Misunderstood Cichlids

From: Vincent Campbell (v.p.campbell@stir.ac.uk)
Date: Wed Aug 29 2001 - 13:29:43 BST

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    From: Vincent Campbell <v.p.campbell@stir.ac.uk>
    To: "'memetics@mmu.ac.uk'" <memetics@mmu.ac.uk>
    Subject: Misunderstood Cichlids
    Date: Wed, 29 Aug 2001 13:29:43 +0100
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    Recently, Ted Dace, offered the following article in defence of his
    questioning of natural selection:

    <In the February 1999 issue of Scientific American, Stiassny and Meyer
    discuss the inexplicable similarity of color patterns on the scales of
    cichlids in separate African lakes.>

    Since I had to go to the library anyway, I looked this article up. I had
    implied he mis-read the article, and I thought I was being prematurely
    unfair, so I thought I'd better check the article.

    On reading it ('Cichlids of the Rift Lakes', by Stiassny, MLJ & Meyer, A, in
    Scientific American, Feb 1999, pp: 64-69), however, I found that despite
    being premature my suspicion was correct.

    Firstly, Cichlids are interesting, the authors state, because of their
    immense diversity. They describe one genus, Tropheus, as being like
    Darwin's finches in their variety, since their lifestyle of hugging close to
    rocky outcrops protecting them from predation means that communities are
    separated sometimes by hundreds of feet resulting in very little if any
    contact between groups, resulting in distinct varieties (p65-6).

    The reasons for the massive diversity of Cichlids comes from a number of
    features. One of these is anatomy- cichlids have two very adapatable sets
    of jaws, such that feeding on different foods produces fish looking really
    very different looking fish. The authors state:

    'The two sets of jaws, fine-tuned according to food habits, allow each
    species to occupy its own very specific ecological niche. In this manner,
    hundreds of species can co-exist without directly competing.' (p.66)

    Another factor is their reproductive behaviour (p.67). Cichilds care for
    offspring long after hatching 'and the protracted association between
    parents and offspring involves elaborate communication' (p.67). The authors
    describe some of the particular lifestyles of species, and then state:

    'The diverse hues (such as those of the colour morphs described earlier)
    have probably arisen because of the preferences of the females. In this
    case, sexual selection, rather than pressure for physical survival, seems to
    have driven the diversification.' (p.67-8)

    [Anyone familiar with Dugatkin's book 'The Imitation Factor' will recognise
    this kind of argument. For those who haven't read it, he also links this to
    memes quite explicitly and interestingly]

    OK, here we get to the crunch part of the article for our debate here. I'll
    have to give a long quote to ensure the correct sense comes across. To set
    the scene, genetic analysis reveals that all the Cichlids stem from 11
    ancestral species in Lake Tanganyika, some later reaching Lakes Victoria and
    Malawi, where all species have evolved from particular branches of these
    ancestral species, even single lineages. The authors state:

    'This scenario implies that almost identical evolutionary adaptations can
    and did evolve many times independently of one another. Cichlids with
    singular anatomical features- designed to feed on other fish or on eggs and
    larvae, to nip off fins, scrap algae, tear off scales, crush molluscs or any
    of myriad other functions- occur in all three lakes. To some of us
    biologists, such features had seemed so unique and so unlikely to evolve
    more than once that we had held that fishes with the same specialisations
    should be closely related.

    If that were so, the predilection to scrape algae (for instance) would have
    evolved only once, its practioners having later dispersed. But algae
    scrapers in Lake Victoria and Lake Malawi have evolved independently of
    those in Lake Tanganyika, from an ancestor with more generalised
    capabilities. The genetic studies show that evolution repeatedly discovers
    the same solutions to the same ecological challenges.' (p.68)

    [Here, as can be seen, they don't suggest something other than evolution is
    at work, and make the same basic case that Dawkins does in his book]

    They go on to point out that palaeoclimatological data concerning Lake
    Victoria shows it virtually dried out less than 14,000 years ago, Lake
    Nabubago (separated by a 4,000 year old sandbar) has 5 unique species of
    Cichlid, and part of Lake Malawi was dry only a couple of hundred years ago,
    and it too has it's own colour morphs. [The rate of speciation is what
    challenges orthodoxy- not similarities between species]. The authors go on
    [accounting for the speciation rate]:

    'These examples, bolstered by recent DNA data from Lake Tangayika, suggest a
    mechanism for the speciation of cichlids: repeated isolation. It appears
    that successive drops in the level of Lake Tanganyika, by as much as 2,000
    feet, facilitated the formation of Tropheus colour morphs and all the other
    rock-dwelling cichlids. Populations that used to exchange genes instead
    became isolated in small pockets of water. They developed independently,
    coming into contact once again as the water level rose- but could no longer
    interbreed.' (p69)

    A visual diagram showing how close in appearance species from the different
    lakes are carried text, the first line of which is 'Distantly related
    cichlids from Lakes Tanganyika and Malawi have evolved to become uncannily
    alike by virtue of occupying similar ecological niches.' (p.68)

    There's no mystery, no hint of other processes besides natural selection,
    only a highly adaptable organism in a rapidly changing environment with
    multiple niches to exploit.


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