MadSci Network: Evolution
Query:

Re: Why is biological evolution considered to work top down and not bottom up?

Date: Wed Feb 25 10:30:16 2004
Posted By: Yvonne Buckley, Post-doctoral researcher
Area of science: Evolution
ID: 1077660690.Ev
Message:

Dear Bruce,
I am a little confused by your question but I'll try and answer what I 
understand you to be asking.
I think you're asking why individual cells in our bodies don't evolve in 
the same time scale as the bacteria and viruses infecting us.  In order 
for evolution to happen there are three conditions which must be 
fulfilled: 1. there is biological variation, 2. these variations are 
heritable and 3. under selection pressure some of these variations are 
more advantageous than others.  Darwin outlined these three conditions 
in "The Origin...".  You are right in that viruses and bacteria evolve 
much more quickly than organisms such as humans with longer generation 
times.  Cells within a human do not however evolve independently.  Each 
cell in our body contains the same genetic code, except for mutations that 
occur within somatic cells, germ cells go on to produce eggs and sperm and 
it is only gene changes in germ cells that are passed on to the next 
generation.  Gene changes in somatic cells are not heritable, violating 
condition 2 for evolution.  So even if one of your skin cells had a 
mutation which made that cell more resistant to infection by a wart virus 
it would make no difference to the germ cells that make sperm containing 
the unmutated gene, so your children would not have greater resistance to 
warts.  If however one of your germ cells happened to acquire a gene for 
wart resistance and that germ cell produced a sperm which went on to 
contribute half of your child's DNA making them resistant to warts, more 
attractive to the opposite sex and more likely to give you grandchildren 
as a result, then we could say that evolution was happening.
I hope this answers your question, I would heartily recommend Richard 
Dawkins book "The Selfish Gene" for further explanation of the levels of 
selection.
Yvonne Buckley

Moderator's Note:
To address the "top down vs. bottom up" part of the question, selection occurs 
in response to all environmental forces, regardless of their position.  
Most organisms are under selective pressure from predators, parasites, 
diseases, environmental changes, etc., such that both "top down" and "bottom 
up" are inaccurate.




Current Queue | Current Queue for Evolution | Evolution archives

Try the links in the MadSci Library for more information on Evolution.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-2003. All rights reserved.