From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: hierarchy and evolution (defining terms)
Date: 6 Nov 92 21:17:25 GMT
Organization: animal -- coelomate -- deuterostome

About a week ago, Larry and I started a discussion of hierarchy in
the process of evolution. This followed an aborted discussion of
hierarchy in the pattern of characters seen in taxa. In a post
(which my news reader has since expired 8-( Larry asks if I was
willing to defend a quote of Gould's. The answer to that is no.

I will briefly outline below what I think hierarchical evolution
is and why I think evolution (the process) actually is, to a great
extent, hierarchical.

Traditionally, the level evolution is thought to occur at is the
population. Frequencies of alleles changed in the gene pool due
to differences in an individuals reproductive success (natural
selection) as well as mutation, gene flow, recombination and drift.

Recently it has been discovered that certain alleles exhibit a
phenomena called segregation distortion or meiotic drive. Driving
alleles exhibit differential reproductive success within the
gamete pool of an individual. For example the t locus (a large
stretch of the 17th chromosome) in mice shows up in 95% of the
gametes of male heterozygotes carrying t. All homozygous t carriers are
dead. Female t heterozygotes do not exhibit drive. Male t heteros
are less fit than "wild type" mice and female t heteros are even less
fit. The t allele remains in populations despite an organismal level
fitness depression due to a gamete level fitness benefit. 

Differential representation in the gamete pool can also be seen
in organisms harboring transposons or B chromosomes (non-homologous
"extra" chromosomes that, in females, nearly always make it into the
egg (not any of the 3 polar bodies) following meiosis.) B chromosomes
are seen mainly in plants I think.

I contend that differential representation of alleles in the gamete
pool is one level of evolution. Differential representation of
alleles in the gene pool is a different level of evolution (one level
"up" from the previous one.) Both of these processes operating is
(in my mind) hierarchical evolution.

I know Larry knows about transposons and t alleles (everyone does).
But, I don't think he buys that they are an example of hierarchical
selection. Larry, do you think there are different levels of
evolution? Hint: can selection act at different levels? If so,
wouldn't demonstrating selection at two adjacent levels constitute
hierarchical selection? Second hint: how is drive modelled mathematically?
How would you build a model combining "normal" autosomal selection and
drive? If you would like I could post one way (that also incorporates
antagonistic sex differences in fitness of the driving allele as 
usually seen in nature) a model could be constructed. (I put together
the above model sometime last month. I'm sure, however, that it has
been done before (it basically just lumps in a couple models right
out of Hartl and Clark). Maybe I can find a published model and we
can discuss it. My model clearly incorporates two different levels
at which selection acts. The first is differential reproductive
success of the driving allele in the gamete pool of the heterozygotes
of the driving sex. The second level is ordinary natural selection
amongst genotypes. I would call this selection at two levels and
hence evolution at two levels. To, this is hierarchical selection.
(If I seem to be repeating myself, I'm sorry... just trying to be
clear.)

Chris Colby 	---	 email: colby@bu-bio.bu.edu	---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
 	--Kurt Vonnegut from "Galapagos"