MadSci Network: Evolution
Query:

Re: Why do humans show so much of the 'white' of their eyes?

Date: Wed Mar 25 13:02:48 1998
Posted By: Brian Foley, Post-doc/Fellow Molecular Genetics
Area of science: Evolution
ID: 890107704.Ev
Message:

	People often get the idea that every organism alive
today is "optimally fit" and that every feature of every
organism has been rigorously selected for.  On top of
this, some people go further to think that each feature
has a well thought out plan, and a purpose.
	In reality, as far as we know, organisms are not
necessarily "optimally fit" today.  They are fit enough
to survive, and that is enough.  An "optimally fit" human
today might have bullet-proof skin, retractable wings,
and thousands of other cool features. Therefore, an existed
feature may not have a strong underlying "purpose". 
	I like to think of "elimination of the weakest"
as much as "survival of the fittest".  But both of these 
are cliches that only tell a very small part of the total
theory of evolution.  The larger picture includes things
like mutation rates, population sizes, selfish DNA, predator-
prey relationships and so on.
	The better question about human eyes might be "Why
are our irises so small."  Nocturnal animals need to have
a huge pupil to let in as much light as possible at night.
Because of this, they might have been selected for large
pupils and large irises to go with them.  Humans spend most
of their time awake during the day and have no need for
great night vision.  Whether we were really under enough 
selective pressure over this feature to actively change pupil
and iris size over time, or whether our genes just happened 
to limit us to a certain iris size and we then adapted to that
by choosing to be awake during the day, is an open question.
I suspect that both cases contributed.

Brian Foley



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