Re: "Macroevolution" from Re: P.Johnson on James Dobson

David Campbell (bivalve@mailserv0.isis.unc.edu)
Fri, 19 Nov 1999 10:24:31 -0500

>In a message dated 11/18/1999 9:26:48 AM, bivalve@mailserv0.isis.unc.edu
>writes:
>
><< Macroevolution has become popular among those rejecting common descent as a
>term for "the biological evolution I do not believe in". In
>paleontological use, it is generally used to refer to biological
>evolutionary processes active at or above the species level, especially any
>that are different from ordinary population-level biological evolution. >>

>Dave,
>
>My questions about macroevolution are not simply a matter of belief. You do
>me an injustice. I don't reject common descent at lower taxonomic levels,
>following the analysis of monophly by Malcolm Gordon. I challenge it,
>however, at the higher taxonomic levels of kingdom, phyla, class, as does
>Gordon. And it is at these higher taxonomic levels that macroevolution, or
>more specifically natural selection, is required to operate. Where is the
>evidence that it did?

True, you are not simply using the phrase "I don't believe in
macroevolution". However, I am not certain how you are defining it. Is it
universal common descent or the ability for natural selection to bring
about major changes or what?

Actually, I think your views could be described as believing in
macroevolution-that there are factors other than the everyday factors like
natural selection and mutation at work in the evolution of higher taxonomic
categories.

>You wrote: "Except for paleontology and molecular systematics. There are at
>least
>transitional fossils between many classes, orders, family, genera, and
>species; depending on the definition of phyla, some forms may be
>transitional between them. Molecular and subcellular structural
>similarities point to universal common ancestry."
>
>My comment: I don't question transitional forms. I question natural
>selection as the mechanism accounting for them, as did Mivart. And I note
>that you do not include kingdoms and phyla in your common ancestry.

Transitional forms imply common ancestry (whatever the mechanism of
bringing about transition), and urkingdoms, kingdoms, phyla, and everything
else is included in universal common ancestry.

Natural selection can give a partial account for transitional forms, in
that they are better adapted than the ancestor for the niche they are
transitioning into. The variation must arise by mutation if it is going to
persist (unless the organism can transmit learned behavior). The fact that
a particular transitional form survives and goes on to evolve into the new
form might be largely attributed to natural selection.

>You wrote: "Natural selection does not try to account for incipient
>structures (except in the case of exaptation, the use of a structure that
>evolved for some
>other purpose). It assumes variation exists and proceeds from there.
>Incipient structures are explained by mutation."
>
>My comment: Natural selection has from Darwin's day onward consisted of two
>parts, internal variation and _selection by the environment_ as you know.
>Yet you have dropped the essential step, selection. Dawkins illustrated the
>need for selection which he called _cumulative selection with his monkeys at
>the typewriter illustration. You are getting rid of Mivart's dilemma simply
>by defining away the need for selection. I don't buy that, Dave.

My comment must not have been clear. I was defining internal variation
away from selection, not defining away selection. Variation is assumed,
not explained, by natural selection. Selection then occurs on these
variants. There are indirect ways in which natural selection can promote
increased variability, but no direct way for it to induce a particular
incipient feature. Once a feature has appeared, selection can act on it to
promote or suppress it.

There are at least three ways in which selection can promote variability,
which might in turn give rise to new features.

First, if predators are effective at forming a search image, variability
helps prevent effective predation. A simplified hypothetical example would
be predators that used color pattern to locate prey. A species with a
single color patterns could be an easy target. However, if there was a
rare mutation in color pattern, these individuals would be overlooked by
the predator and become more abundant. As this form becomes dominant,
hungry predators may learn to look for this form instead (or a formerly
rare mutation in the predator may become more abundant). This seems most
likely to work on low levels. Shell banding and coloration in land snails
is one well-studied example of this.

Secondly, selection is most intense if there is a lot of competition. If
conditions are very easy, even detrimental mutations can survive. Some of
these mutations (or mutation from those mutations) might turn out to be
useful in other settings. On the other hand, very difficult settings may
prevent competition because everything is simply trying to survive. Again,
a mutation may persist enough to encounter favorable selective conditions,
even though under ordinary conditions it would be outcompeted. Both of
these patterns seem to fit the fossil record and lab experiments.

Finally, there may be some genetic feedbacks related to stress. Heat-shock
proteins help promote stability of various molecules, but are also used in
emergency management (hence the name). If the heat-shock proteins are all
being used to deal with stress, the molecules normally stabilized by them
may assume different configurations. In other words, previously hidden
mutations may be revealed and available for selection to act upon.
Similarly, one possible mutation that persists under particularly benign or
hostile settings may be a mutation in DNA copying, repair, or error
checking. Such mutations promote high mutation rates, which would increase
the likelihood of new features appearing.

Conversely, it is possible that selection has generally promoted a decrease
in variability and mutation rates. Measuring variability remains
contentious, and we do not have old enough fossil DNA to be very
informative in testing this, but some models do seem to correspond with the
observed pattern. At the first appearance of a major innovation,
variability is probably more advantageous because of the many opportunities
and the high probability of finding some open niche. However, as niches
are filled, it becomes more advantageous to be good at one thing and
outcompete any newcomers. Also, especially in multicellular organisms,
high genetic instability can be highly detrimental (cancer). An additional
factor is the evolution of genetic feedback systems. Greater interaction
among genes promotes greater stability and makes variation more costly.
Such patterns would account for the high variability seen early in many
lineages, followed by greater stability (e.g., late Precambrian to Cambrian
diversification of animals or the high variability in early post-extinction
faunas).

David C.