Re: Part 1 (of 3): What are major aspects of evolutionary theory?

On Sun, 11 Dec 2005 23:19:03 -0800, anon1@xxxxxxx wrote:

>> Not every cell reproduces.
>
>Agreed. But in an asexually-reproducing strain, virtually *every* cell
>that accumulates sufficient resources to reproduce, does in fact
>reproduce. It's extremely rare that a strain of asexual critters that
>has reproduced for billions of years would suddenly fail to reproduce
>(due to a mutation that affects DNA replicase or some other mechanism
>needed only for reproduction).
>
>And in a multi-cellular animal or plant, the fact that some somatic
>cells are programmed *not* to reproduce is irrelevant to what happens
>to in the gonads.
>
>And the fact (or urban legend) that in women's ovaries, only a fixed
>number of eggs are made, and then the cells in the ovaries completely
>stop reproducing, is pretty much irrelevant to the question as to
>whether the woman as a whole makes babies, because there are in general
>**plenty** enough eggs to last through a normal life before disease or
>starvation due to dental loss ends the life. (Human women getting a
>career and using birth control to postpone making babies to avoid
>interfering with career advancement, then living past menopause without
>having yet started a family, is somewhat of an exception compared to
>most of nature. Generally a female animal either makes lots of babies
>before menopause, or doesn't live that long.)
>
>And the fact that sometimes a man or a woman can't find a mate, so
>makes lots of eggs or sperm but never gets achieves fertilization, is
>irrelevant to the question of whether they make the eggs or sperm in
>the first place. At a cellular level, the ovaries etc. do indeed make
>all the haploid half-babies as usual. When ocean-dwelling critters shed
>large cloudes of sperm in the ocean, which drift around looking for
>eggs, it's hard to say whether they should have counted as "already"
>done their job or not.
>
>To keep it simple, I'll stick to asexual strains such as bacteria,
>where it's clear that virtually all cells that get fully fed to where
>they have resources to reproduce, *do* reproduce, and those that don't
>get resources simply live in stasis indefinitely until they either get
>killed off somehow or they finally achieve resources to allow
>reproduction. So the thing that varies is whether they live long enough
>and get enough resources to reach the point where they are ready to
>reproduce, not whether upon reaching that point they actually do
>reproduce. I'd attribute failure to reproduce *not* to reproductive
>failure per se but merely to death at an earlier point in the life
>cycle. Summary: Every cell that reaches that point in life cycle, does
>indeed then reproduce. The stochastic element appears elsewhere in the
>life cycle, not at the instant of reproduction.
>
>> The ability to pass an allele to the next generation depends on
>> survival (which is generally stochastic)
>
>Yes, but that was earlier in the life cycle.
>
>> and whether or not the individual reproduces (which is generally
>> stochastic)
>
>No. If the cell survives to that point, it *does* split into two.
>
>> and how many offspring are produced (which is generally stochastic)
>
>There are always exactly two daughter cells after prokaryotic fission.
>
>> and whether each offspring itself is capable of surviving to
>> reproductive age (which is generally stochastic).
>
>That's *after* reproduction has already occurred successfully. To
>rewrite history by saying that it never reproduced in the first place
>is a lie.
>
>> If the cell is eukaryotic and diploid and reproducing sexually via
>> meiosis and fertilization, then you have even more probabilistic
>> factors to worry about.
>
>If a cell reaches the point where it's ready to undergo meiosis, it
>generally *does* do that, successfully virtually 100% of the time. At
>that point is *has* reproduced in the sense of making sperms or egg.
>Generally there's a surplus of sperm, so only the eggs count when
>deciding success or failure of fertilization, and generally at least
>one sperm does indeed find the egg, so fertilization runs to completion.
>(In two clades of eukaryotes, Animalia, and Plantae, it's a little more
>complicated sometimes. Let's skip those special cases, OK? In all the
>other clades, it's like I said.)
>
>> Life is uncertain.
>
>I agree. Life, the 99% of the life cycle when reproduction isn't
>happening, is uncertain. But the reproduction itself, either
>prokaryotic fission, or simple non-animal non-plant sexual
>reproduction, is pretty much a sure thing if life gets to that point in
>the first place.
>
>Now if you conflate the 99% of life cycle, and the 1% of reproduction
>itself, into a single calculation of reproduction of the daughter cells
>at exactly the same life-cycle point as we started the calculation but
>exactly one generation down the line of descent, *that* conflated
>calculation is indeed stochastic. But the stochastic part of it is the
>life, not the reproduction. But if that's what you meant to be talking
>about, you should have made it clear up front, instead of using the
>word "reproduction" all by itself and claiming *that* was stochastic.
>
>> The only way to describe it accurately is with probabilities.
>
>If by "it" you mean the entire life cycle, all the way around the cycle
>exactly once, back to the same phase you started but now one generation
>later, then I agree.
>
>> I do believe you and I are talking about very different universes. I
>> claim mine more closely resembles the world of biology.
>
>I believe you are screwing up the language to make it look like you are
>saying something other than what you intend, and what you *say* is
>wrong, but what you secretly believe is the same as what I say, but
>unfortunately you failed to communicate that. I believe in the life
>cycle of a bacterium, most of the deaths which occur are *not* at the
>moment of reproduction (making daughter cells), but are somewhere else
>in the life cycle, and likewise most of the extended delays are not due
>to something wrong with reproduction itself, but due to something
>earlier in the life cycle where not enough nutrients were built up to
>be ready yet to reproduce, so it never gets to the point where
>reproduction would occur, not that reproduction itself fails.
>.

I still believe we are talking about two very different universes,
only one of which resembles real biology.

Take a steady-state ecosystem including, say, one million asexually
reproducing organisms, whether bacteria or amoeba or whatever. Wait a
year after which time these organisms will have gone through many cell
divisions. Because the ecosystem is in a steady-state, you will end
up with one million organisms. Trace the ancestry of these one
million back to the original population and you will get a tiny
fraction of the original successfully reproducing.

The fact is that most systems are usually in something close to a
steady state. As Darwin (and Malthus) noticed a few years ago, the
propensity to procreate far exceeds the reality of procreation. So
your very lengthy analysis of how mitosis works and how many
unfertilized eggs and sperm exist and everything you say above is
completely meaningless for evolution. The only thing that matters is
having ones particular alleles passed down to future generations.

At the end of your screed you do say ">If by "it" you mean the entire
life cycle, all the way around the cycle exactly once, back to the
same phase you started but now one generation later, then I agree".
The only thing that makes sense biologically in reproduction is the
entire life cycle, from adult organisms in one generation all the way
to adult organisms in the next generation. There are events that
occur constantly throughout that complete cycle that are either random
or depend on so many uncontrolled and external factors that they are
best described as random. On the other hand, the probability of
successfully completing the cycle also does depend on the phenotype
and genotype of the original parent. Hence, drift plus selection in
the fact that different genotypes have different probabilities in
their ability to successfully reproduce (going the full cycle). The
probability distribution, defined over the space of genotypes, is
non-uniform.


.

Relevant Pages

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  • Re: Questions about the life and death of prokaryotes
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  • Re: Part 1 (of 3): What are major aspects of evolutionary theory?
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