Isolating mechanisms

[Murray Fletcher]

On Mon, 28 Aug 1995, Richard Jensen wrote:

> I am curious to know how readers of this list respond to the following.
> In an article in TREE (vol. 10, pages 294-299), James Mallet refers to
> the concept of isolating mechanisms as "...one of the most extraordinary
> pieces of philosophical trickery ever foisted successfully on a community
> of intelligent human beings."  Further, the use of this expression is
> "...anachronistic; few researchers on speciation or hybrid zones
> seriously now use these terms."
>
> I will take the position that isolating mechanisms are a legitimate
> aspect of our investigation of speciation.  Unless there is something
> operating to restrict gene flow between populations, we are not in a
> position to argue that the populations belong to different species.
> Understanding what it is that prevents gene flow (the isolating mechanism)
> is important.
>
> Any thoughts on this?
>
> Richard J. Jensen      |   E-MAIL: rjensen at saintmarys.edu
> Dept. of Biology       |   TELEPHONE: 219-284-4674
> Saint Mary's College   |   FAX: 219-284-4716
> Notre Dame, IN  46556  |
>

A very elegant demonstration of an isolating mechanism operating in
leafhoppers was presented at the 8th International Auchenorrhyncha
Congress in Delphi in 1993 by Hildegard Strubing. Dr Strubing showed
a videotape she had made of mating behaviour between two closely
related species of cicadellids. Female leafhoppers use a
characteristic substrate vibration to "call" the males. The
videotape showed the female making her call and the male responding
and approaching. Finally the male parked himself next to the female,
stretched his abdomen under her abdomen and connected. The pair then
rotated so they were facing away from each other with the tips of
their abdomens held tent-like between them and they settled in that
position for sperm transfer. The videotape then went on to show the
same sequence with the male of a different, but closely related,
species. The same female made the same call, the male responded and
approached. When he was alongside the female, his abdomen reached
under hers, just as the other male's had done and they connected.
However, when the two turned to face away from each other in the
usual mating position, the connection came loose and every time
they attempted to mate the connection was broken. The tape then went
on to show another successful mating by the same female with the
"correct" male.

This demonstrated to me the importance of the morphology of the male
genitalia as an isolating mechanism for cicadellids. The male
genitalia have long been recognised as providing extremely valuable
characters for specific diagnoses throughout the Auchenorrhyncha but
this video demonstrated the biological relevance of this for
isolating the species. It also showed that the "calls" produced by
closely related species had little impact on species isolation.

Murray Fletcher
NSW Agriculture
fletchm at agric.nsw.gov.au