January 3, 2010

Brendan Kelly
National Marine Mammal Laboratory
National Oceanic and Atmospheric Administration
Juneau, Alaska 99801

Dear Brendan Kelly:
I am heartened to read in Brendan Kelly, Andrew Whiteley and David Tallmon, The Arctic Melting Pot NATURE vol. 468 no. 7326 page 865 that it is recognized that hybrid vigor in the first generation may be supplanted by outbreeding depression in subsequent generations.  It’s not that I think this is such a great thing, but it’s good to see it out in the open like that. 

The thing is that outbreeding depression between species or sub subspecies is only the tip of the iceberg.  All right.  Poor choice of words.  While I am being inappropriate, your article reminded me of one day as a child when I said to my father, “A bear walks ten feet south, ten feet east and ten feet north and is right back where he started.  What color is the bear?” 

He gazed at me a moment with his hazel eyes.  I swear that at moments like that you could see the light from his mighty brain.  Then he burst out laughing and said, “The gummint done it.”  Feigning illiteracy was a mild joke he would play sometimes. 

Puzzled, I asked, “But what color was the bear?”


“No he’s a polar bear.  You see he starts at the north pole …”



“The gummint said we couldn’t have all them white bears livin’ together.” 

And now it has come to pass.  So I thank my stars that I have lived long enough, or maybe an hour or two too long.

Anyway, outbreeding depression can occur between wild populations.  Well understood.  But it also regularly occurs within wild populations.  This has been amply demonstrated in an article by Sibly.  Look at nobabies.net and check out the posting I did last Christmas Eve. 

Then, to make matters more extreme, in the same place check out the Helgason reference.  In humans at least outbreeding depression is demonstrable within families.  By the time you get out to 5th cousins fertility is less than if the kinship is a bit closer. 

I doubt that the relationship the Iceland study lays out is totally fixed.  My guess of the week is that the relationship drifts over time.  As you pointed out, outbreeding depression increases with generations. 

Well if polar bears are mating with grizzly bears then I think you can take for a good bet that the mating pools of both the polar bears and the grizzly bears are getting stirred up within species.  That is not what they need; widespread outbreeding depression just when the population is being stressed by climate change. 

Alas, I suspect that looking after those bears is going to be very frustrating.  Do let me know what you think. 


M. Linton Herbert MD

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