Hybridization of Bird Species

Allen Smith allens at yang.earlham.edu
Mon Apr 27 07:28:49 EST 1992


In article <540 at tdatirv.UUCP>, sarima at tdatirv.UUCP (Stanley Friesen) writes:
> In article <6290029 at hpindda.cup.hp.com> alanm at hpindda.cup.hp.com (Alan McGowen
   ) writes:
> |  The particular point that I thought
> |interesting was that the authors establish a lower bound of approximately 10%
> |on hybridization of wild bird spiecies.
> |
> |   In other words, at least 9.42% of all wild bird spiecies breed outside the
   ir
> |spiecies and create hybrids, some of which a fertile.  I find this sort of
> |surprising.  The notion of spiecies for birds is more sociological and less
> |genetic than I had thought.
>
> I think this conclusion is slightly overstated.  The presence of hybrids
> does *not* make the species concept 'sociological' rather than genetic.
> The fate of the hybrids is important.
>
> Do they survive to breeding less than the parental types?
> Do they breed less successfully than the parental types?
>
> In general I would suspect that the answer to both questions is 'yes'.
> If so, the two parental forms may still be good biologically defined
> species, with effective, if somewhat leaky, isolating mechanisms.
>
> It is a common misconception that the biological species concept requires
> failure to produce fertile offspring.   This is false, all that is
> required is that effective gene transport is blocked between the forms
> in question.
> |
> |   talk.origins:  this should make selection and hence evolution of bird
> |proceede much more quickly.
>
> I doubt it, though there may be selection for improved, less leaky,
> isolating mechanisms (especially ones that prevent mis-mating).
>
> The high level of hybridization in birds may point to a spurt of
> speciation only a few millenia ago (say during the last glaciation),
> with only incomplete isolating mechanisms so far in place.
> |
> |   sci.environment: either birds have less genetic diversity than we thought
> |or their diversity is more widespread (and hence more easilly conserved).  In
> |any case it strikes me that this implies birds are more environmentally
> |robust than we might suppose when reasoning by analogy from other
> |vertibrates.
>
> Not necessarily.  Hybridization does *not* necessarily imply preserved
> variability - the hybrid forms may often fail to effectively reproduce.
> Thus the added 'variability' would be quickly eliminated.
>
> As I suggest, it may be more of an indication of a higher rate of speciation
> in the 'recent' past than previously.
>
> [Though there is evidence for stable hybridization between oak species
> over periods of millions of years, so I may be wrong].

        Actually, from what I know of the subject, differentiation by
species via the hybridization success test is sometimes a matter of
definition. There's one "species" ranging from someplace around Florida to
Washington State, in which the ends can't interbreed. But the middle
sections can do so, and the hybrids are quite successful.
        Also, how does one decide where the species divisions are in the
past? One can't exactly do a breeding experiment. Sometimes, enough DNA is
isolatable to run a DNA hybridization check, but where does one draw the
line? Admittedly, one way would be to look for hybrids. But what if the
two species in question are seperated by time?
        -Allen



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