I appreciate Loren Haarsma's comments of 7th August on my earlier
post. The delay in replying is a result of two day's leave.
"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."
There is something useful to discuss here: as limits of some
form ARE recognised within macroevolutionary theory. So, for
example, it would appear that no new phyla have appeared since
the Cambrian explosion, so something is acting to block change
here. Evolutionary radiations seem to apply at the species and
genus level. Transitions at higher taxonomic levels are the ones
which are crucial to answering the question as to whether there
are block-type limits to variation. It is true that empirical data
are over short timescales - but this means we either have to be
clever in designing experimental work, or that we must accept that
our theoretical ideas cannot progress beyond hypothesis.
" ... we need to extrapolate. As we all know, extrapolation is
one of science's greatest dangers. ... When we do it, we need to
justify it."
Agreed. I hope, then, that you will concur with the views
expressed by Phillip Johnson that much of the literature
advocating neoDarwinism is defective - making grandiose
extrapolations from the data we have without the crucial
justification for those extrapolations. When there is agreement
that there is an urgent need to reform educational literature,
we will have made some progress.
"First, there is the genetic data ..."
My own view is that homologies are consistent with both
common ancestry theories and explanations involving creative
design. "Normal science", in a Kuhnian sense, can take place
using such data within both paradigms.
"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 artificial 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?"
This brings us back to Basic Types - where observational
evidence for very close developmental pathways exist. It can be
hypothesised that all the organisms within a Basic Type are
related by common ancestry - despite the variations which, as you
say, go well beyond those accomplished by artificial breeding.
A great gulf separates one Basic Type from any other - and this
is where I would call a halt to extrapolation.
Reference has been made on this Reflector to the writings
of Professor Brian Goodwin: he is one of the few biologists to
spell out the implications of recognising Basic Types in the
living world - and to suggest that much neoDarwinian literature
is failing to address the real issues in biology. I applaud him
for making an effort to redefine these issues and to translate
them into research programmes. However, he is very much a lone
voice here in the UK.
Best wishes,
*** From David J. Tyler, CDT Department, Hollings Faculty,
Manchester Metropolitan University, UK.
Telephone: 0161-247-2636 ***