MadSci Network: Genetics
Query:

Re: What genetic changes take place in a species as their environment changes?

Date: Sun May 2 18:35:15 2004
Posted By: Gabriel Vargas, Assistant Adjunct Professor
Area of science: Genetics
ID: 1080792657.Ge
Message:

Classical Darwinian natural selection theory would stipulate that 
environmental change would put selective pressure upon the genetic 
variants already present.  There are no genetic changes that happen per 
se of the environmental changes.  There is selective pressure which would 
lead over time to selection of favorable characteristics which give the 
organisms that possess those favorable features a selective advantage.  
Letís for example take the hypothetical case of the ancestor of the 
giraffe.  If during a period of environmental change, the trees which 
ancestral giraffes fed from lost their lower leaves, there would be a 
selective pressure for those ancestral giraffes who have longer necks to 
survive because those creatures would be more able to feed, survive and 
procreate.  Thus it is not that environmental changes bring about genetic 
change but that the environmental change leads to an increase in 
selective pressure for those particular genes already present that led 
some of the ancestral giraffes to have longer necks.  Increased selective 
pressure over many thousands of years would then lead to genetic change 
whereby these genes which were present initially in a few individuals are 
now widespread and there is now a new species-the giraffe.  The major 
point is that selective pressure for a particular characteristic over 
time leads to changes in gene frequencies over the population as a whole 
but those genes were present initially.  Natural selection must have 
variety from which to select from.  If all the ancestral giraffes were so 
short that they were unable to feed from the taller trees they would have 
gone extinct.

hope this helps
gabriel vargas md/phd

References:
George C. Williams Adaptation and Natural Selection


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