Origins of Salmon

Ken Triebold ktriebol at
Sun Nov 19 06:42:57 EST 1995

A question about Salmon has been on my mind for quite some time, and I
thought that perhaps someone reading this newsgroup could provide an
answer.  Here goes:

A type of Salmon (I think Pacific Salmon) has a four year life cycle.
It begins its life in a fresh water stream, spends most of its life in
the ocean, then returns to the same fresh water stream in the fourth 
year of its life to spawn, then dies. The population of Salmon is 
distributed such that every year there is a class of Salmon in each of
the four phases of the life cycle. We could call them Freshman, Sophomore,
Junior, and Senior classes.

I have two questions about this arrangement.

1. As Salmon evolved, it would seem to me that all members of the family
   would be born in the same year. This would leave a system where all
   Salmon would be born in one year, then no other Salmon would be born
   until the fourth year. How did it evolve to the 4-class system present

2. The 4-class system present today does not allow inter-breeding between
   the classes. If that is true, why haven't the four separate classes
   evolved into different species? Has anyone been able to detect any
   genetic diversity between the classes?

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