Salamanders are amphibians. Most amphibians reproduce in the water: the eggs are laid in an aquatic environment and give birth to swimming larva that breathe through gills. Later, like the frog tadpole, this larva goes through a metamorphosis, transforming into a lung-breathing organisms that generally lives on dry land for the rest of its life. Most salamanders follow the same pattern, though the metamorphosis is less spectacular than in the frog because adult salamanders resemble the larval form more closely, in particular the retain a well-developed tail.
A strange family
In North America there is a strange family of salamanders which has no lung in the adult form. These mature salamanders breathe through their skins, but this is only possible if the skin is kept constantly moist. This means these salamanders must live in humid environments well protected from droughts. The genus Ensatina is a member of this family. The Ensatina lives in or on the layer of decomposing leaves on forest floors; at times it also burrows into the topmost layers of soil. The Ensatina feeds on the abundant insect populations in this habitat, which is moist enough year-round to allow it to breathe properly. This salamander is also remarkable for the fact that it lays its eggs in soil rather than water. Ensatina eggs go through the same developmental stages as those of other salamanders, but the metamorphosis takes place within the protective envelope of the egg; the young thus begin their lives in an adult-like form.
Unlike salamanders which have an aquatic larval phase, Ensatina can live and reproduce far from brooks and ponds. This is undoubtedly why they have conquered such a vast territory. We find Ensatina in the western portion of North America, from near Vancouver, Canada to southern California. In California, two population groups can be distinguished, coastal populations and populations in the Sierra Nevada mountains. These groups are separated by the wide, relatively dry Central Valley, where the Ensatina cannot survive; the hostile valley environment forms a natural barrier between these populations.
A scattered family
Depending on their size, color, and other, less visible, characteristics, we can identify several types of Ensatina, which have all been given scientific Latin names: oregonensis, picta, xanthoptica, eschscholtzi, platensis, croceator, and klauberi. The shaded portions of the map shoy the entire area propulated by Ensatina. The drawings indicate the territories occupied by each type; each has its own well-defined area. Nonetheless, there are rather vast territories where we do not find the typical forms listed above; instead, we find varying degrees of intermediate types. For example, a portion of the zone inhabited by picta is fringed by an area where intermediary forms betweein picta and oregonensis live. This indicates that, through crossbreeding, the animals found within this area have varying proportions of both picta and oregonensis genes. Scientists describe this as genetic mixing between two typical forms; because such mixing can occur, they are considered members of the same species. Likewise, there are intermediary forms between oregonensis and xanthoptica, xanthoptica and eschscholtzi, oregonensis and platensis, platensis and croceator, and croceator and klauberi. We therefore conclude that these seven forms all belong to the same species: these are known as geographical races, or subspecies. To be precise, they all belong to the species Ensatina eschscholtzi and the various subspecies are called Ensatina eschscholtzi eschzcholtzi, Ensatina eschscholtizi croceator, and so on.
The family ancestor
Since the oregonensis subespecies most closely resembles the other salamanders in this family, we think that the intire Ensatina group is derived from an ancesgtral form much like oregonensis that lived north of the specie´s current habitat. Moving south, the ancestral form colonized new territories. Because the conditions (particularly the climate and plant life) in these environments differed from those in its original habitat, natural selection favored mutants that were best adapted to these new territories. This has led to the differentiation of subespecies. In addition, these subspecies differ more from the ancestral form the farther south they are found.
When Ensatina reached California, it continued its march southward along two fronts, along each side of the Central Valley. The coastal, western Ensatina differentiated into the xanthoptica and eschscholtzi subespecies, whereas the platensis, croceator, and klauberi arose from the eastern, mountain branch. And remarkably, the southernmost Ensatina, the eschscholtzi and klauberi share a narrow territory east of San Diego that is devoid of intermediate varieties. This shows that each of these forms have differentiated to such a degree there is no crossbreeding between them. Scientists say these subespecies are separated by a "reproductive isolation barrier". In other words, if ther were not linked by the chain of subespecies -eschscholtiz, xanthoptica, oregonesis, platensis, croceator, and klauberi- we would consider them two distinct species! Furthermore, if for any reason, such as a climatic change for example, the northernmost subespecies disappeared and broke the chain, the species Ensatina eschscoltzi would indeed be split into two new species. This differentiation of two evolving lineages into independent species is called "speciation".
Prof. Jean Génermont
University of Paris.
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