speciation
Carination again
29-03-2010 07:17
Some time ago I wrote about ‘evolution at work’ and
the phenomenon of carination
(see also this
and linked posts). During my trip in Peru I had
the opportunity to visit another locality where
carinated snails occur. It is in the Rio
Jequetepeque valley near Tembladera.
On the west side of Tembladera (located at the eastern end of the lake), Scutalus cretaceus (Pfeiffer, 1855) occurs. It was found hibernating on large rocks. East of the village S. baroni is found on rock-faces (pictures of a living specimen here).
When I was in Tucumán, I saw some lots - collected by Weyrauch - showing the transition between the two species. One series is shown above, but you’ll notice the different forms also in the lot shown below.
Since I strongly have the impression that Weyrauch mixed shells from an area (and also from different visits) into one lot, it is probable that these transitional forms were not found at one spot (and at one time).
Anyway, this raises questions like:
- How strongly demarcated is the hybrid zone between the two taxa?
- Is this zone determined mainly by a change in habitat? What is the role of the microclimate? Are there differences in soil or rock types? Can any of these factors considered to be the selective, driving force?
- How extended is the population of S. baroni and is there also hybridization with S. debilisculptus Weyrauch, 1967 which occurs at slightly higher elevation in the valley?
- Why is carination such a locally restricted phenomenon? And what is the evolutionary advantage?
On the west side of Tembladera (located at the eastern end of the lake), Scutalus cretaceus (Pfeiffer, 1855) occurs. It was found hibernating on large rocks. East of the village S. baroni is found on rock-faces (pictures of a living specimen here).
When I was in Tucumán, I saw some lots - collected by Weyrauch - showing the transition between the two species. One series is shown above, but you’ll notice the different forms also in the lot shown below.
Since I strongly have the impression that Weyrauch mixed shells from an area (and also from different visits) into one lot, it is probable that these transitional forms were not found at one spot (and at one time).
Anyway, this raises questions like:
- How strongly demarcated is the hybrid zone between the two taxa?
- Is this zone determined mainly by a change in habitat? What is the role of the microclimate? Are there differences in soil or rock types? Can any of these factors considered to be the selective, driving force?
- How extended is the population of S. baroni and is there also hybridization with S. debilisculptus Weyrauch, 1967 which occurs at slightly higher elevation in the valley?
- Why is carination such a locally restricted phenomenon? And what is the evolutionary advantage?
Carination revisited
10-12-2008 16:56
While searching some literature on Chile for a
friend, I found an interesting paper on the ecology
of a Bostryx species from the desert in the northern
part of the country. It also sheds some new light on
the phenomenon of carianation (subject of an upcoming
paper) and relates to the specialization / speciation
discussion briefly summarized
here
and
here.
In 1970, the paleontologist Herm described a new species, Bostryx variabilis, from Pleistocene marine sediments near Antofagasta.
Map showing the area just north of Antafagasta (from Herm, 1970)
To explain the occurrence of this terrestrial species amidst marine facies, he postulated a sea level regression-transgression. The species is carinated (keeled) and partially uncoiled.
In 1981, Alan Craig -a geographer- did field work in the same region and made some observations which questioned Herm’s hypothesis. He found many specimens of B. variabilis in nearby localitions, but they were restricted to places where also fossilized lichens occurred. Moreover he found transitions to a Recent species, B. mejillonensis (Pfeiffer, 1857).
Isotopic dating yielded an age of 2180 +/- 50 yr B.P., making the Pleistocene sea level change as postulated by Herm unnecessary for the type locality.
But how to explain the occurrence of B. mejillonensis-variabilis at this place? Eventually it appeared that Bostryx mejillonensis was only found in localities where meteorological circumstances sustained epiphytic colonies of lichens on cacti at the top of a hill nearby the type locality of Herm. At the slopes below the elevation where the seasonal fog banks have influence, the hyperarid desert constitutes a barrier for the dispersal of snails. The hill top thus forms an ecological island. This finally led Craig to hypothesize that B. mejillonensis had been forced outside their habitat into the surrounding desert from where they were unable to escape. Rapid changing environmental conditions would have ‘stress-induced’ a speciation process which led to the aberrant form of B. variabilis. Support for this hypothesis were field observations by Craig of runnels, caused by occasional winter rains, leading to downslope transportation of snails.
Area just north of Antafagasta with the localities mentioned by Herm (1970) and Craig (1985) indicated. The distribution area of lichens indicated by Craig marked on Morro Moreno in white (source: Google Earth).
Given the hypothesis briefly outlined above, my question is: is carination as observed in other places (e.g., here and here) also best explained as a ‘stress-induced speciation process’? If so, are there analogous ecological circumstances that force snails to transform into aberrant, carinated and -sometimes even - uncoiled shells?
References:
Craig, A.K., 1985. Speciation and age revision of the Atacaman snail Bostryx variabilis Herm. - Quarternary Research 23: 382-387.
Herm, D., 1970. Bostryx variabilis n.sp., eine Landschnecke aus dem Altpleistozän von Mejillones, Nordchile. - Mittheilungen Bayerischen Staatssammlung Paläontologie und historische Geologie 10: 189-198.
In 1970, the paleontologist Herm described a new species, Bostryx variabilis, from Pleistocene marine sediments near Antofagasta.
Map showing the area just north of Antafagasta (from Herm, 1970)
To explain the occurrence of this terrestrial species amidst marine facies, he postulated a sea level regression-transgression. The species is carinated (keeled) and partially uncoiled.
In 1981, Alan Craig -a geographer- did field work in the same region and made some observations which questioned Herm’s hypothesis. He found many specimens of B. variabilis in nearby localitions, but they were restricted to places where also fossilized lichens occurred. Moreover he found transitions to a Recent species, B. mejillonensis (Pfeiffer, 1857).
Isotopic dating yielded an age of 2180 +/- 50 yr B.P., making the Pleistocene sea level change as postulated by Herm unnecessary for the type locality.
But how to explain the occurrence of B. mejillonensis-variabilis at this place? Eventually it appeared that Bostryx mejillonensis was only found in localities where meteorological circumstances sustained epiphytic colonies of lichens on cacti at the top of a hill nearby the type locality of Herm. At the slopes below the elevation where the seasonal fog banks have influence, the hyperarid desert constitutes a barrier for the dispersal of snails. The hill top thus forms an ecological island. This finally led Craig to hypothesize that B. mejillonensis had been forced outside their habitat into the surrounding desert from where they were unable to escape. Rapid changing environmental conditions would have ‘stress-induced’ a speciation process which led to the aberrant form of B. variabilis. Support for this hypothesis were field observations by Craig of runnels, caused by occasional winter rains, leading to downslope transportation of snails.
Area just north of Antafagasta with the localities mentioned by Herm (1970) and Craig (1985) indicated. The distribution area of lichens indicated by Craig marked on Morro Moreno in white (source: Google Earth).
Given the hypothesis briefly outlined above, my question is: is carination as observed in other places (e.g., here and here) also best explained as a ‘stress-induced speciation process’? If so, are there analogous ecological circumstances that force snails to transform into aberrant, carinated and -sometimes even - uncoiled shells?
References:
Craig, A.K., 1985. Speciation and age revision of the Atacaman snail Bostryx variabilis Herm. - Quarternary Research 23: 382-387.
Herm, D., 1970. Bostryx variabilis n.sp., eine Landschnecke aus dem Altpleistozän von Mejillones, Nordchile. - Mittheilungen Bayerischen Staatssammlung Paläontologie und historische Geologie 10: 189-198.
Prime and remnant species
27-11-2008 21:32
An interesting concept that is part of a thesis to be
defended next week (more on it then), is that of the
distinction between ‘prime’ and ‘remnant’ species
(Gittenberger & Kokshoorn, 2008).
In several genera, there is one (sub)species that has a relatively wide range and a common occurrence. These evolutionary successful taxa may be dubbed the ‘prime’ species of a genus. In contrast, species with a relatively small distribution range might be called ‘remnant’ species. However, data on the phylogenetic relationships will be necessary to be able to differentiate between (1) recent speciation and (2) a secondary situation, with stability or decrease in range.
‘Remnant’ species may also be considered as rare, endemic species which might be of potential interest for conservation. Rabonowitz (1981) has made a classification of rarity in plants, and it is interesting to apply this here in analogy. He distinguishes seven forms of rarity in three categories: range size, habitat specificity and local abundance. His scheme is:
Two orthalicid species that spring to mind which may exemplify the concept of ‘prime’ species are Plekocheilus (Eurytus) floccosus (Orbigny, 1835) and Drymaeus (D.) expansus (Pfeiffer, 1848). They range both on the eastern slopes of the Andes, from Bolivia to Peru/Ecuador. A picture of the latter species (unfortunately, no better in stock at the moment) is here:
References:
Gittenberger, E. & Kokshoorn, B., 2008. Evolutionary inequality in Chondrina and Arianta (Gastropoda: Pulmonata): ‘prime’ and ‘remnant’ species. In: B. Kokshoorn, 2008. Resolving riddles and presenting new puzzles in Chondrinidae phylogenetics. Leiden, Ph.D. thesis: 59-64.
Rabinowitz, D., 1981. Seven forms of rarity. In: H. Synge (ed.), The biological aspects of rare plant conservation. New York, Wiley: 205-217.
In several genera, there is one (sub)species that has a relatively wide range and a common occurrence. These evolutionary successful taxa may be dubbed the ‘prime’ species of a genus. In contrast, species with a relatively small distribution range might be called ‘remnant’ species. However, data on the phylogenetic relationships will be necessary to be able to differentiate between (1) recent speciation and (2) a secondary situation, with stability or decrease in range.
‘Remnant’ species may also be considered as rare, endemic species which might be of potential interest for conservation. Rabonowitz (1981) has made a classification of rarity in plants, and it is interesting to apply this here in analogy. He distinguishes seven forms of rarity in three categories: range size, habitat specificity and local abundance. His scheme is:
| Range size
|
Habitat
specificity
|
Local abundance
|
Prime/remnant
|
| Large
|
Wide
|
Small
|
|
| Large
|
Narrow
|
Large
|
?‘Prime’
|
| Large
|
Narrow
|
Small
|
|
| Small
|
Wide
|
Large
|
|
| Small
|
Wide
|
Small
|
|
| Small*
|
Narrow*
|
Large*
|
‘Remnant’
|
| Small*
|
Narrow*
|
Small*
|
‘Remnant’
|
Two orthalicid species that spring to mind which may exemplify the concept of ‘prime’ species are Plekocheilus (Eurytus) floccosus (Orbigny, 1835) and Drymaeus (D.) expansus (Pfeiffer, 1848). They range both on the eastern slopes of the Andes, from Bolivia to Peru/Ecuador. A picture of the latter species (unfortunately, no better in stock at the moment) is here:
References:
Gittenberger, E. & Kokshoorn, B., 2008. Evolutionary inequality in Chondrina and Arianta (Gastropoda: Pulmonata): ‘prime’ and ‘remnant’ species. In: B. Kokshoorn, 2008. Resolving riddles and presenting new puzzles in Chondrinidae phylogenetics. Leiden, Ph.D. thesis: 59-64.
Rabinowitz, D., 1981. Seven forms of rarity. In: H. Synge (ed.), The biological aspects of rare plant conservation. New York, Wiley: 205-217.
Specialization and speciation (cont'd)
16-04-2008 19:13
The other conclusion of Patten that make me think, is
that "lowered genetic variance associated with
specialization may also lead to extended periods
during which generalists beget specialists and
specialists beget more specialists but during which
few generalists evolve. (...) Accordingly, over time
populations will edge steadily towards the specialist
pole of the generalist-specialist continuum".
The question is here, in malacological terms, what makes a land snail a specialist and how do you know? Is that by populations adapting to a narrow ecological niche? Than one could infer their stand on the generalist-specialist continuum from their distribution pattern. A range-restricted species would be more 'specialist' than a wide-spread species. Or could one say that certain 'life styles' (e.g. arboreal versus under stones or feeding on rocks) are to be considered more specialized? What is then, to speak in cladistic terms, to be considered plesiomorph and what apomorph? And what is the role of convergence*?
Patten also argues that "there should be a net accumulation of specialists over time...to a point. Because of their reduced ability to evolve, specialists are more likely than generalists to pay the price when, say, climate or habitat changes rapidly." This is an interesting hypothesis, especially when fossils are good represented in a group and they could tell us something about the position on the generalist-specialist continuum. I'm afraid that land snail fossils are poorly represented (at least in the Neotropics) and not very informative (often only the shell shape). So, at least for the moment, there are more questions than crisp and clear insights.
Reference:
Breure, A. S. H. & E. Gittenberger (1982). The rock-scraping radula, a striking case of convergence (Mollusca). Netherlands Journal of Zoology, 32, 307-312
The question is here, in malacological terms, what makes a land snail a specialist and how do you know? Is that by populations adapting to a narrow ecological niche? Than one could infer their stand on the generalist-specialist continuum from their distribution pattern. A range-restricted species would be more 'specialist' than a wide-spread species. Or could one say that certain 'life styles' (e.g. arboreal versus under stones or feeding on rocks) are to be considered more specialized? What is then, to speak in cladistic terms, to be considered plesiomorph and what apomorph? And what is the role of convergence*?
Patten also argues that "there should be a net accumulation of specialists over time...to a point. Because of their reduced ability to evolve, specialists are more likely than generalists to pay the price when, say, climate or habitat changes rapidly." This is an interesting hypothesis, especially when fossils are good represented in a group and they could tell us something about the position on the generalist-specialist continuum. I'm afraid that land snail fossils are poorly represented (at least in the Neotropics) and not very informative (often only the shell shape). So, at least for the moment, there are more questions than crisp and clear insights.
Reference:
Breure, A. S. H. & E. Gittenberger (1982). The rock-scraping radula, a striking case of convergence (Mollusca). Netherlands Journal of Zoology, 32, 307-312
Specialization and speciation
15-04-2008 16:41
The title refers to an interesting recently
commentary in the Journal of Biogeography*, where
Michael Patten argues that speciation and
specialization are the same phenomenon. I'm not going
to repeat his arguments here, but some of his
conclusions are mind-provocative.
The first is that a simple geographic barrier becomes inadequate to 'explain' divergence between populations. If subtle differences in climate lead to subtle difference in vegetation, which in turn leads to different behaviour. In most cases one have to think in geological timescales to imagine how these subtle differences lead to larger differences and effective speciation comes into play.
I always wonder how these general theories work out in malacology. Just an example that puzzles me, possibly speciation still at work.
Imagine you see the shells above. The only thing you know is that they all occur in the same region, more or less at the same altitude, but at two sides of a river. I have plotted the (approximate) localities (taken from labels and collected some 40 years ago) on this map:
Now the question is, what would you call it? One species? Four subspecies? Two subspecies? I'm inclined to say the latter, but this case clearly needs further investigation. It would be nice to study these populations genetically to see to what extent the morphological differences can be explained. The only problem is: these animals occur high up in the Andes, Río Rimac valley, 3300-3400m. Any volunteers for collecting?
Reference:
Patten, M. A. (2008). The intersection of specialization and speciation. J. Biogeogr, 35, 193-194.
The first is that a simple geographic barrier becomes inadequate to 'explain' divergence between populations. If subtle differences in climate lead to subtle difference in vegetation, which in turn leads to different behaviour. In most cases one have to think in geological timescales to imagine how these subtle differences lead to larger differences and effective speciation comes into play.
I always wonder how these general theories work out in malacology. Just an example that puzzles me, possibly speciation still at work.
Imagine you see the shells above. The only thing you know is that they all occur in the same region, more or less at the same altitude, but at two sides of a river. I have plotted the (approximate) localities (taken from labels and collected some 40 years ago) on this map:
Now the question is, what would you call it? One species? Four subspecies? Two subspecies? I'm inclined to say the latter, but this case clearly needs further investigation. It would be nice to study these populations genetically to see to what extent the morphological differences can be explained. The only problem is: these animals occur high up in the Andes, Río Rimac valley, 3300-3400m. Any volunteers for collecting?
Reference:
Patten, M. A. (2008). The intersection of specialization and speciation. J. Biogeogr, 35, 193-194.