Received: by alpheratz.cpm.aca.mmu.ac.uk id OAA13084 (8.6.9/5.3[ref pg@gmsl.co.uk] for cpm.aca.mmu.ac.uk from fmb-bounces@mmu.ac.uk); Wed, 29 Aug 2001 14:10:32 +0100 Message-ID: <2D1C159B783DD211808A006008062D3101746063@inchna.stir.ac.uk> 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 X-Mailer: Internet Mail Service (5.5.2650.21) Content-Type: text/plain; charset="iso-8859-1" X-Filter-Info: UoS MailScan 0.1 [D 1] 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
-- The University of Stirling is a university established in Scotland by charter at Stirling, FK9 4LA. Privileged/Confidential Information may be contained in this message. If you are not the addressee indicated in this message (or responsible for delivery of the message to such person), you may not disclose, copy or deliver this message to anyone and any action taken or omitted to be taken in reliance on it, is prohibited and may be unlawful. In such case, you should destroy this message and kindly notify the sender by reply email. Please advise immediately if you or your employer do not consent to Internet email for messages of this kind. Opinions, conclusions and other information in this message that do not relate to the official business of the University of Stirling shall be understood as neither given nor endorsed by it.=============================================================== This was distributed via the memetics list associated with the Journal of Memetics - Evolutionary Models of Information Transmission For information about the journal and the list (e.g. unsubscribing) see: http://www.cpm.mmu.ac.uk/jom-emit
This archive was generated by hypermail 2b29 : Wed Aug 29 2001 - 14:15:10 BST