Re: How do body parts evolve?

On 12 Apr, 23:46, "Perplexed in Peoria" <jimmene...@xxxxxxxxxxxxx>
wrote:
"Jack" <cawo...@xxxxxxxxx> wrote in messagenews:1176408627.572520.131240@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
All of the calculations of evolutionary probability I have seen assume
inherantly that each mutation provides a benefit and is therefore
retained by the advantage it brings (natural selection).

Close. But it would be much more accurate to say that each mutation which
gets 'fixed' in the population provides a benefit. No one denies, of
course, that a lot of mutations are harmful, and therefore are eliminated
by selection.

A more subtle point is that quite a few mutations get fixed by 'drift',
rather than by selection. This may not matter too much for your argument,
which is based on whether advantages can accumulate, but it is very important
for the people who look at DNA sequences and decide what is related to what.

Is there any
robust and proven demonstration that body parts can indeed evolve or
elongate to perform or better perform a desired function?

There is plenty of artificial selection (animal breeding) evidence that
body parts can be elongated or shortened by selection. One famous example
is dachshunds, a breed of dogs artificially selected to hunt badgers.

Evolution of new body parts or organs takes a lot more time, so I don't
have any examples.

Consider a tail-less monkey: A tail would be beneficial but how could
a tail evolve when the first mutation produces no advantage? I don't
mean a small advantage, or a negligable advantage but actually zero
(possibly even a disadvantage). Even with 100 co-incidental
cummulative mutations in the area of tail growth over 100 generations
(vanishingly unlikely) you end up with a small lump - all pain, no
gain.

Actually, you have chosen a bad example here, since the story is that
tail-less monkeys are known to have evolved from ancestors that had tails.
That might mean that your tail-less monkey could evolve a new tail with
just a few mutations. ;-)

And if you change the question to how did tetrapod animals in general
evolve tails, it turns out that tails go a very long way back - back to
when we were still fish!

He's chosen a good example if the exercise is to see how something
secondarily lost can be tertiarily regained. Maybe it can't, and
doesn't happen.

I'm thinking what happens if it becomes useful to have a limb in a
certain place when there once was one there before? It seems that a
replacement structure can arise, but it is unlikely to be a homologous
restoration.

The example being the bottle nosed whale. One was recently stranded
in the Thames, and witnesses were mistaking it for a shark because it
had a dorsal fin shaped exactly like one you would see on a shark.
That is exactly the evolutionary scenario Jack is talking about -
something appearing where there was nothing before, and where a
miniscule instance of such an organ would make no survival difference
at all.

Note how whales have failed to lose some vestigial bones over the same
timescale in which they have evolved a whole new stabilisation 'fin'
de novo.

I can only conjecture that is easier to evolve new sticky-out bits if
they are on the mid line (cf. camels' humps). With the obvious
exception of breasts. So perhaps if an animal with no tail needed to
come up with one, it might make it out of fat, of out of stuck-
together hair like rhino horn.

Not only that, but some animals without tails, including us, actually have
tails at an embryonic stage of development. So the question becomes whether
it is advantageous for an embryo to keep its tail into childhood and adult
life. It isn't just the adult form of the organism which evolves. It is
the entire life history that evolves.

This is not just a quibble. It really does matter. Just about every
example of new organs or new body parts passes through a stage in its
development when even small differences may matter - they may matter a lot!
So, if someone is going to set up as a critic of evolution, they need to
learn a lot of biology just to know whether an example is a good one.

It seems to me that each mutation would have to produce startling
results in order to have any effect on survival which would not be
buried in the noise of other factors (chance, climate).

Yeah, it might seem that way. But it is just not so. Even small differences
in fitness shine through the noise. Over enough generations, they make a
difference. The key is the fact that species population sizes are fairly
large. It turns out that the dividing line between the things that make
enough difference to matter and the things that really are swamped by noise
is pretty close to one over the population size. If there are ten million
of a particular kind of beastie out there, then any difference which saves
a beastie's life once every ten million lifetimes will be noticed by selection
and will (over thousands of generations) become fixed in the population.

But if the population is only ten thousand individuals, then that beneficial
(at the one in ten million level) mutation probably will not be fixed by
selection. There are literally thousands of research papers dealing with
this fact - proving it or using it. Scientists are neither stupid, nor rare.
Objections generated by amateurs like yourself have been thought about by
many generations of people who know what they are doing.


.

Relevant Pages

  • Re: How do body parts evolve?
    ... inherantly that each mutation provides a benefit and is therefore ... retained by the advantage it brings (natural selection). ... a tail evolve when the first mutation produces no advantage? ... cummulative mutations in the area of tail growth over 100 generations ...
    (talk.origins)
  • Re: Behaviorism vs. evolutionary psychology
    ... is: mutation + natural selection. ... laws are primed to evolve. ... What is a specific constraint you have in mind, ...
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  • Re: How do body parts evolve?
    ... inherantly that each mutation provides a benefit and is therefore ... retained by the advantage it brings (natural selection). ... a tail evolve when the first mutation produces no advantage? ... tail-less monkeys are known to have evolved from ancestors that had tails. ...
    (talk.origins)
  • Re: How do body parts evolve?
    ... inherantly that each mutation provides a benefit and is therefore ... retained by the advantage it brings (natural selection). ... Consider a tail-less monkey: A tail would be beneficial but how could ... a tail evolve when the first mutation produces no advantage? ...
    (talk.origins)
  • Re: How do body parts evolve?
    ... inherantly that each mutation provides a benefit and is therefore ... retained by the advantage it brings (natural selection). ... Consider a tail-less monkey: A tail would be beneficial but how could ... a tail evolve when the first mutation produces no advantage? ...
    (talk.origins)