a. Explain the two strategies that led to the differences in morphologies (coloration patterns, etc.) between the coastal populations and the Sierra Nevada populations.
Since the salamander population along the Sierra Nevada Mountains was close to the forest, these animals had a better chance of survival provided they could hide themselves effectively or blend in with their environment. They, therefore, started evolving along these lines and became adept at camouflage. Morphologically, they are dull in color, with markings to help them blend in. As they slowly moved south, their markings became more distinct and they got better at camouflage techniques.
On the other hand, the coastal population has evolved using the strategy of mimicry. They have become brightly colored like the poisonous newt species living there and have adapted their behavior along a similar pattern. They advertise their resemblance to newts and attract attention towards themselves as a warning to predators (NOVA, 2007).
b. The Great Central Valley lies in the middle of the salamander distributional range, separating the coastal and forest populations. Explain a mode of speciation that might address this pattern.
When a population is split into two by a geographical barrier (valley, in this case), isolation initially occurs due to physical barriers or distance. However, as the two populations continue to evolve along different lines, after a period they become very different and can no longer interbreed. This mode of formation of different species is known as allopatric speciation. In such cases, the barrier between the two populations makes it difficult for them to mate and the gene flow between the two populations is greatly reduced (Berkeley.edu).
c. At the southern end of the distribution, two of the “morphotypes” are able to hybridize, producing offspring with hybrid-type morphology. Applying the biological species concept, are the two parental “morphotypes” considered separate species?
According to the biologcal species concept, a species is comprised of a group of individuals that can breed together and are reproductively isolated, that is, cannot breed with any other group. Since these two “morphotypes” can interbreed and produce offspring, and they are reproductively isolated (they can only breed with other morphotypes and amongst their own morphotype), they are part of the same species.
However, at the extreme Southern end, where the ranges overlap, the last two morphotypes E. klauberi and E. eschscholtzii do not interbreed. As per the biological species concept, these two should be classified as separate species due to their inability to interbreed produce offspring (PBS).
Focusing on the two southern morphotypes, what type of mode of speciation might be occurring?
The fact that all the subspecies breed with their neighbors except at the southernmost end, where the two morphotypes E. klauberi and E. eschscholtzii do not breed with each other, shows that ring speciationhas occurred. At one end of the ring, there are two distinct forms that do not interbreed; however, along the sides of the ring, several diverse sub-species occur which interbreed with their neighbors (Irwin, 2002).
Explain one possible isolating barrier that may result in the morphotypes becoming completely separate species.
Extrinsic post-zygotic isolation may also occur. This means that natural selection selects against the hybrids or hybrids cannot find appropriate mates. This can also be an isolating barrier that may result in separation of species over time (Kisdi, 2012). In the region where two morphotypes occur together but do not and cannot interbreed, it is possible that gametic isolation may occur: the structure of the gametes becomes different and cross-fertilization is no longer possible.
d. Given the distributional patterns of the morphotypes, describe an additional mode of speciation along with a possible isolating barrier that may result in distinct biological species.
Along the sides of the riing in places where there is a slight overlap (except at the end), parapatric speciation seems to occur. This means that, even though the gene flow is not completely absent and the barrier is permeable, the two populations prefer to mate with their own kind rather than to interbreed. The two morphotypes may exhibit a preference for their own type while mating. Since they are morphologically and behaviorally different, this would be a type of behavioral isolation (Scoville).
e. Briefly explain how each of the following mechanisms of evolution could be playing a role: natural selection, genetic drift, gene flow.
Natural selection: the two lines evolved in the manner in which their survival was best assured. The Sierra Nevada line became adept at camouflage, while the coastal line mimicked the poisonous newts and scared predators away. In both cases, the animals that were not appropriately camouflaged or not particularly good at mimicry were prone to predation. So with time, natural selection weeded out the organisms that could not adapt well and left the ones in which the characteristics of camouflage and mimicry were particularly strong. As both lines evolved, they also kept moving southwards, so that the gradient of sub-species from north to south is from least to most evolved. In terms of morphology, the species in the south have more distinct markings and are better suited for blending in or mimicking newts. Through natural selection, the traits that were more beneficial for survival have been made more prominent. Where the populations interbreed to create hybrids, the hybrids can neither camouflage themselves nor mimic newts properly; as a result, natural selection does not allow them to survive for long or perpetuate their genes.
Accumulation of genetic changes in an isolated population over an extended period such that at the end of the period the members of the population are markedly different from the initial ones is known as genetic drift (BioMed Central, 2011). In this case, genetic drift occurred on both sides of the valley and the gene pools of both populations changed; when they met at the extreme southern end, they could no longer interbreed and were separate species.