Re: Preventing loss of genetic diversity

Dear Karsten,

You pose an interesting question, one that comes up fairly regularly 
within the conservation community in one form or another.  I'm trained in 
ecology, not population biology, so I checked with a friend better 
schooled in the nuances to make sure my instincts were right on this.

The short answer is that hybridization isn't just a way to ensure that a 
species' genes are still represented in the world at large.  There are 
considerable negative ramifications of hybridization that render your 
suggestion (to allow interbreeding between Kiwi subspecies or tiger 
subspecies) a tricky proposition indeed.  

Outbreeding depression is the biggest negative ramification of 
hybridization.  Without more training in population biology, I can't 
really provide you with a good definition for outbreeding depression, but 
I suspect there's lots of good, detailed information out there if you want 
to read more.  The basic idea is that hybrid animals run into three main 
problems:  (1) coadapted gene complexes are potentially lost in 
hybridization, (2) local adaptations might render the hybrid offspring 
unfit (or less fit, to be fair), and (3) (having nothing to do with 
outbreeding depression) loss of legal protection in some 
regions.  

Here's what that means, in a nutshell:  (1) coadapted gene complexes - 
as you probably know well, genes are not simple.  A tiger and a lion 
aren't just two kinds of cats, they're two kinds of cats with all kinds of 
genetic stuff going on that make them lions and tigers.  Or different 
subspecies of tigers, or kiwis, or orangutans.  In fact, hybrid orangs are 
known to have problems (health-related, I think) due to the loss of 
certain coadapted gene complexes that are present in non-hybridized 
individuals.  The roll of the dice of hybridization may break up essential 
gene complexes, which can spell trouble for the resulting offspring.  The 
idea is that we may not understand how all these gene complexes work, but 
they definitely work better when they're not broken up.

As for (2) local adaptations, that's a little simpler to explain.  
Mountain ibex are a great example.  At some point (not sure how), highland 
ibex began interbreeding with lowland ibex.  An ibex is an ibex, right?  
Not so.  The different ibexes have their babies at different times.  So 
the hybridized ibex living in the highlands have their babies too early in 
the season, while the weather was too cold.  Presumably that timing would 
be fine in the lowlands, but up high, the babies freeze, thus ending that 
experiment in hybridization.  

And for (3) loss of legal protection, this is a non-scientific issue with 
significant implications.  Hybrids are not recognized under the US 
Endangered Species Act.  This means that if someone decided that the 
endangered eastern hoolie hoolie bird (I'm making up this example - red 
wolves are an active debate along these lines - some say they're 
coyote/wolf hybrids) populations were going down the tubes, so why not 
just breed them with the abundant western hoolie hoolie birds (another 
subspecies), and then we'll have some hoolie hoolie genes still in the 
population in the East.  Well, those resulting hybrid offspring would not 
technically be protected under federal law.  So folks could go to great 
effort to produce additional birds, but their habitat (the hybrids') 
wouldn't be protected from further loss or degradation, which is probably 
what put the hoolie hoolie bird in trouble in the first place.

So the biggest concern I have with just letting really rare 
species/subspecies interbreed with more common species/subspecies to 
maintain genetic information "out there" is that the focus then shifts off 
the root problems pushing the rarer species to the brink.  I'm wary of 
technical solutions to conservation problems that don't address the core 
issues of habitat loss and fragmentation.  If we don't address the 
ultimate causes of species loss, any resulting hybrids will run into the 
same problems as their predecessors eventually, as even more habitat is 
rendered unsuitable, and then what will we cross them with?  Habitat 
protection has to be the bottom line.  

The purist in me wouldn't see a lion/South Chinese tiger hybrid as a 
beautiful way to maintain rare tiger genes.  I would see that liger as a 
reminder that we didn't react strongly enough, soon enough to maintain 
those tigers' genes in the tigers themselves.  

Some would argue with me on this, that's certain.  With a bit of research, 
you're likely to find lots of debate on this topic.  I am a purist, a 
splitter - I think a subspecies is a subspecies and blurring the lines is 
dangerous business.  To me, our focus should be on doing everything we can 
to protect species and subspecies as we find them, intact, separate, 
complete.  If we can't do that, I don't think that knowing that a few dodo 
genes remain in another bird species/subspecies would be consolation for 
the loss of the dodo.  

Keep asking these questions and looking to others for answers.  This 
bioethics stuff is never-ending and a tremendous challenge.  I'm still 
refining my arguments on this and other similar issues.  Thanks for the 
opportunity to think more about it.  I know I've oversimplified the issue, 
so definitely keep researching all sides to this topic.

Best wishes,
Ruth