Re: Evolution, "adaptation", and what's currently adaptive

Gerold Firl (
9 Sep 1996 20:54:53 GMT

In article <50o647$>, (Bryant) writes:

|> Some molluscs can (I think octopi were mentioned). But eyes are
|> expensive. Without real fitness benefits to maintain their presence, all
|> the "phylogenetic constraint" in the world won't keep them around, as the
|> study of cave vertebrates shows: mutation accumulation destroys them.
|> (This too is described in _Exploring Evolutionary Biology_.)

|> In article <50ncrt$>,
|> Gerold Firl <> patiently explained his comments about
|> nature's "complete gamut" of eye structures:

|> >Now, before you start strumming your teleological harp, lets get this
|> >straight: the genes of the fish do not "know" that they're in a cave,
|> >or that it's dark. What happens is that eventually a mutation occurs
|> >which produces an eyeless fish, and not only does this new
|> >configuration prove to be non-deleterious, it is actually advantageous
|> >- "adaptive", one might even say.

|> Because of the reduced metabolic investment in a pointless trait, you mean?

That's what I had in mind, but after considering your question, now
I'm not so sure. Why are eyes expensive? I can imagine that there
might be slightly higher caloric requirements for growing eyes than
simple skin, but that seems like a very minor advantage. It seems
quite possible that other benefits of eyelessness are more important.
Some possibilities:

1. DNA integrity/storage requirements: Cross-over will eventually
disrupt the eye sequences, thereby killing the organism when sight is
adaptive. In the cave environment, that constraint is lifted. (Can
this be considered "advantageous", or simply neutral? Probably depends
on whether those areas of the genetic sequence previously devoted to
eye structures get re-used.)

2. Linkage between the eye and vision centers in the brain: if the
presence of an eye necessitates the devotion of some portion of the
brain to vision, then those neurons could be better used elsewhere.
Maybe the eye has to go before those brain cells can be freed-up for
other tasks.

3. Injury, disease, parasite infection: eyes are more vulnurable than
other tissues to external insult; if you go bumping into things in the
dark, eyes are not a good bumper.

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=-=-=-=-=-=-=-=-=-=-=-=-=-=---- Gerold Firl @ ..hplabs!hp-sdd!geroldf