Received: by alpheratz.cpm.aca.mmu.ac.uk id TAA15892 (8.6.9/5.3[ref pg@gmsl.co.uk] for cpm.aca.mmu.ac.uk from fmb-bounces@mmu.ac.uk); Thu, 30 Aug 2001 19:54:48 +0100 Message-ID: <002701c13185$05e83540$9124f4d8@teddace> From: "Dace" <edace@earthlink.net> To: <memetics@mmu.ac.uk> References: <2D1C159B783DD211808A006008062D3101746063@inchna.stir.ac.uk> Subject: Re: Misunderstood Cichlids Date: Thu, 30 Aug 2001 11:53:04 -0700 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.50.4133.2400 X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4133.2400 Sender: fmb-bounces@mmu.ac.uk Precedence: bulk Reply-To: memetics@mmu.ac.uk
> 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.
>
> Vincent
There's no mystery regarding the independent evolution of similar jaws for
exploiting various kinds of food found in each lake. What's mysterious, and
what I alluded to in my reference to this article, is the uncanny
resemblance in color pattern on the scales of the cichlids. It's not at all
clear from the article why living in similar environments would cause these
amazingly similar appearances.
http://bioinf.man.ac.uk/chris_t/images/cichlid_page.gif
(Thanks to Chris to for this link.)
Ted
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