Please explain natural selection...

Stephen Gisselbrecht gisselbr at husc8.harvard.edu
Wed Jun 9 14:22:56 EST 1993


In article <1v4t1s$jth at access.digex.net> stephen at access.digex.net (stephen balbach) writes:
> ...
>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
>species.
>
>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?  
> ...

	Well, the problem lies in the idea that "there is a certain
mice'ness that can not be selectively bred out."  If you bred only large
mice, over time your mice would get larger.  Eventually, they would get
large enough to be sick.  Okay so far.  Now, we have to look at the
relative fitness of various mice.  If you keep only breeding the largest
mice, there is an absolute selection for large size--large animals are
more "fit", under these weird and artificial circumstances, than more
healthy animals.  But healthy animals are obviously going to produce more
progeny and have a higher survival rate than sick ones, so now there is a
selective advantage to any modification that enables the larger animals to
be healthy.  As time goes on, more and more of the population will carry
each modification that arises.  If enough of these modifications
accumulate, then the mice will be able to get even bigger.  Eventually,
even if sperm from your "mega-mice" can still fertilize normal mouse eggs
and vice versa, breeding between the two strains will become impractical
if not impossible. (Imagine trying to breed a chihuahua and a great
dane...)  Voila--speciation, at least by one definition, has occurred. 
Without interbreeding, they'll only drift further apart.

	The point of all this is that there is no essential "mice-ness". 
What we perceive as such is actually a collection of traits which, while
certainly interdependent, are for the most part separable.  New species
arise when a gradual drift in one or some of these traits is combined with
an event that creates separate breeding populations.  It also helps to
remember that "species" is a totally manmade concept; it's not necessarily
relevant to what organisms are actually doing, and you should ignore it if
it's giving you more grief than enlightenment.

	Okay, I think I'm done.  I'm no evolutionist, so tear it to
shreds--I have no ego invested. 

					steve gisselbrecht
					cell & dev. bio.
					harvard medical school



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