ligule origins

[John Grehan]

Bill Shear wrote
  Could we agree that the ultimate "cause" would be that plants with
>a ligule were more successful at producing offspring than plants without,
>hence the persistance of this structure?

This is the point where I would not agree. I could see the possibility that
plants with a ligule were more successful, but I don't see this as necessarily
being the case, or necessary to explain the persistance of this structure. I
belong to a minority point of view that accepts some genetic changes
may occur such that a "mutation" may appear and spread through a
population without involing differential reproduction.

>Why were these plants more successful?  Perhaps because the functions
>performed by the ligule allowed them to devote more of their resources to
>reproducing themselves, or enhanced their chances of survival in a
>particular environment.  In this sense, the function of the ligule is
>indeed a "cause" of its presence.

I would not see it that way. I would see the above functions as being
consequences of having the ligule in the first place. If, however,
the ligule arose through a "random" mutation that was not spreading
through the population of its own accord, natural selection would
provide an alternative mechanism for its spread. In this case there
is no information on the nature of the genetic changes that took
place at the time of its origin (as far as I am aware).

>
>Naturally in the absence of any direct tests of the effect of the ligulate
>condition on survival and reproduction, such statements remain hypotheses.
>But since similar hypotheses have been supported by direct tests in the
>past, we might have a good deal of confidence in this one.

What is the foundation for having confidence that the ligure was established
through its functionality?

>
>I find it something of a stretcher to believe that the ligule, a structure
>that has persisted almost unchanged in lycopods for 385,000,000 years, and
>appears to have a function in living lycopods, is any more vestigial than
>legs on tetrapods.  And indeed, the account reported by Haynes provides a
>number of perfectly plausible (testable) ideas of its current and probably
>past function.

An human appendix has function, even though its vestigial with repsect to its
earlier condition.

>
>What is Croizat's evidence that the ligule has the potential to develop
>into a shoot?

Croizat cited Warmer's studies that show there is no formal distinction between
trichomes, telomes, and phylomes. There is probably more to this than I
can present without citing his text in full, but he discusses trichomes in a
section from pp. 1037-1043

John Grehan