Evolution doesn't produce complexity?

Steve McGrew 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 
days).

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 
precursor.

Guru:  If you teach young students that evolution leads to increased 
complexity then this is only one small step from the creationist postion, in 
my opinion. 

*****
Guru:  A single allele *will* become fixed in a population in the absence of 
selection.

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 
recombination.

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 
molecular crossover.

Guru: Recombination is not required in order to exchange genetic information 
via plasmids. 

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?

Steve

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