes confusingly enough typologism) or should they consider the weight of features by simple enumeration ('populationism') to define different species?

 

This also ties into to cladistic vs. classical taxonomy, with cladists preferring the former tack and classicists the latter.

 

I realize my analysis of the competing philosophies is probably pretty incomplete and perhaps even flawed, but I think the question is salient regardless of any philosophical or historical quibbles. Feel free to correct me of course. That's how I learn.

 

 

I think many ways to define could be employed. For instance, what information geologically do you have at a discovery? Another is going from what we can of history what would be the phenotype plasticity for a species at any giving time, of course this would relate to population in terms of size I would think to some extent. For instance, I don’t know of many studies that trails populations of some specie in different areas looking for say a certain percentage appearance of say a phenotypic characteristic in regards to biochemistry. I don’t know how this applies directly save when exactly does a specie become separated in regards to sexual reproduction really. Its also hard to state exactly how evolution would "attack" a species. Such as what was the amount of radiation by said species vs. the period of time a particular adaptation was taken place, which I think ties back into sexual reproductive barriers with other species.

 

I tend to shy away from essentialism and nominalism, I think they are philosophical positions chiefly. Plus taxonomy does not seem to have overall as of yet a solid irrefutable method employed by all biologists, more or less it seems to be advancing regularly with research really. Binomial nomenclatures are neat, but mathematical taxonomy I think failed early on simply by lack of tools and methodology we have today. on a side note I read somewhere that humans only diverge from mice by about 300 genes, is this true?

 

The more bones the better I would suggest also, for using an entire skull is probably better then trying to compare single ribs. Unless of course we know for sure how morphogenesis in each case is attained for a unitary organism in all related fields of physical information. I think evo-devo will actually aid understanding human evolution.

[lucaspa]

If we're going to go by Mayr's notion (which I like), there are only two valid species concepts: The Biological Species Concept, and a typological species concept.

 

You do realize that there is also a Genetic Species Concept, which is used for microorganisms? This has come into play in the neandertal/sapiens debate since both mtDNA and genomic DNA has been recovered from neandertal samples.

 

So the question is, should paleontologists look at some specific 'diagnostic' features (known as essentialism, or sometimes confusingly enough typologism) or should they consider the weight of features by simple enumeration ('populationism') to define different species?

 

This also ties into to cladistic vs. classical taxonomy, with cladists preferring the former tack and classicists the latter.

 

Cladistics isn't really about assigning individual species. It's about determining a lineage and relationships between species. That is, are species cousins/siblings or is there an ancestor-descendent relationship here. That is what the debate is about in the new Erectus-Habilis paper in Nature: Is erectus the descendent of habilis or are they both sibling species from a common ancestor?

 

Hennig first proposed cladistics and argued that phylogenetic relationships and classification ABOVE the species level should NOT reflect degree of adaptation or overall similarity.

 

Of the 3 kinds of similarity -- homoplasty (convergence), similarities based on common primitive characters, or shared derived (advanced) characteristics -- only the last is going to provide the basis of making phylogenetic conclusions. Hennig's principle is that monophyletic groups are defined by shared, uniquely derived character traits. Therefore, Hennig 1) proposed a method for inferring the true branching patterns of evolutionary history and 2) presented an opinion on criteria for classification. These very different proposals are known as cladistics. Monophyletic groups are called "clades" and monophyletic groups are nested within more inclusive monophyletic groups.

 

Most systematists accept the method of inferring phylogeny, but not all subscribe to the cladistic philosophy of classification.

 

So, CDarwin, you seem to have mixed the two responses: accepting the methodology and accepting the philosophy of classification.

 

There are limits to cladistics:

1. Scoring characters. How many independent characters are there? Second, determining which character states are homologous can be very difficult. The computer programs available that use cladistics use discrete character states and cannot handle measurements that vary continuously. (in the Nature paper on erectus-habilis, the authors asssert that some character states are not really character states)

2. Homoplasy is very common.

3. The process of evolution may make it difficult to determine relationships. If the divergence happened long ago, finding homologous characters can be difficult. OTOH, if the species arose by adaptive radiation in a short period of time, then there was little opportunity for the ancestors of each monophyletic group to evolve distinctive derived character states shared by 2 or more taxa.

 

Now, back to determining whether individual fossils or a small group of fossils represents an individual speciation by the Morphological Species Concept.

 

When you have enough samples of a species, you can make a bell-shaped curve and track the gradual separation into 2 separate bell-shaped curves over time. This is done in the following papers:

1. Williamson, PG, Paleontological documentation of speciation in cenozoic molluscs from Turkana basin. Nature 293:437-443, 1981.

2. PR Sheldon, Parallel gradualistic evolution of Ordovician trilobites. Nature 330: 561-563, 1987.

3. Kellogg DE and Hays JD Microevolutionary patterns in Late Cenozoic Radiolara. Paleobiology 1: 150-160, 1975.

 

When you don't have enough samples to reliably do that, then the cladistics method is used. You look for different "derived" characters between the fossils. A difficulty comes in that transitional individuals are often "mosaics". That is, they have some features of the ancestor and some of the descendent. Rarely do you see in-between features.

 

Remember that cladistics is based on multiple morphological criteria. For instance, on group inferred the phylogeny of 11 species of sea urchins using 81 morphological characters! They then checked it against 380 base pairs of the gene for 28s ribosomal RNA. This provided an independent check for the morphological data. And it worked!

 

For instance, I don’t know of many studies that trails populations of some specie in different areas looking for say a certain percentage appearance of say a phenotypic characteristic in regards to biochemistry.

 

There are many such studies. It's more common to look at appearance (morphology) because sampling the biochemistry is a LOT more work. However, once amino acid sequencing of proteins became common and cheap, there were dozens of studies done looking at regional differences within the proteins.

 

I think many ways to define could be employed. For instance, what information geologically do you have at a discovery?

 

That is often included in the particular papers of a fossil. For instance, in the Nature paper on Erectus and Habilis at Turkana, much is known from other studies. Figure 3 looks at a cross section of the geology: "Figure 3 | A composite stratigraphic section of strata in the Ileret area. Components are taken from sections PNG-06A and PNG-08A of ref. 27, adding only the Chari Tuff to section PNG-06A. 40Ar/39Ar ages are from ref. 26. The placement of calvaria KNM-ER 42700 and partial maxilla KNM-ER 42703 are shown. Section in meters (m)." In the References you find:

Feibel, C. S., Brown, F. H. & McDougall, I. Stratigraphic context of fossil hominids from the Omo groups deposits: northern Turkana Basin, Kenya and Ethiopia. Am. J. Phys. Anthrop. 78, 595–622 (1989).

Manega, P. C. Geochronology, Geochemistry, and Isotopic Study of Plio-Pleistocene Hominid Sites and the Ngorongoro Volcanic Highlands in Northern Tanzania. PhD thesis, Univ. Colorado (1993).

 

Another is going from what we can of history what would be the phenotype plasticity for a species at any giving time, of course this would relate to population in terms of size I would think to some extent.

 

Phenotype plasticity doesn't affect the major characteristics of bone. Phenotype plasticity involves differences in musculature. This shows up in bone looking that the insertion sites of tendons, but doesn't affect the major parameters. I think what you meant was the diversity within the population in terms of morphology.

 

I don’t know how this applies directly save when exactly does a specie become separated in regards to sexual reproduction really. Its also hard to state exactly how evolution would "attack" a species. Such as what was the amount of radiation by said species vs. the period of time a particular adaptation was taken place, which I think ties back into sexual reproductive barriers with other species.

 

Again, there is no hard and fast line. Reproductive isolation happens over generations. The genes for hybrid fertility are known, and thus changes to those genes results in hybrid sterility. However, reproductive isolation can happen before then. The most common method is simply mate recognition: the members of one population simply don't recognize the members of another population as mates.

 

The Morphological Species Concept simply realizes that with enough morphological change, reproductive isolation will have occurred. The MSC is actually very conservative. It is likely to underestimate the number of species. In one famous example of snails, the shell of 2 different species are identical and, if all we had were the fossil shells, the MSC would say they are the same species. But one species is hermaphroditic and the other uses normal sexual reproduction. Two separate species.

 

Plus taxonomy does not seem to have overall as of yet a solid irrefutable method employed by all biologists, more or less it seems to be advancing regularly with research really. Binomial nomenclatures are neat, but mathematical taxonomy I think failed early on simply by lack of tools and methodology we have today.

 

on a side note I read somewhere that humans only diverge from mice by about 300 genes, is this true?

 

Define what you mean by "genes". Do you mean that humans have 300 genes that mice don't have, or do you mean that humans only have 300 genes with different alleles from mice? On the basis of alleles (forms of genes), we differ from mice by far more than 300 genes.

[CDarwin]

You do realize that there is also a Genetic Species Concept, which is used for microorganisms? This has come into play in the neandertal/sapiens debate since both mtDNA and genomic DNA has been recovered from neandertal samples.[/Quote]

 

Mmm, Mayr would call that a method for defining species within a typological species concept. You're basis is still degree of difference, though not phenotypic differnce as I said and probably shouldn't have.

 

Of course he was talking about sexually reproducing "true" speices, not the asexual lot.

 

As for the rest of your post: I see what you're saying. That's about all I can say there, it seems.

[lucaspa]

Mmm, Mayr would call that a method for defining species within a typological species concept. You're basis is still degree of difference, though not phenotypic differnce as I said and probably shouldn't have.

 

I'm not sure what work of Mayr you are reading, but you did get him accurately!

 

"The equivalent of biological species of sexually reproducing organisms does not exist in asexual organisms. ... In such cases, one is forced to fall back on the typological species definition and recognize these species, so-called agamospecies, by the degree of difference." Mayr, What Evolution IS, pg 173.

 

Of course, this is because Mayr has already boxed himself in on page 167 by saying "Are these new species concepts legitimate? To summarize my conclusion, they are not. ... Up to now, only two qualifying concepts have been proposed: ... "

 

So, having limited himself to only 2 concepts, Mayr has no choice but to put prokaryotes in one of them!

 

You can argue both that genetics are a variant of "typology" and a "different concept". After all, the genetic concept does use "clumps" of different alleles and thus defines a "population" of "clades", which in turn is a modification of the biological species concept. (according to Mayr pg 167) So you can persuasively argue that the Genetics Species Concept (which is applied only to asexually reproducing organisms) is a third species concept that is a hybrid between typology and biological species.

 

As for the rest of your post: I see what you're saying. That's about all I can say there, it seems.

 

Hmmm. I'm not sure what that means. It seems to me that you are not convinced, but cannot find a way to disagree. If it helps (or hurts), I got the discussion from Futuyma's Evolutionary Biology. I really think you would both enjoy that book and find it immensely useful. It will provide the knowledge base you need to ask all the questions you have (and many you haven't yet thought about) on evolution.

[CDarwin]

Hmmm. I'm not sure what that means. It seems to me that you are not convinced, but cannot find a way to disagree. If it helps (or hurts), I got the discussion from Futuyma's Evolutionary Biology. I really think you would both enjoy that book and find it immensely useful. It will provide the knowledge base you need to ask all the questions you have (and many you haven't yet thought about) on evolution.

 

Oh, I just meant that I had nothing to add. I believe I might like to read that book someday when I don't have an English class to battle with.