|MadSci Network: Genetics|
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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