Richard Wells wrote:
"Mmmmmm. Molecules and Invasions...interesting - though a bit confusing to one that is not part of the molecular illuminati, but definitely interesting. I am a little surprised at the apparent relatively recent separation of the two Oxyuranus scutellatus members...I think you should go over your data again or apply some other bit of molecular gymnastics because I have seen both scutellatus and canni - and I considered them to be parapatric species - and if my suspicion is correct, then I think they may have differentiated more than your present data indicates."

Me:
I think you are unfamiliar with the scientific fact that molecular evolution occurs independently of morphological evolution. Basically, many molecules that the molecular systematists study are selectively neutral, or nearly so. Mutations in these molecules occur rather randomly, so that the chance that a particular mutation will be fixated in a population is virtually entirely due to chance. Therefore, like the atomic clock, the "molecular clock" really measures (albeit much less accurately than the atomic clock) roughly how long ago two species may have shared a common ancestor. mtDNA is very useful for this purpose. In fact, mtDNA from a Neandertal man suggests that Neandertals last shared a common ancestor with modern humans about 500,000 years ago, whereas all living human populations last shared a common ancestor about 150,000 years ago. However, that does not necessary mean that Neandertals are a different species than modern humans. To determine that, one has to look for evidence to see if Neandertals can interbreed successfully with modern humans. Since Neandertals are extinct, that determination is quite difficult and the taxonomic status of Neandertals therefore remains controversial as well as unresolved.

Because molecules evolve independently of morphology, sometimes a species can diverge rather quickly from its closest relative in a relatively short period of time. Conversely, two or more populations of a slowly evolving species may display a great deal of mtDNA divergence (or a great deal of nuclear DNA divergence) and yet be morphologically similar. A real world example is presented below. In an earlier post,

http://forums.kingsnake.com/view.php?id=609826,622134

I reproduced part of an actual branching diagram (based on mtDNA) from a published paper but I removed all of the names. I then challenge anyone to tell me how many species are represented in that diagram. Not surprisingly there are no takers.

In the diagram below I will review the answer. The names I use differ from those in the original paper, as I personally do not agree with the split of the genus Scaphiopus into the genera Spea and Scaphiopus. As one can see from the diagram, the species Scaphiopus bombifrons and S. intermontanus have only recently diverged from one another, and yet they are recognizable as distinct species. On the other hand, the populations of S. hammondii in San Diego and Alameda Counties, California show much greater mtDNA divergence than that between S. bombifrons and S. intermontanus. This is illustration that a little mtDNA divergence does not necessarily mean that there cannot be a disproportionately large amount of morphological divergence. WW et al.'s data may accurately reflect the timing of the split between the taxa they studied, but their data cannot tell us how different two taxa may be, simply because mtDNA data is not a measure of morphological evolution.

I have also chosen to classify S. multiplicatus as being conspecific with S. hammondii. This is done to highlight the fact that despite the considerable mtDNA distances between hammondii and multiplicatus, these two taxa are still recognizable as close relatives and they were for a long period of time regarded as different, allopatric populations of the same species. In sum, it is not possible to learn from mtDNA data whether two populations are the same species and how different they may be morphologically. These are questions that must be answered by other sorts of data. What mtDNA is very good at is to reveal past migratory patterns and which populations of a species may be the oldest, as well as revealing which particular species may be basal or even ancestral to another, provided of course that the researchers choose their outgroups carefully.

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