Tuesday, July 22, 2008

Ring species

What are ring species?
I think it's about time someone explained ring species and how it blurs pretty much every species concept. Why are ring species so difficult to categorize? For starters, distant points of sample from the group are often genetically, phenotypically, and behaviorally distinct from one another while the closer the points of sample are to one another, the less distinct the two. The group does not have to be a "ring," in fact, three of the prime examples of "ring species" do not complete the ring. These three common examples are Herring Gulls, Greenish Warblers, and salamanders of the genus Ensatina. Today, I'll only be talking about Ensatina. The paper I'll be talking about can be found here.

The researches are using ring species to gain insights into speciation. They do this by comparing the hybridization cline and hybrid deficit. The idea is that the narrower and more pronounced the cline, the less genetic flow between the two species exists reinforcing the accumulation of non-compatible alleles.

Summation of the boring part:
They looked at a mid-ring secondary contact location since primary contact locations will have had no separation in gene flow. They are, therefor, looking at how hybridized the two subspecies are. By comparing known distinguishing markers including morphological and geneological data from those at extreme points from the cline to those near the midpoint of the cline on outwards, a map of the distribution of hybrids could be made. They also used mitochondrial DNA to establish maternal lineage of each salamander and nuclear DNA for comparison purposes. These various data were then graded 0 to 1 for morphological characteristics and 0-16 for genetic characteristics. They then collapsed the data into a one dimensional transect by using a best fit. Following this, Hardy-Weinberg equilibrium was assessed.

Good part:
The cline zone between the 1981 study and the 2001 study were nearly the same indicating cline stability with narrow zones of hybridization. There are also very few F1 hybrids (less than 1%), but near cline centers, backcrossed hybrids were above 50%. There are also high levels of linkage between parental alleles indicating very little gene flow between the groups and high selection against hybrids. The low number of F1 hybrids also indicates that either the two groups rarely cross-breed or F1 hybrids are prone to much higher mortality than either parent population.

The absence of hybridization in some regions of the ring complex indicate that speciation, at least in some areas, has already been completed and these groups could easily be considered distinct species were they not in the ring complex. Evidence of frequent isolation and secondary contact between subspecies also indicates that selection plays a large role in compatibility upon secondary contact.

My comments:
We have known, for may years, that Ensatina species are highly divergent at distant points in the ring. We have also known that at many allopatric areas, subspecies frequently interbreed producing a stable zone of hybridization. The zone of this study, however, sheds new light on exactly how zones of secondary contact in partially speciated groups can behave. In this case, the zone of hybridization, being fairly narrow and very distinct indicates very little gene flow between species and strong negative selection of F1 hybrids which further reinforces the cline. Though the researchers did not directly say it, I would think that in a relatively short time, we may see complete distinction between these two groups. I would also consider this to be more incomplete peripatric speciation with secondary contact before complete speciation. In any event, Ensatina and other ring species are continuing to give us insight into the process of speciation. I wonder if the idiots at Conservapedia are going to try and discredit this paper, or if Luskin is going rant and scream about something missing, perhaps the specific genes?

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