Thanks to David Tyler for his excellent post on this topic. I hope I can
craft a worthy reply.
DT> [. . .]
> My concern here is WHENEVER SOMEONE
> IS ADVOCATING A MODEL OF THE REAL WORLD, THE ONUS RESTS ON THEM
> TO JUSTIFY THAT MODEL. Where neoDarwinists advocate no limits
> to variability, they must carry the responsibility to defend this
> position. Otherwise, neoDarwinism must remain a hypothesis
> awaiting testing of its fundamental assumptions. Similarly,
> where PCs and YECs propose a model of the animate creation which
> involves limited variability, they carry the responsibility to
> justify that position. My response to Gordon is to say that we
> must start with the data we have - and ask what do we make of it?
>
> Looking back over the messages, I find several specific arguments
> for limits (from myself on 25 July, 26 July and from Arthur
> Chadwick on 2 August. Apologies if I've forgotten other
> contributions). These come under three headings:
>
> (a) Artificial breeding. Breeding without mutations comes up
> against limits (cow milk yields, sugar beet content) and problems
> of non-viability without human help; breeding with mutations show
> that genetic information can be juggled and mixed in many ways -
> but new information (to break through limits) is elusive
> (Drosophila, E. Coli). Mutations have never been thought of as
> a significant part of plant or animal breeding programmes.
>
> (b) Natural breeding. Speciation is possible, but descendants
> of ancestral forms exhibit a limited range of features (ciclid
> fishes, Hawaiian plants of the Compositae family). It is
> possible to group organisms into Basic Types - which have an
> objective link obtained via hybridisation studies. These are
> usually, but not always, at the Family level. Basic Types have
> similar developmental pathways.
>
> (c) Complexity. Complex structures are not able to vary far
> without a drastic loss of function.
>
> Evidences for variation are the common property of macro-
> evolutionists and non-macro-evolutionists. We must object if
> people appeal to evidences of variation without addressing
> alternative models for understanding the data. It seems to me
> that those evidences we do see (summarised above) are best
> understood within a framework of limited variation. I have yet
> to see a neoDarwinian explain in a convincing way that these data
> are indicators of no limits to variation.
Macroevolution does not require "no limits" on variability, only that the
actual limits are broad enough to allow, under certain specialized
circumstances, changes of phyla/class/order in the space of a few tens of
millions of years. That's a mighty tough one to _prove_ when we've only
got observational data for a few thousand years and laboratory data for a
few decades.
The examples you cite of artificial breeding, natural speciation, and
complexity theory -- combined with our increasing understanding of
molecular biology and embryology -- provide us with SOME ideas of what the
"limits on variability" are. We have seen some examples of rapid and
dramatic, though still limited, change. We have seen many examples of
"stability." But since our present data set is quite small compared to
the possibilities inherent in an entire ecosystem over millions of years,
we need to extrapolate.
As we all know, extrapolation is one of science's greatest dangers.
(Perhaps second only to fraud. :-) Unlike fraud, however, it is often
a necessary part of the process. When we do it, we need to justify it.
First, there is the genetic data which supports common ancestry even at
the level of class and order. (E.g. homologies of coding and non-coding
sequences, gene location, cladistic reconstruction, etc.) Results from
developmental biology tend to support this. Of course, this data can be
explained in many versions of progressive creation which include
supernatural actions to overcome "limits in variation." The genetic data
is not incompatible with limits in variation. However, macroevolutionary
theory _predicts_ this pattern of genetic homologies whereas any theory
which limits variation to the family/genus/species level does NOT make
this prediction. Hence, it provides some justification for the
extrapolation.
Second, there seems to be no sharp line between microevolution and
macroevolution. Consider the genetic and chromosomal differences between,
say, sheep and goats, or cows and camels -- which most of us on the
reflector would, I believe, classify as microevolution. Even these are
beyond the "limits" which we have so far induced by articial breeding of
animals. To call these "microevolution" is already somewhat of an
extrapolation. Where then do you draw the line of limits, and how do you
justify where you draw that line?
Thanks again for your post,
--------------------------------------------------------------------------------
In theory, there is no difference between | Loren Haarsma
theory and practice, but in practice | lhaarsma@opal.tufts.edu
there is a great deal of difference. |