> Derek McLarnen wrote:
>
> > Stephen Jones wrote:
>
> > > Dawkins is right that naturalistic evolution can only
> > > happen by Neo-Darwinian mechanisms.
>
> > A bit simplistic, but essentially right.
>
> Since all three of you agree on this, could one of you
> elaborate a little? What are Neo-Darwinian mechanisms?
> Which mechanisms are excluded?
What I did after I wrote the following, but should have done
before, was to read some of your previous correspondence to
get a feel for your level of knowledge. Consequently, I fear
that I may have written "below" you. I have not changed it,
however, since someone else may benefit from the style I
have used.
Neo-Darwinian evolutionary theory claims that the most
significant cause of evolution is the adaptation of
populations of living organisms in response to environmental
changes. It further claims that adaptation is a result of
natural selection acting differentially on phenotypically
distinct members of a population, i.e. individual organisms
with inherited physical and genetic characteristics that
allow them to pass on the maximum number of their genes to
succeeding generations.
Underlying this is the theory that genes exist only to
replicate. Those genes that are best at replicating
themselves into succeeding generations will obviously appear
most frequently in succeeding generations.
Now let's look at the mechanisms, starting at the genetic
level and working towards the population level.
At the genetic level we are looking for those mechanisms
that generate genetic diversity. Mutation of genes
(substitutions, transpositions and insertions of base pairs)
is the most commonly quoted of these mechanisms but it is
probably the least frequent to occur. Much more common are
chromosomal changes (deletion, duplication, inversion and
translocation of chromosome parts). More common still are
"crossing-over" events that occur during mitosis and
meiosis. These serve to shuffle the genetic code into new
combinations. Occasionally, whole chromosomes will split or
combine. All of these mechanisms promote genetic variability
within populations which, in turn, generates the phenotypic
variability on which natural selection can act.
Natural selection is a higher-level neo-Darwinian mechanism.
(I have included sexual selection, kin selection and
species selection in the more general term of natural
selection. Some may argue that species selection is not
neo-Darwinian and I don't feel strongly enough about it to
argue against them.) It is simply that in an enviroment with
limited resources (almost all natural environments), those
individual members of a population best able to utilise
those resources will, on average, live longer, be healthier
and generate more fertile offspring carrying their genes
than members of the population less well able to utilise the
environment's resources.
If the characteristics that enabled the individual to
utilise its environment's resources are genetically
inherited rather than environmentally acquired, then those
characteristics will show up more frequently in succeeding
generations of the population, simply because, again on
average, the individuals that have those characteristics
will pass them on to more offspring.
Other high-level neo-Darwinian mechanisms are allopatric,
parapatric and sympatric speciation, coevolution and
extinction. Note that examples of sympatric speciation have,
as far as I am aware, not yet been found in nature.
Now let's look at some mechanisms that affect evolution but
are not neo-Darwinian. You should note that, while these
mechanisms have an enormous impact on the direction and
tempo of evolution, they are not actually evolutionary
mechanisms, but the mechanisms that are at work when
evolution is *not* happening in a population.
Stasis is a good first example since it is so prevalent in
the fossil record. This is simply evolution at its minimum
extreme, occuring when there is little or no genotypic or
phenotypic change in a population. The environment may
provide practically unlimited resources. The population may
be maximally adapted to the environment which is stable. The
population may be less than maximally adapted but the
genotype changes that would enable it to be better adapted
have not occurred, or are not possible from the existing
genotype.
Another non neo-Darwinian mechanism is habitat tracking. The
standard neo-Darwinian response to a changing environment is
that the populations inhabiting that environment either
adapt or go extinct. However, the proponents of punctuated
equilibria have shown that the most common response to a
changing environment is that the populations affected simply
move or spread (if they can) to a nearby environment that
suits their existing adaptations.
Genetic drift is also arguably not a neo-Darwinian
mechanism. This is best seen in the comparison between two
non-interbreeding populations of the same species living in
very similar environments. It will be noted, over time, that
the populations will start to vary from each other, not in
response to environmental imperatives, but simply as a
result of the accumulation of different random mutations and
chromosomal changes that, while they may affect appearance,
do not significantly effect environmental fitness.
All non-Mendelian inheritance mechanisms (including
Lamarkian inheritance) are not neo-Darwinian, nor are they
known to occur.
Another mechanism that is not neo-Darwinian, but is
specifically relevant to this Reflector, is supernatural
selection. The claim is that a supernatural entity (claimed
to be "God" among this group, but probably claimed to be
Allah by a similar Islamic group) is and/or has been
supernaturally directing evolution to some divine purpose.
The commonly assumed purpose is usually the creation of
humankind. This is a hypothesis for which no testable
supporting evidence has yet been forthcoming, though it
continues to be widely believed.
There are also a number of supernatural creation hypotheses
which are neither neo-Darwinian nor even evolutionary, so I
see no need to describe them further.
What I have written here is incomplete and may not be up to
date with current thinking. However, your request and my
response may generate enough additional responses that we
will both end with a better understanding of what are and
aren't neo-Darwinian mechanisms.
--RegardsDerek
-----------------------------------------------------| Derek McLarnen | dmclarne@pcug.org.au || Melba ACT | derek.mclarnen@telstra.com.au || Australia | | -----------------------------------------------------