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Re: Digit Loss



  Just some random thoughts, nothing directed at anyone, nothing
personal....

------

Daniel Bensen (dbensen@gotnet.net) wrote:

<Plates and horns are _far_ simpler structures than digits.
Horns are undifferentiated tissue, whereas a digit needs muscle,
bone, skin, scales, and nerves in the right places. The
mutations necessary for each of these separate factors all grown
together are quite complex, and the chances of them occurring by
chance are slimmer than the chances of digit-deletion.>

  Horns are four-fold or more in structure, at least: underlying
bone, surface blood vessels and soft-tissue, laminar tissue at
the base with skin, and the caratin itself, also strongly
innervated with blood. Horns could probably flush with blood as
much as most ceratin surfaces, as seen even in birds.

<But Ichthyostega's fingers were modified (thickened)
fin-struts, they didn't spring out of nothing.>

  They are not thickened actinofibral bases. This is true of the
panderichthyids. They are in fact composed of knobby, jointed
phalangeal bones, true digits, but probably not terrestrial
load-bearing.

<Each intermediate step has to confer either positive or neutral
effect upon the organism.>

  Each step has a chance of conferring a positive, negative, or
neutral effect. It's iffy, but is three-fold. A neutral effect
is nothing. A positive effect can even be selected against. We
might actually have need of an appendix, but since we are no
longer herbivores, it is unlikely we need this organ. It is not
required, but stays; therefore it is being positively selected
for. Eye color confers no human benefit; however, in a
population, such as nordic Europe, this neutral trait may be
positively selected for in spite of itself.

<For instance, why haven't any vertebrates evolved another pair
of limbs (or legless animals re-evolve them)?>

  Why? After selecting against them, leg-less vertebrates do
well enough that it is not likely limbs are required for
anything. In a python, the prescence of hindlimb spurs are
neutrally selected for in that they do atrophy to enlarge, but
are retained for a functional purpose.

<I can imagine all sorts of advantages inherent in having two
extra hands, or hooves, or claws.>

  I can't. Pardon, but how would these features benefit a
population in a Darwinian or Lamarckian paradigm? In such a
structure where manipulation of tools, development from
arboreality to walking (not running) is selected for as a
benefit to the wandering brain, such features would in fact be
detrimental and negative to the development of the population.
Carnoivores with hooves? Cats with short nails? A six-legged
horse? Why?

<The reason is that the sequence of mutations necessary to grow
another set of limbs is extremely unlikely to spring up by
chance. A single mutation in that sequence may be likely, but
that single mutation may not be useful all by itself.>

  Mutations are aquired as they benefit an organism. The
organism is dead and not aquiring if the mutation is negative in
effect. The mutation rate in a neutral system is null. There is
no mutation.

<So the tyrannosaur lineage stopped using their forelimbs, which
atrophied, then found some use for the two-fingered remnant,
which began to hypertrophy.>

  Arms lack sentience (sorry, couldn't resist); neither do
genes. I think Tracy said this, before. The full equippage of an
arm is not selected _for_, so the arms do not developed at
embryology to the degree they do in, say crocodiles or birds.
The ability to function in spite of this derives a positive
mutation, as something becomes selected for: spreading fingers,
robust humerus and muscular system, etc. So, neutral effect
followed by positive effect, is more likely to persist in a
state where something does not appear to be selected against.
With a perfect, robust arm, this can be considered more
parsimonious than the legs of a snake; which are being selected
against, which can be suggested from the deformed and stubby
form of the elements in question.

----

  Take a monkey and make him hit do a random act to your car. If
this act does not effect the car's performance in any way, then
it is a neutral effect. If this makes the car run sweeter, hum,
purr, and rev like a lion, then it's a positive effect. If the
car blows up, then it is a negetive effect. In either of the
first two cases, have thge monkey do another random act to your
car. Purring, nothing, or explosions, repeat. Now your car is
evolving. Nature is not sentient, and the monkey is your genome.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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