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Big Story There isn't a Biggest Story for Today, yet.
| Re: N. rachovii Lower Shire River MZMW 09-4 |
Subject: Re: N. rachovii Lower Shire River MZMW 09-4
by bturner on 2009/9/1 19:14:17
I did not mean to imply that chromosomal divergence was more important than other kinds of genetic differences, including mtDNA sequences. Merely that chromosome differentiation comes quickly to mind when one thinks of rivuline killifishes in general. N. rachovii from Beira has a very low chromosome number, and I was interested in knowing if the Shire R. form was different in any way. Classically, chromosomal divergence is supposed to lead to postmating isolating mechanisms, and has been treated by several authorities (most notably MJD White) as a "magic bullet" of speciation. In recent times, the strength of this relationship has been questioned, and it appears to be less straightforward than initially thought.
However, I certainly agree that the geographical isolation of many Notho habitats means that it is hard to interpret their divergences in terms of the classical species
category. I would take this one step further: It is difficult to apply the biological species concept meaningfully to allopatric populations in general. So, for comparisons of geographic isolates, I am comfortable with a phylogenetic species concept. However, I would argue that the argument that isolated populations are different species gets better as the number of divergent features gets larger.
For geographically contiguous or sympatric populations, however, I think that some of your reservations hold. There is a growing list of cases, for example, in which individuals that are members of the same gene pool can have highly divergent mtDNA matrilines (due, perhaps, to a past fusion of differentiated populations, or to ancient hybridization).
In these situations, geography and sampling are everything: If a form is (more or less) continuously distributed, and populations from different extremes of the distribution have different mtDNA matrilines, it is not a good idea to regard them as distinct species until one has examined some "in between" populations. The SE Brazil populations of Kryptolebias marmoratus (sensu lato) are a good example of this problem. We know that they are divergent from Caribbean populations in color pattern, and that they are sharply different in some mtDNA sequences as well. Based on the former, my colleague Wilson Costa has treated the SE Brazil populations as a separate species, K. ocellatus. The molecular data are consistent with this. However, there is a long stretch of the South American coast that has not been sampled (roughly from Recife to Rio) and there is plenty of potential for intermediate populations to exist, perhaps some with both matrilines.
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