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

> "Selection pressure" is a perfectly good term for the factors that
> drive directional selection in a particular direction. No, there is
> no physical pressure, but it is quite comparable to the notion of a
> "diffusion force" that pushes material to flow from high concentration
> to low. It is a convenient shorthand to describe factors that cause
> processes to move in a particular direction.

Where "direction" means toward a particular goal, namely fixation of
all relatively advantageous allelles by means of extinction of all
relatively bad allelles at the same sites, ignoring all neutral sites,
right? (Caveat: Not a *teleological* goal!! Just an "attractor" or
"sink". If you turn the "fitness landscape" upside down, so you're
flowing toward a sink rather than climbing toward a peak, it makes more
sense. We already use this metaphor in thermodynamics, where we talk of
something falling down a potential well, or being jostled over a wall
by thermal energy, or tunneling through a wall if quantum mechanics is
applicable, although perhaps tunneling is really nothing more than
thermodynamics of vacuum virtual particles as with Hawking radiation.)

With diffusion, we talk about "pressure", but we might equally talk
about "gradient". There's a density gradient across a permeable
membrane, which causes particles to tend to travel from the more-dense
region toward the less-dense region, i.e. travel "down-gradient". It's
just that the membrane slows down such diffusion, compared to
free-fluid diffusion which is unimpeded. (OT: The funny thing about
permeable membranes is that they often allow only certain sizes or
types of particles to pass, so if you put one type of particle on one
side and another type on the other side, it's only the *partial*
pressure of the passible particles that matters, not the *total*
pressure in the fluid, so osmosis can actually build up a *total*
pressure difference across a membrane by equalizing the partial
pressure of passible particles while *not* equalizing partial pressure
of the non-passible particles.)

However it occurs to me that the whole metaphor of flow or pressure or
hill-climbing etc. here is wrong, because we're talking about change in
statistics of populations, not change in characters of individuals. The
individuals don't climb the hill (or flow toward the sink), only the
averages do. Lines of descent don't (in absense of new mutations) climb
the hill (etc.). Lines of descent either survive or don't survive,
either fan out to lots of descendents or stay narrow with only very few
descendents or none at all. The so-called "fitness landscape" which
life "climbs" over billions of years refers to individuals, i.e.
individuals nowadays are mostly higher up the hill than their distant
ancestors were, because surviving individuals nowadays are because of
lines of descent that happened to be so fortunate as to get new
beneficial mutations from time to time. When we talk about fixation of
a good allelle, or extincation of a bad allelle, or selection pressure,
we're merely talking about massive replication of individuals already
high on the hill and massive deaths of individuals already low on the
hill, not anybody actually moving from a low point to a high point.
Selection pressure moves the centroid of the population up the hill, by
killing of lows and replicating highs, but doesn't move any individual
or any line of descent. It's only a new mutation, or recombination of
good allelles that didn't previously appear within the same individual,
that establishes a single individual even higher up the hill than ever
before, thereby bootstrapping selection-pressure to the ability of
eventually shifting the centroid toward that new "base camp higher up
the slope of Everest".

Maybe an amoeba climbing a gradient (such as toward some food that it
can "smell" off in some direction, stronger smell at one end of the
amoeba than at the other end, hence food must be in the direction where
the smell is strongest) is a better metaphor: An amoeba moves by two
processes that somewhat alternate: First, the amoeba feebly extends its
cell membrane further along the path to its target, with only a tiny
bit of protoplasm moved into the new/extended pseudopod. Then the
amoeba flows the bulk of protoplasm from its rear toward its front
end, shifting the center of mass from where it was before to where it
is now closer to the goal. Eventually some extremely rear pseudopods
are completely drained of protoplasm and are withdrawn. Extending a new
pseudopod represents new mutations and recombinations that never
occurred before, while flowing protoplasm represents shifting the
statistics away from less-fit phenotypes toward most-fit-so-far
phenotypes, and withdrawl of rear-empty pseudopods represents
extinction of the least-fit allelles/phenotypes. Because fitness
involves simultaneous expression of many genes, with fixation or
extincation of each gene independently, but selection based on the
"whole ball of wax" rather than individual genes per se, and it's only
single allelles not phenotypes that can become extinct, i.e. old
phenotypes can re-appear due to recombination, the amoeba metaphor
isn't exact. But for asexual life, where there's no recombination so
it's impossible for old phenotypes to re-appear after they're gone
(except by very unlikely chance reverse mutation), and where again
there's no recombination so new phenotypes appear *only* by new
mutation, it may be very close to an exactly correct metaphor. Perhaps
when explaining evolution to beginners, we should explain only the
asexual case at first, using the amoeba metaphor, and then only after
it's well understood we move to the much more complicated sexual case?

OK, now I have a good metaphor for the sexual-reproduction case:
Ultra-conservative family values and housing market: When a young
couple gets married, they usually buy their own house, near one or both
of their parents' houses, but they don't live in either of their
parents' houses. Once they buy a house, they *never* move, and *never*
divorce. There is never any motion of an individual up the fitness
landscape, and children are always similar to their parents but not
identical to either. A community moves up the fitness landscape not by
moving houses or people moving to a new house, but only by bearing
young which establish new households in new locations. Those young
couples who happen to establish homes where there are no jobs, starve
to death (no welfare to bale them out, and parents already drained of
resources), whereas those young couples who happen to establish homes
where there are fine jobs have plenty of money to support large
families. As a result of differential jobs and differential family
size, the center-of-mass of communities tends to miagrate toward where
the best jobs are located. Getting married and buying a new house near
where their parents lived represents meiosis. Making babies within a
single household, and raising them until they're ready to go find a
mate, represents mitosis. Meiosis happens only once per generation.
Difficulty finding and getting together with a mate who lives far away,
and logistical difficulties of visiting relatives who live too far from
each other and from your new home, represent reproductive barriers that
tend to divide species into "ring species" and eventually into totally
separate species.

Am I now the master of metaphors?

(Splitting reply here, as I shift to a meta-topic raised by the turkey
known as "r norman".)
..

.

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