next message in archive
next message in thread
previous message in archive
previous message in thread
Index of Subjects
Index of Subjects
Steve Shaw wrote:
> On 20-Oct-06, at 12:57 PM, David & Alison Webster wrote:
>
>>
>> Steve Shaw wrote:
>
>
>> Evolution is a random walk; either in small circles, into greener
>> pastures or off of a cliff (metaphorical cliff in the case of flying
>> animals).
>
> Not sure of the entire metaphor here but I don't think so, for the
> random walk idea (which has a specific meaning as well). The
> direction that evolutionary species adaptations can take is actually
> very limited, depending very much on what was programmed into the
> original species by its particular genetic construction, interacting
> with its local 'environment' during development (epigenetic
> factors). So an insect's compound eye can't just suddenly evolve into
> a camera eye (and hasn't, although there have been successful moves in
> that direction by some day-flying Leps and even by the firefly beetle,
> of previous haiku-fame in the post of Brian's). A couple of families
> of fly, parasites, have gone on to lose their wings, but none has gone
> backwards to re-develop two wings again like those of nearly all the
> other insects, and like the paired-wing dipteran ancestors (possibly
> Mecoptera, scorpion flies). That's how strong developmental/genetic
> constraints can be, but even this is not irreversible in principle.
> As you probably know, mutations have turned up or have been induced in
> the fruitfly _Drosophila_ that can transform whole organs from one
> type to another, for instance from an antenna to a leg, and from a
> haltere back to a second set of wings on the thorax, so called
> homeotic mutations. So there is strong evidence of developmental
> control of transformation at a much higher level, of the sort that
> presumably occurred when an ancestral scorpionfly had its back wings
> converted to fly-type halteres way back in the Triassic, in the
> opposite direction, where they've stayed ever since. Copies of the
> genes involved in this control are found throughout metazoan animals
> and are now much studied (the genes are 'homologous' -- have a common
> ancestor -- rather than being completely identical, and the number of
> copies present in the genome varies).
Hi Steve, Oct 21, 2006
The random element was with reference to changes in the underlying
genetic code which I understand, perhaps incorrectly, to change at
random within the constraints imposed by DNA composition. About 95% of
this genetic code is, if I recall correctly, identical for all organisms
which is presumably a reflection of an astonishingly long half-life and
not proof that some sequences are immune from change.
At the level of expression, the scope of changes that could be
implemented are further constrained by structure & physiology and in
that sense are non-random. The visual model that I find helpful in this
context is the repeatedly branched structure (tree of life if you wish).
All organisms are out on some limb (e.g. compound eyes) and going back
to some previous fork is, I agree, not among the probable options;
remotely possible in the same sense that all water molecules in a glass
of water could suddenly all move in the same outward direction at once
and evaporate in a poof.
These rather spectacular homeotic mutations (_bithorax_ for 4 wings
and 8 legs; _antennapedia_ for a leg insead of an antenna and
_proboscipedia_ for legs instead of mouth parts) are special cases but
are good examples of walking off of a metaphorical cliff. In common with
many modifications of genetic code, these changes confer disadvantage as
opposed to advantage and do not persist. Many modifications apparently
affect subroutines that are no longer called and have no effect at all.
And once in a blue moon some modification, such as yellow and black
coloration, emerges in some isolated population of a generalist fly
species. And because this change confers more advantage than
disadvantage in the specialized situation of prolonged exposure to
predation while working flowers for nectar and pollen, this change will
persist in this isolated population. And those strains of this initially
generalist feeder, that become specialized feeders of nectar and pollen
will tend to reproduce successfully and in time form a new branch. And
because this was a largely underexploited resource this branch spreads
and branches further, becoming in time the assembly now known as the
Syrpidae.
To mutate a spelling and taxonomy slightly, one might say that all
lifeforms belong to the Super-family Serendipidae.
Yours truly, Dave Webster, Kentville
>
>> In the case of relatively large insects, that spend appreciable
>> time, in full view, working flowers (Syrphids and Stratiomyids), is
>> it not reasonable that chance modifications, such that some strains
>> resembled dangerous insects, would confer some survival value ?
>
> Not all syrphids or strats are large, but otherwise it is a perfectly
> reasonable idea so far, and the 2nd and 3rd steps to resemble the
> model even more closely are also reasonable.
>
>> If all flies had taken this turn, then yellow with black would have
>> come to mean 'harmless food' and bees and wasps would have assumed
>> some other coloration.
>
> Good point -- one type of mimicry supposedly is only useful when it is
> a rare commodity; if everybody does it, the insect eating birds learn
> the rule very easily and it loses its usefulness (red-black is also a
> common warning coloration). This suggests why bee-wasp mimicry by
> flies should be uncommon, which perhaps explains why its is rare in
> most families, which in turn points up the question of why it is so
> common in syrphids: I think the original question remains valid.
> Next we ought to mention Batesian and Mullerian mimicry but that's
> getting deeper in... Also, Lep people know about this stuff in
> certain tropical butterflies: I think it is danaids (danaiids?) that
> use mimicr