Evolution doesn't produce complexity?
stevem at comtch.iea.com
Sat Jan 11 12:03:09 EST 1997
Has the Earnest Student in the following conversation chosen a false Guru?
How are the two failing to understand each other?
Guru: The vast majority of evolution is probably not due to natural selection
(unless you define evolution as natural selection - but nobody does that these
Earnest Student: You have defined evolution as "change in allele frequency",
but you have said that allele frequency is defined as the histogram made by
counting the various allelles in the population for each gene. So, your
definition of evolution would exclude a *redistribution* of alleles that
leaves the histogram unchanged. A quantitative definition of evolution or
rate of evolution should take account of changes in the *distribution* of
alleles, not just the histogram of allele frequencies.
Guru: The current definition of evolution is "change in allele frequency".
Are you claiming that the last generation of biologists were so stupid
that they made up a useless definition?
Earnest Student: I am looking for terms to distinguish between "random walk"
type genetic drift and the type of genetic change that leads to complex
organisms. A random walk, without natural selection, will never in a trillion
lifetimes of the universe produce even a nematode from a single-celled
Guru: If you teach young students that evolution leads to increased
complexity then this is only one small step from the creationist postion, in
Guru: A single allele *will* become fixed in a population in the absence of
Earnest Student: I thought I understood genetic drift, until you said that in
a population not subject to selection or recombination a single allele will
become fixed. You can't maintain a small population without applying *some*
sort of selection! When you say, "A single allele *will* become fixed in the
absence of selection", you must not be counting random sampling as selection--
or you must be assuming that I am not counting random sampling as selection.
Guru: It never occurred to me that you would consider drift to be an example
of selection. It will never occur to any other scientist either. It seems to
me that you are unfamiliar with genetic drift. Drift will always lead to
fixation of one or other of the various alleles, it takes a long time in large
populations but can occur very quickly in small populations.
Guru: Do you think that chromosome segregation and sorting is the same as
"recombination"? Recombination refers to the physical exchange of information
between homologous DNA molecules. Your statement doesn't make sense to me
using my definition of recombination - in fact it seems to be way off base.
Earnest Student:Yes, chromosome sorting and segregation is certainly a type of
Guru: No, it is not. At least not in any scientific textbook I ever saw.
Earnest Student: My working definition of "recombination" has been any process
that forms a new genotype by combining some of the genetic information from a
parent with genetic information from another source. It would seem very
artificial and restrictive to limit our concept of recombination only to
Guru: Recombination is not required in order to exchange genetic information
Earnest Student: I would think genetic information exchange via plasmids *is*
a type of recombination in the broad sense.
Guru: It is not recombination in any sense. I write textbooks on this stuff
and I've never heard of such a definition.
Earnest Student: Please tell me if I understand you correctly: When you
referred to the rate of evolution, you meant the rate of change of a histogram
of allele frequencies in a population. The way to represent this rate
mathematically would be as a vector, where each element would represent the
rate of change of a specific allele. You think of "genetic drift" as the
variation of the histogram with respect to time, driven primarily by random
errors in replication at the single-nucleotide level and by elimination of
large numbers of randomly selected individuals from the population by causes
unrelated to fitness. If we call the locus of the population in the
histogram's vector space the "evolutionary path" of the organism, then you
would say that selection according to fitness (as opposed to the random
selection occuring in genetic drift) has a negligible influence on the
evolutionary path. Do I have it right?
Guru: [angry silence]
Earnest Student: I'm sure you would agree that selection operates at many
places in the life cycle of an organism: competition between sperms, ability
to form a viable zygote, ability to repair mutations, ability to make a few
errors in replication, ability to got through all the stages of embryo growth,
ability to survive right after birth, ability to live to reproductive age,
ability to attract one or more mates, ability to attract high fitness mates,
ability to produce offspring with a mate, ability to recognize others of the
same species, AND, the simple good luck not to be one of those individuals
that has to be eliminated in each generation to keep the population small
enough to fit into its available niche. Aren't all of these are accompanied
by processes that fall within the broad definition of selection ?
Guru: [breaks off the conversation in disgust]
The Earnest Student wonders if the whole conversation was just a bad
dream. Why did the conversation go sour?
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