Re: Limits of Kinds - Isoetes

R. Joel Duff (
Mon, 17 Nov 1997 20:46:45 -0600 (CST)

>At 03:13 PM 11/13/97 -0600, Joel wrote:
>>This may not be all that surprising though. Isoetes has a highly reduced
>>vegatative morphology which such that any two species of the 150 species
>>world wide would be difficult to tell apart from one another based on
>>leaves, roots, or corm structure alone. The few definitive charaters
>>involve the sporangia and spores.
>Then a paragraph later...
>>Some characters remained virtually
>>unchanged over 200 million years while other exhibit a wide degree of
>>change over time.
>and later...
>>Note also that lack of morphology change does not mean there is a lack of
>>genetic change over time. Morphology and underlying genetic change are not
>>always tightly correlated.
>What exactly do you mean by "a wide degree of change over time"????


You are right to call me on this one. You are likly to think my meaning of
"wide degree of change" to not seem very "wide." It might have been better
said that some characters are more stable than others. The overall gross
morphology of the plants recognized as Isoetes has changed little since the
early Triassic and yet sporangial and leaf axil characters associated with
dessication tolerance have appeared at later dates. Spore morphologies
have diversified and corm development patterns have apparently changed.

In addition it is only fair to admit there has been some differences of
opinion regarding the status of this genus in the fossil record. It
appears to me that many have taken fossil forms of this genes and though
they cannot truly distinguish them from the modern genus they have ascribed
them to a fossil genus Isoetites. I think one of the reasons (if not the
overriding reason) is that it is thought that a fossil of that age can't
belong to a modern genus and so any difference seen is enough to cause it
to be reclassified. The paper I referred to breaks with some of the
previous classifications in ascribing the modern genus to the early

Interestingly, although there are plants even in the early Triassic that
appear just as today's Isoetes through the Triassic, there is a rather
strong fossil record of a multitude of other herbaceous lycopod lineages
(Tomiostrobus, Lepacyclotes, Lycostrobus, Cylostrubus, Pleuromeia) of which
at least several appear to be derivatives of an Isoetes stock. Retallack
(1997) suggests that the representatives of the modern genus were presnet
as far back as the early triassic and that they gave rise to a multitude of
other lineages that all went extinct by the end of the Triassic. So
although Isoetes may have given rise to other Isoetes-like lineages, some
remained evolutionarily static with respect to their overall morphology.
Again though the morhology is similar we have no way of knowing if this
genus has remained genetically static. I would suggest not because Isoetes
actually exhibites a high rate of sequence divergence (in all three
gonomes) relative to other extant lycopods though is remarkably genetically
uniform among the ca. 150 modern species but that is a whole other topic
for another time.

Joel and Dawn Duff / | ' \ Spell Check?
Carbondale IL 62901 ( ) 0
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