Microevolution and Ecology of Salamanders and Frogs
Ecology and Evolution of the Caucasian Salamander
My publications on this subject
There is the region at the eastern coast of the Black Sea called Colchis. It is composed by south-western slopes of the Great Caucasus mountains, north-western slopes of the Lesser Caucasus and a lowland between these two mountain systems. The region has mild, humid climate and largely covered by forests. In contrast, the areas that lie north, south and eastwards from Colchis have continental climate. Forest landscapes of Colchis survived aridisation of climate in early Pliocene (about 6 millions of years ago) and the Ice Age. Many plants and animals, which live in this region, are relicts - their ancestors were wide spread throughout the Near East in Miocene, when this area was covered by humid subtropical forests. About 25 % of vertebrate species which are found in forests of Colchis are endemic for the Caucasus Isthmus.
Recent genetic studies support the point of view that forest islands which survived in Colchis during several millions of years triggered diversification and speciation processes. A good example is the Caucasian salamander ( Mertensiella caucasica ), which is found exclusively at the north-western slopes of the Lesser Caucasus and adjacent mountain areas in north-eastern Turkey. This species was wide-spread in Europe 4-5 millions of years ago, but disappeared from the largest part of its range possibly during the Ice Age. Currently, most of populations of this salamanders live in streams that belong to the basin of the Black Sea and only a little geographic population inhabits streams associated with the river Mtkvari (Kura) flowing down to Caspian. These two areas were separated by treeless landscapes during the Ice Age and possibly long before that.
The analysis of the mitochondrial DNA structure of the salamanders from both areas revealed, that they harbor strongly different motochondrial DNA haplotypes. This reflects separation between the lineages since several millions of years (most likely 6-7 millions of years). Moreover, they have fixed differences in the structure of the nuclear DNA. Therefore, in a strict sense, the Caucasian salamander is composed of two genetically isolated species - one from the western, another from the eastern part of the range. The salamander is a strognly specialised animal, which lives exclusively at small mountain brooks with rapid current, cold water, and plenty of shelters. Such specialisation and low mobility maintained isolation of salamander from western and eastern habitats for millions of years, although the distance between them did not exceed several dosens of kilometres. On the other hand, even in isolated habitats the salamanders lived in very similar habitats. As a result, no clear morphological differences developed in two isolated parts of the range, in spite of a very long period of isolation.
There are of course some morphological variation throughout the range of the salamander. For instance, animals from the extreme south-western Georgia have reduced pigmentation (and were described as a separate subspecies), salamanders from Zigana pass in Turkey have asymmetrically positioned yellow spots on the back, and salamanders from Batumi botanical garden have enormously large larvae. However these variations, fixed by selection dependent on local ecological conditions, do nothing with the separation between the eastern and the western species.
This is in line with many other studies of salamanders throughout the world which demonstrated that the level of morphological differences between subspecies or closely related species usually does not reflect their genetic closeness or genetic borders.
Actually, there are two main distinct processes triggering speciation: selection, which increases reproductive fitness of an individual in particular habitat; and gene drift which increases genetic differences between isolated populations irrespective to reproductive fitness of genotypes. Two populations, which are isolated long time, gain differences both by gene drift (because they endure the drift separately) and by selection (because the habitats they live in commonly differ). Drift operates with the random sample of genes, only few of that influence morphology and ecology of animals. Selection operates with a very small part of genome. As a result, the differences gained by selection are easy to see but difficult to record with genetic methods. Differences gained by drift are difficult to see but easy to estimate studying the genome. At early stages of speciation it is not surprising that some races show deep morphological but minute genetic differences, and another way around. It depends on how strong the races are isolated (if strong, they have a lot of genetic differences) and in how different habitats they live (if in different, they have much morphological differences). When speciation is completed, both genetic and morphological differences slowly accumulate in time and dissimilar genera or families are usually also genetically distant.
1. Tarkhnishvili, D.N. & I.A.Serbinova, 1993. The ecology of the Caucasian salamander ( Mertensiella caucasica Waga) in a local population - Asiatic Herpetological Research, 5: 147-165
2. Tarkhnishvili, D. N., 1994. Interdependences between populational, developmental and morphological features of the Caucasian salamander, Mertensiella caucasica . - Mertensiella (Bonn), 4: 315-325.
3. Tarkhnishvili, D. N. and Gokhelashvili, R. K., 1994. Preliminary data of the age structure of a Mertensiella caucasica population. -Ibid: 327-334.
4. Tarkhnishvili, D. N., 1996. The distribution and ecology of the amphibians of Georgia and the Caucasus: a biogeographic analysis. -Zeitschrift fur Feldherpetol. 3: 167-196.
5. Tarkhnishvili, D. N. and Serbinova, I. A., 1997. Normal development of the Caucasian salamander ( Mertensiella caucasica ). -Advances in Amphibian Research in the Former Soviet Union, 2: 13-30.
6. Tarkhnishvili, D.N. and Serbinova, I.A., 1998. Giant larvae of the Caucasian salamander. -Advances in Amphibian Research in the Former Soviet Union, 3: 187-191.
7. Tarkhnishvili, D. N., Thorpe, R. S. and Arntzen, J. W., 2000. Pre-Pleistocene refugia and differentiation between populations of the Caucasian salamander ( Mertensiella caucasica ). Molecular Phylogenetics and Evolution 14: 414-422.