Hybrid Swarms, was Re: Species and varieties, bulbs and trees

totototo@telus.net totototo@telus.net
Sat, 09 Jun 2007 19:38:37 PDT
There are many instances where a population smoothly intergrades 
between two extremes that, in the absence of intermediate forms, 
would unquestionably be considered distinct varieties or even species.

Such populations present great difficulties in classification. It is 
hard to decide if they represent two distinct taxa with hybrids in 
the middle, or alternatively represent a single taxon that simply has 
extremes of variation.

The problem, she thickens, because those two possibilities may be 
mere mental constructs with no reality to them. The correct 
interpretation of such an intergrade may depend on the history of the 
populations, which is unknown.

Examples: IIRC, an old forester once told me that Abies lasiocarpa in 
the west smoothly intergrades across Canada with another, eastern, 
species that is quite distinct. No one knows with any certainty which 
scenario the overall population represents - is it two species with a 
hybrid swarm or is it one species with wide variation, perhaps in the 
process of splitting into two species?

Another example is Cyclmaen coum coum and its supposedly distinctive 
subspecies C. c. caucasicum. Extensive field work sponsored by the 
Cyclamen Society has shown that there is a continuous variation 
between the two extremes, to the point that there are lots of 
intermediates impossible to classify as one or the other.

Sometimes you have to wonder if Mother Nature takes offense at human 
attempts to pigeonhole her creations and deliberately sets up these 
taxonomic pitfalls...

Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate

on beautiful Vancouver Island

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