About Shells 
Shell anatomy, terminology, classification, nomenclature, etc.

 Species, subspecies and forms, How do we measure speciation?

Feb 28, 2013     A Conch-L discussion.

2/19/2013 Marlo wrote:

"Not to stir up a hornet’s nest, but what’re current opinions? Is H. salleana a separate species from H. cinerea, a subspecies, or simply a geographic morph (a form)? If they interbreed (as Harry suggests), can they be separate species?"

John Tucker:

"Just because two taxa interbreed does not necessarily mean that they are not different species. It certainly makes it difficult to apply the biological species concept. However, the number of species that do interbreed from time to time is legion. Competent taxonomists in all fields can cite examples. Many toads in the United States that are considered separate species by herpetologists interbreed in areas of disturbed habitats which is nearly everywhere now. 

I understand this does not help with the Hastula problem. But it might suggest that it is going to be a difficult problem to get an absolute answer to."

David Campbell:

"The WoRMS database cites Terryn Y. (2007). Terebridae: A Collectors Guide. Conchbooks & NaturalArt. as an authority for recognizing both Impages and Hastula and for putting both of those in Impages as valid species.  However, Mol Phylogenet Evol. 2012 Jul;64(1):21-44. doi:
10.1016/j.ympev.2012.03.001. Epub 2012 Mar 14.  Macroevolution of venom apparatus innovations in auger snails (Gastropoda; Conoidea; Terebridae).  Castelin M, Puillandre N, Kantor YI, Modica MV, Terryn Y, Cruaud C, Bouchet P, Holford M. includes in its abstract the statement: "The non-monophyly of most terebrid genera analyzed indicates that the current genus-level classification of the group is plagued with homoplasy and requires further taxonomic investigations."

It doesn't seem to have analyzed salleana."

Harry Lee:

"I address the zoogeography at (not operative) and a later draft appears in Lee (2009: 125).

The distinction between the two taxa was first elucidated by Joe Morrison in Texas Conchologist, and, as was recently pointed out by a member of this forum, this work is available on-line,

 

In the late 1970's I agreed with Joe on all counts except the NE and N Central Florida shells are clearly referable to H. salleana. Soon thereafter, while examining many dozen lots in my collection and the FLMNH, I found forms intermediate between the two in SE Florida and (Caribbean) Nicaragua and wrote this up for the Shell-O-Gram. Regrettably that issue in not available on-line. I concluded that the two were allopatric and closely-related, certainly meeting criteria for subspecies: a Carolinian one (Jax to SE FL, reconstituting itself in W Florida and inhabiting almost all the continental shoreline of the Gulf of Mexico) and a Caribbean one, extending well into Brasil. The zoogeographic interfaces provided evidence of incomplete reproductive isolation.

As for generic assignment, I suppose it's essential to first decide which of the two candidates more closely resembles our gray augers. Although there are important non-conchological characters, e.g., the proboscis and radula, I think shell characters might be sufficient.

Hastula H. and A. Adams, 1853 is based on (type species) Buccinum strigilatum Linnaeus, 1758: 741  by the subsequent designation of Cossmann (1896: 53).  Impages E.A. Smith, 1873 is based on (type species) Terebra caerulescens Lamarck, 1822 [= Buccinum hecticum Linnaeus, 1758: 741, same page as B. strigilatum above!]

"Terebra" cinerea and "T." salleana look more like Impages hectica than they do Hastula strigilata, so we must now decide if Impages is subordinate to Hastula or does it merit full generic recognition. All of a sudden I have a sense of déjà-vu; as I recall, this issue that has been bandied about for some time. I think any decision in this regard must be informed by a more holistic (read anterior digestive tract) data matrix. This cop-out appears in the terebrid literature repeatedly.

Marlo, I know you like definitive and "accepted" systematics, so maybe you'll conform to the WoRMS/Terryn fiat, but there certainly isn't any evidence that the later work utilized any criteria other than conchological for its generic parsing. Thus there is good reason to hang on to the earlier Hastula for the topical species-level taxa.

As the Question-man used to say: 'my two cents worth.' "

2/20/13 David Campbell:

"It's worth bearing in mind that Terryn is responsible for the WoRMS classification using Terryn's book as the reference, so the two together constitute one opinion rather than an independent support for its use.

The DNA paper definitely suggests that more work is needed; of course, that's true for almost any organism. Whether a particular group deserves to have a genus name or not is ultimately a subjective decision. Analyses can will tell whether a particular group seems coherent or not, and we can say that "if A is considered a genus, B is a similar group and probably should be treated in a similar manner". But there's no magic amount of difference in DNA, anatomy, or shell that must or must not be a genus."

Marlo:

" I framed this string in terms of controversy and got little response. Where’re the hornets? So, here’s a little stirring in hopes I’ll learn what it is I misunderstand. 

First I want to just address the species question. Are they separate or the same? If there’s one lesson I’ve gotten from Conch-L and the literature, it’s that we all get to decide for ourselves once we’ve considered others’ opinions. So, with this in mind, first let me explain my prejudices. There can be tremendous physical differences within species (color, shape, size, sculpture, radula, etc.) driven by many factors. If two are quite similar in general appearance, habitat, behavior, diet, etc. then they are the same species until proven not to be so by an objective and widely accepted measurement (many offered, many failed, and we are not quite there yet). I like the definition “they can mate and produce viable offspring.” But, that’s almost impossible to test in a practical sense. So, being one who does not trust human observation, and prefers reliance on technology, for me only DNA analysis based upon an established common convention (if we ever get one) should define speciation. For me, there are no “subspecies*” in a biological sense. It’s only a poor nomenclatural convention. By DNA analysis, once the boundary is crossed, speciation has occurred. Either inside or outside the measurement boundary. And, all that are within the boundary are one species (lump, lump, lump). A species can have allopatric as well as polyphenic or polymorphic forms, but they are all one species. I reject the “subspecies” terminology because it infers a not-within-the-species-breeding-group and/or a lesser standing or rank, rather than being the same, but displaying different characters or behaviors. Strictly speaking, the first described “species” name is the “nominate subspecies” and the later identified forms are the “autonymous subspecies” and all are equal “subspecies” within the species rank, able to breed if mixed in the same population. It is only the accident of the timing of publication of names that distinguishes nominate and autonymous subspecies nomenclature within a species.

So, since Harry accepts H. c. salleana as a “subspecies,” then by my definition it is an allopatric form of H. cinerea until DNA analysis demonstrates otherwise. So, it comes down to nomenclature. I prefer Hastula cinerea form salleana.


[Note: I am not saying DNA analysis is the only measurement. A group of characters (protoconch, sculpture, radula, color, habitat, reproduction, etc.) taken together can, as a whole, show that there is sufficient dissimilarity that separate species are at hand. But, a few slight differences in size, shape, degree of expression, color, etc. of a few characters are too easily explained by allopatric genetic drift, diet, habitat, sex, polymorphism, and inadequate breath of the writer’s examination of populations (or just plain biased observations) to be accepted as the basis for speciation.] 


*I know the ICZN accepts “subspecies” as a rank within the classification system. And, I can accept the utility of doing so. It is the terminology to which I object. The term “form” would be far more accurate on its face and the botanical nomenclature clearer; namely Hastula cinerea form salleana."

2/20/13 Doug Stemke:

"I am a molecular biologist myself and teach a course in Biodiversity. I find that I have to give a balanced answer to how much weight to give DNA. The classic example is the Monk flower. A single gene apparently separates a red flower from a white flower. It may in fact come down to a single base pair in the 'red gene' or even a single base pair in the control of the gene that causes the gene to be on or off. That would hardly seem to be enough difference, from a DNA level, to justify two species. But the difference in a practical sense is red flowers get pollinated by birds (hummingbirds) white flower rely on insects to pollinate them. They do not exchange DNA. Presumably each color flower will then begin to gather separate mutations and over time will ultimately diverge. But at the point at which those two organisms are no longer mating they have become separate species. DNA in this case where an algorithm is used to identify species would potentially mislead classification whereby a Biological Species concept probably is the best way to classify a species.

We have to admit, we like to classify things (as shell enthusiasts we are worst than most). Biological systems defy neatly classifying organisms. And that complication extends to questions as to when is something a genus, family, class etc. DNA does inform us what is most closely related to what unless you are in my world of microbes where organisms swap DNA all the time. Talk about a mess! 

Personally, when it comes to identification of my shell collection, I blow with the wind. Let the Conchologists mix it up and follow their lead. They may ultimately be right, they may ultimately be wrong. It is an artificial human endeavor applied to a biological systems where there are few 'Laws'. Think about it, even central 'Theories' of Biology have fallen flat. The Cell Theory (the smallest living entity is a cell, what of slime molds, Phycomycota, Streptomyces). 'One Gene, One Protein theory' It fell when we realized that different tissues transcribe genes in different tissues. So if we fundamentally have trouble holding onto theories in living systems, with a few exceptions, notably of the Theory of Evolution. Ergo in living systems it is going to be fundamentally difficult to have universal ways to speciate all organisms." 

Ed Johannes:

"It may be irritating that our endeavors to explain the "messy" biological world have fallen flat or constantly changing as new discovers are made. However, I prefer this to a world where everything is solved or most of the earths biota is described, or more likely extinct, leaving many biologists wondering what to do with their time. I love the fact that I can go out and conduct surveys in the Pacific Northwest where I discover on average 2 new freshwater or terrestrial mollusks. The possibilities that new species or ideas can still be discovered get me out of bed everyday."

2/21/13 Harry Lee:

"Great commentaries! Doug's report of a minuscule alternation of the genome (e.g., the single gene alteration of the Monk Flower petal coloration) initiating reproductive isolation of a morph has its parallel in malacology and seems quite relevant to Marlo's predicament. Chiral-reversal in certain terrestrial pulmonate snails (probably driven by a single gene) or change from planktotrophy to lecithotrophy in marine prosobranchs (Marien?) seem to be analogous to the Monk Flower story. Each of these situations is accompanied by a small morphological change (one character state), but in each instance reproductive isolation can be inferred, convincingly so in some instances, e.g. clausiliids in the Carpathians and Euhadra in Japan.

The issue of "instant speciation" may be moot, but it seems to me "the DNA" is giving us a clue to sympatric speciation. It is not unreasonable to expect acquisition of further alterations in the genome of each divergent stock to generate further morphologic and physiologic divergences and ultimately enough accrual of distinctive characters to satisfy even the most devout lumper.

Speciation is a dynamic process. It has to begin somewhere, and, by strict criteria, is not complete in that instant. That's why the concept of "subspecies," with all its arbitrary encumbrances, is a tenable construct to define a phylogenetic relationship between the initiating event, be it allopatry (the topical Hastula taxa) or a determinative clausiliid or Monk Flower mutation (sympatry), and the emergence consensus sibling species. Is this so messy?"

David Campbell:

"As with any feature of biological organisms, DNA sometimes provides nice clean distinctions between forms and sometimes doesn't. Perhaps the most consicuously genetically messy mollusks are those with hybrid polyploidy. Multiple sets of DNA, each with different histories but potentially able to get mixed and matched. Lasea, Campeloma, sphaeriids, various thiarids, freshwater Corbicula, etc. The apparent ancient lineage sorting problems cropping up in freshwater cerithioideans (and probably others) is another example. If an ancestral population has more than one version of a gene, over time isolated descendant populations are expected to lose all but one (ignoring mutation producing new ones). However, that can take a long time, so that a population has two very different versions of a gene within it. And an occasional hybridization event can put odd genes into a population. A definitive answer on DNA patterns requires thorough sampling of multiple genes across the range of the species of interest.

A further problem on subspecies is that, given the lack of official standardization, old below-species names are to be credited as subspecies. Sometimes these were merely intended as recognition of individual variation, the modern concept of variety, but they get credited as subspecies. Those wanting to have or to sell as many kinds as possible like to recognize more forms. Thus, there is a need to distinguish between what is thought to be a type of individual variant (such as an albino) and what is thought to be distinctive of a large population that eventually intergrades with other populations."

Marien Faber:

"thank you for the Monk Flower (did not hear about this one before) and summary. Great to see confirmed, in a way, what I've been thinking ever since I took SEM photos of Iniforis turrithomae and I. casta, now more than 30 years ago. Of course there was nothing smart about that because the alternative option (that the planktotrophic I. turristhomae and the direct developer I. casta came from very different lineages and just happened to converge to identicality in every teleoconch character) totally lacks parsimony. 
Talking gastropods: the fun part of all this is that you can have two bona fide species with exactly the same genome - if one accepts that by -practical - definition the switch in larval development is the start of a new species. I do believe, however, that successful speciation (mostly the switch from r to K strategy, perhaps also the reverse) is near entirely peripatric (ranges are closely adjacent but do not overlap)."

Harry:

"I thought r and K reproductive strategies were part of a continuum. It seems to me, at least in the example you cite, that planktotrophy and lecithotrophy are all-or-nothing alternatives.  I guess epigenetic factors may drive both the above life-history strategies, but the evidence for strictly genomic determination of chirality is pretty convincing.  As for peripatry vs. sympatry: the former certainly helps restrict gene flow, but is it necessary in any of the scenarios we've discussed."

Tom Eichhorst:

" I hesitate to step into these deep waters, but just to mix things up a bit. The widespread European Theodoxus fluviatilis can be found in freshwater as well as brackish water within tidal influence - thus at times salt water. Specimens from these two extremes have been shown to be genetically the same species, but they cannot interbreed because neither can tolerate the habitat of the other. This is surely a setup for allopatric speciation, but where they are on that genetic trail is unknown. Another interesting case, now talking about planktotrophy and lecithotrophy, is Nerita melanotragus of New Zealand and eastern Australia. This snail was known as a planktotrophic spawner, until Przeslawski, 2008, 2011) discovered that sometimes the egg capsules hatched out lecithotropic juveniles. It seems the majority of the time the eggs hatch typical planktotrophic veligers, but every now and then they develop in the egg and hatch out as crawl-away juveniles. If further research bears this out, it means a chance to actually observe as a major phenotype change takes place, and eventually maybe parapatric speciation. 

These critters do not read our biology texts and so do not know or always follow the rules. Sure is interesting though...

Tom Eichhorst

Przeslawski, Rachael. 2008. Temporal patterns of gastropod egg mass deposition on southeastern Australian shores, Marine and Freshwater Research, 59: 652.
Przeslawski, Rachaeil. 2011. Notes on the egg capsule and variable embryonic development of Nerita melanotragus (Gastropoda: Neritidae), Molluscan Research, 31 (3): 152-158."

Marien, responding to Harry:

"I think it is more all-or-nothing like, as once demonstrated in the paper by Johannesson (1988) on Rockall (peripheral!) littorinids. And there is always a distinct morphological gap between the two protoconch types, and nothing inbetweenish. Regarding chirality, that's more problematical in terms of speciation, because the next step: getting offspring, is so much harder. Hence the low incidence. For instance in 60 My of Conus history, there was only one lefty (unless your name is Ed Petuch), whereas left handed specimens are less rare (and there has been one case of a cluster of left-handed cones - in the Mediterranean if I remember correctly -but that went nowhere either)."

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