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Please explain natural selection...

stephen balbach stephen at access.digex.net
Wed Jun 9 09:45:16 EST 1993

I am reading _Blueprints_ by Maitland Edy and Donald Johanson (_Lucy_) and
I don't think I quite understand the concept of natural selection.

I understand that when a mutation in an animal forms that mutation can be
carried by many generations but not be readily apparant thus explaining
why blending does not occur.  And I understand that when a trait forms
that is in some way advantageous to an animal it may become dominate within a

However one could conclude from this that if I bred only large mice that
over time my mice would become larger and larger - but eventually the mice
will only get so large before they become sick, or sterile or still born -
there is a certain mice'ness that can not be selectivly bred out.

So how does nature form entirely new animals?  I understand natural
selection to the point that it favors certain traits under certain
environmental conditions, but it does not seem to explain how entirely new
species are formed?


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