[Odonata-l] Morphology and niche overlap

[Dennis Paulson]

Hi, Mike.

For a long time it amazed me that dragonfly appendages and hamules  
seemed so unvariable within a single species, even all across a huge  
range, when so many other traits vary both individually and  
geographically (size, color, habitat, probably behavior, although we  
have quantified that so rarely). And, as you said, the variation  
AMONG species can be dramatic, so much variation has been possible  
over the evolutionary history of a group.

But when you think about it, if there is a lock-and-key relationship  
between male and female structures, then any mutation away from the  
normal shape would probably not work quite as well in mating, and  
such mutations would be unlikely to be successful. Even if that  
particular mutation arose at some low frequency (i.e., more than  
once), a male and a female in a population would have to have  
mutations that just happened to fit together, and those two  
particular individuals would have to come together by chance in a  
large population. Those low probabilities when multiplied make such  
an event extremely unlikely. But then when such mutations do happen  
to coincide (when populations are in the many thousands, this might  
be possible even at an extremely low rate), then you might have the  
rapid evolution of another species. I believe this is what Mark  
McPeek and others have postulated to explain the evolution of a group  
of similar species of Enallagma in North America.

With odonate groups that don't have lock-and-key mating (I think that  
would include many if not most libellulids), I would predict more  
variation in male appendages and hamules. It seems clear these  
structures function just to keep a tight grip on the female during  
mating, and there is obviously a great variety of solutions to that  
problem if there doesn't need to be the kind of fit implied by lock  
and key. I don't think there has been much attempt to look for such  
variation, actually. We need assessments of variation of the sorts  
you mentioned. In which genera are there coexisting species with  
exactly the same morphology in genitalic structures? In which genera  
do all coexisting species differ in those structures? Is there  
variation in this phenomenon within a family? We distinguish many  
species by these structures, but there have been precious few studies  
to show if their function is to prevent interspecific mating.

You touched on another very interesting question about odonates as  
well. Niche theory has always predicted that there should be some  
difference in two closely related species to allow them to coexist,  
yet that doesn't seem to be the case in many odonate genera. Are  
odonates examples of animals that can be used to refute the absolutes  
of both niche theory and allopatric-speciation theory?
-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382

On Mar 26, 2006, at 11:55 AM, Mike Ferro wrote:

> I was thinking about speciation with loose overlap of niches but
> strong separation of reproductive morphology and it occurred to me
> that dragonflies would be a wonderful group in which to look for this.
>
> Concerning the male genetalia of, say Somatochlora: How much variation
> of the male genital morphology exists in a species? How much variation
> of the male genital morphology exists among the species in the genus
> (it certainly looks like quite a lot in Dragonflies of NA)? I have no
> idea how one would go about quantifying the latter.
>
> Basically, would we see male genetalia with clearly defined
> (non-overlapping) differences among species, but poorly defined
> (overlapping) niches among species of the genus, or vice versa, or
> neither?
>
> I guess my idea is that there may be strong pressures from sister
> species to keep a certain morphology clearly defined while not so much
> pressure on other aspects of the life of the beast.
>
> If anyone has done a study on this, I'd love to hear about it. Any
> comments are appreciated.
>
> Cheers,
>
> Mike Ferro
> spongymesophyll at gmail.com
>
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