In message .com,
Raphanus writes
I've read in several places (can get references if necessary) that oaks
( Quercus spp.) hybridize very easily. If so, wouldn't - over
millennia - speciation be lost and all oaks blend to a single
homogeneous oak? If hybridization is so easy - what keeps the species
apart? How does tell if a particular "wierd" specimen is a hybrid or a
new species? Am I expecting botany to be too exact?

There are other processes which may operate to limit introgression.
Apart from selection against hybrids such processes include hybrid
sterility and hybrid breakdown.

I don't know much about oaks, except that hybridisation is widespread
within the oak subgenera, but I can give you examples from another
genus.

Hybrid sterility - the hybrid between Malva sylvestris and Malva durieui
is fairly easy to produce. However it is triploid and highly sterile,
with a seed set of under 1%. The germination rate is also low - from
circa 200 seeds I only managed to raise 2 plants this year.

Hybrid breakdown - the hybrid between Malva sylvestris and Malva linnaei
is also fairly easy to produce, and has appreciable seed set (~ 5-10%).
Germination is again poor - from circa 100 seeds I raised 8 plants. But
these plants appear to highly sterile.


If DNA analysis was extremely inexpensive (say, $ .10) would botanical
taxonomy be void of ambiguity?


No, unless you can find a convenient locus. Gene trees are not perfectly
correlated with species trees, and closely related species may not be
separated absolutely by DNA sequences. Examples that come to mind - from
ornithology - are the Larus argentatus (herring gull) and Loxia
curvirostra (crossbill) complexes.

Introgression and hybrid speciation also complicate matters.
--
Stewart Robert Hinsley