Jerry_Tangren wrote:

>The Northern Oriole is not simply a change in man's interpretation, the
>populations involved did NOT "remain unchanged." The split is on the basis
>of a change in the nature of the hybrid zone on the Great Plains. In
>locations where previously hybrids only existed, both Bullock's and
>Baltimore now co-exist. This is a highly dynamic situation and illustrates
>exactly the point you are trying to make.
>
Tweets:

As far as I know this is not the case. The hybrid zone did not change
appreciably between the 60's and 70's and apparently between the 70's and
80's. There is a tendency for Bullock's characters to persist further east
of the hybrid zone than do
Baltimore type characters west of the hybrid zone. If anything,
there is evidence for an eastward invasion of Bullock's genes,
but not the opposite. In parts of the northern Great Plains both orioles
are present and few are intermediate in appearance, implying that
the two species segregate there. A study done ten years ago in Colorado
found significant positive assortative mating in orioles (like mates with
like), with few hybrids but this is not a site that had been studied in
detail in the past. The hybrid zone appears not to have changed, but our
knowledge did. In the southern Great Plains mating is random with respect to
morphology and all birds in the hybrid zone are intermediate in morphology
(its a sight to see!).
In general, the hybrid zone maps well on to a gradient of rainfall
(decreases to the west), with Bullock's being the xeric specialist.
Presumably shifts in the rainfall gradients due to droughts etc, may shift
the hybrid zone. Annual differences in spring migration may also cause short
term changes at sites within the hybrid zone, if these cause certain
populations to arrive earlier and procure the available territories. Its
interesting to ponder if perhaps hybridization is being selected for in this
case. If Baltimore is a mesic (wet) specialist and Bullock's is a xeric
(dry) specialist, then it may not be such a stretch to suggest that the
hybrids do well in the 'dry mesic' intermediate zone. Outside of this area,
hybridization will be selected against of course. Hybridization may be a
way for an animal to pass on more of its genes to the next population than
its contemporaries.




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