Imagine a kind of bush that grows outward in all directions from a single
"stem" that is very short. Think of height as representing nichese of
increased optimum organismic complexity and the distance from the point
where the bush branches out in all available directions from the stem as
representing time, so that, at a short period of time after the origin of
live at the stem, there are only a few short branches, but some of them are
horizontal, some are vertical (or nearly so) and some are at in-between
angles.
Since I'm representing complexity *also* by a distance from the baseline, we
have, in effect, two scales overlapping each other, one a polar scale and
one a rectangular scale. On the polar scale, the distance from the center
represents time, but, on the rectangular scale, the vertical distance from
the baseline represents complexity.
The result is that, at any time, there is a hemisphere of outermost niches,
with those near the ends of the hemisphere representing niches in which
optimum organisms are very simple, and those at the top of the hemisphere
representing the most niches with the highest optimum complexity that
organisms have so far reached.
Now, what I'm saying is that, as time progresses, evolution will create
organisms in all available directions from the center point, and a great
many of these will be very simple, and it will keep creating (and,
sometimes, recreating) simple organisms through time, as new niches open and
as niches change. But, it will also, in the upward direction, keep evolving
new organisms of greater and greater complexity, so that, over time, the
entire hemispherical "bush" expands, filling more and more simple-organism
niches, but also working upward into more and more complex-organism niches.
Get this clearly in mind for a moment before going on.
It's not accurate in one important respect: The bush branches in many
directions from each point it has so far reached, so that, simpler forms of
life, even if they are far from the stem, will sometimes "spin off" forms of
life that are more complex (i.e., further upward than they are themselves)
IF a niche opens directly above them and stays open for a long enough period
of time. And, sometimes more complex forms will become simpler if an
especially promising niche opens below them. What organism gets the niche
depends on local conditions and how easy it is for organisms to "move" into
the niche.
But, in general, while all this sort of thing is going on, the *top* of the
bush (like other parts of the outer "surface" of it) are growing further
away from the stem. But, in the case of the top of the bush, this is also in
the direction of greater complexity (assuming that there is some advantage
to doing so). Life does not *necessarily* spread in all directions, but, if
there are niches in external contact with the bush-of-life's "surface,"
evolutionary pressure will push organisms into these niches, whether they
involve increasing complexity or not.
The point is, *some* of them *do* involve increasing complexity. There is no
"magic" about complexity, it's just *one* direction in which there *are* in
fact additional niches for life to fill, so it expands to fill them. There
are also additional niches for *simple* organisms (represented by horizontal
branches near but above the baseline). And, of course, there are plenty of
niches in various diagonal directions.
But, explaining how organisms evolve horizontally does not seem to be much
of a problem to people. The clever thing about evolutionary theory is that
it offers not only an explanation for organisms that are at simple levels,
but also those at increasing levels of complexity. This is why I made the
remark about the point of evolutionary theory and complexity. And, it
doesn't apply to all organisms. This does not mean that evolutionary theory
in general does not apply to them, but merely that the real significance of
the theory is in how it deals with and explains and requires increasing
complexity (under suitable conditions). If someone had come up with an
evolutionary theory that showed how simple organisms could become *other*
simple organisms via some mechanism, it would be nice but not impressive if
it did not *also* explain how less-complex organisms can become, over many
generations, perhaps, more-complex organisms. *That's* where the really neat
stuff happens, that's where information content goes up, that's where it
explains how *we* got here.
Semantic notes:
I don't describe evolution as RM & NS, because "random" is too ambiguous,
and because "mutation" suggests some sort of abnormal event, like changes
wrought by genetic exposure to strong radiation, etc. So I simply call it
variation, no matter how it comes about, though I assume naturalistic
purpose-free mechanisms of various sorts. I replace "natural selection" with
"culling," because there is no actual "selection" going on, except in a
metaphorical sense. But, even "culling" suggests a deliberate process,
which, in standard evolutionary theory, it isn't. "Natural reduction of the
unfit" is more accurate, but too cumbersome. Maybe "variation and natural
culling" would be a better way to describe it, rather than "random mutation
and natural selection."
Think of a situation in which apples come along on a conveyor belt, and it's
someone's job to pick out and discard the bad apples. This is culling. But,
if we think of the person as picking out the *good* apples and putting them
somewhere else, this would be selection. The result is the same, but the
mechanism is different. Culling focuses on characteristics that are
"unsatisfactory," while selection focuses on characteristics that *are*
"satisfactory." Nature's "selection" is more like "culling" because it does
not focus on characteristics that are survival values, but on
characteristics that are *disvalues*, that are flaws (relative to the local
environment). In effect, the environment is watching the conveyor belt of
life go by and it removes those variations that are not "fit" to continue
on.