Hybrids

From: Thom Quinn (swo@execpc.com)
Date: Wed Jul 23 1997 - 14:28:29 MDT

I would like to start a thread on hybrids and speciation

        Hybrids between two related species have been a common
occurrence in biology. It is a point where the way nature really works
and the way biologist view nature break down. The Biological Species
Concept would suggest that organism that can interbreed successfully
would be within the same species....but clearly organism that have
different behaviors, physical features, and even environments have been
known to breed when brought together either naturally or with human
intervention.

        Hybrids are considered to be flukes, accidental pairings, which
lead to unfit progeny. This is often the case between distant, although
related animals. One must only look to the mating of a Horse and Donkey:
a viable, strong creature, but sterile. The genes of those parents are
lost forever and it is an evolutionary dead end. See FIG 2.

        We normally think of speciation occurring via a SPLIT in gene
pools, via environmental barriers or behavior restraints. Two species
move apart from one another and will never be a part of the same gene
pool again. At least, that is the usual thinking when we talk about
reproductively isolated species. See FIG 1.

        What if the Hybridization occurs between two sister species, and
the offspring are better adapted, have higher variability, and are
fertile. These creatures would not be evolutionary dead ends, instead,
they would be evolutionary BEGINNINGS! FIG 3 shows just that. For a
hybrid to take root in the parent species niche, we would expect this
pairing happens often.

        I did a short review of the hybrid literature this morning, but
I did not see anyone suggest that this could very well be mechanism for
speciation in ANIMALS, but a recent blurb in SCIENCE, Vol. 277, July 18,
1997 (p 316) suggests that the native crayfish population has been
invaded by an exotic crayfish population; however, both population are
being replaced by the fertile hybrids.

Let me all know what you think.

Thom Quinn

                                                                        
     
                                                                        
        
           Species B Species C
                \ /
                 \ /
                  \ /
                   \ /
                Species A
 

     FIG 1: Traditional Speciation

             Hybrid BC
                |
     Species B --- Species C
            \ /
             \ /
              \ /
               \ /
            Species A

FIG 2: Occasional Hybrid

                    Species X
                       |
                       |
            Species B --- Species C
                  \ /
                   \ /
                    \ /
                     \ /
                   Species A
   

          FIG 3: Speciation Event

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