"Robert Karl Stonjek" :
> Each species had its own characteristic type of web. But the scientists
> found that in several cases, separate species of Tetragnatha spiders on
> different islands constructed extremely similar orb webs, right 
> down to the
> number of spokes, and the lengths and densities of the sticky spiral that
> captures bugs.
> 
> Was this an example of similar environments producing the same complex
> behaviour, or did the spiders with corresponding webs share a common
> ancestor?

JE:-
This problem is known as
analogy Vs homology. Analogy refers to
similar complex traits that have evolved
from unrelated individuals while
homology defines different structures 
that are closely related, e.g. fins, limbs
and wings. In the spider web example
"similar environments producing the same complex
behaviour" in unrelated species may provide
a good example of analogous evolution. 

The classic case of analogy is the
eye of the octopus (invertebrate) and 
the mammal (vertebrate). It can be
proven that both eyes evolved
independently from different 
embryological tissues. However
both eye types have an iris, 
lens and light receptor cells
connected to a brain using the optic 
nerve. In the octopus eye no blind
spot needed to exist because all the 
light sensitive cells point the correct
way: towards and not away from, the 
lens. In the mammal vertebrate eye 
these cells evolved facing the wrong way 
so a hole was required to allow the nerves
from each light sensitive cell to connect 
to the brain via an optic nerve.
The analogous eye structures within these
independently evolved eyes resemble each other 
very closely.

It seems if you want to build an eye
not many options exist. It also appears
god had an off day (or was just plain
bored) when he/she/it designed the vertebrate
eye :-) These unrelated but very similar forms 
of eyes may demonstrate just one of a finite
number of possible eye types can evolve.
OTOH all eyes probably evolved from just 
the one ancestor: a simple depression that 
contained light sensitive cells. In 
this most primitive sense, all eyes are 
homologous.

I think that that large number of
cases of demonstrated analogy supports 
the view that life on earth would have 
evolved about the same way even if it 
was restarted and made to evolve 
itself all over again. 

Regards,

John Edser
Independent Researcher

PO Box 266
Church Pt
NSW 2105
Australia

edser{at}tpg.com.au
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