[Taxacom] "Accelerate gradualism" in early angiosperms

Kenneth Kinman kennethkinman at webtv.net
Sun Dec 6 23:12:13 CST 2009


Dear All, 
         Among what some might label as Darwin's
so-called "failures" are what he himself called an "abominable
mystery"---the relatively sudden appearance of angiosperms in the fossil
record.   But it really isn't all that surprising that it was then a
great mystery, since it remains somewhat of a mystery even today
(although less so). 
         But the early evolution of angiosperms is an
excellent example of the large continuum that exists from extremely slow
gradualism (such as coelacanth fish or horseshoe crabs, on the one hand,
to the extremely rapid saltatory "punctations" which polyploidy in
particular sometime provides (especially in plants).  However, much of
what occurred in early angiosperm evolution was somewhere in between,
displaying what is sometimes called "acclerated gradualism". They are
still examples of gradualism, even though they may be accelerated above
normal levels. 
        The origin of the angiosperm flower itself was
perhaps not the foremost reason for that group's evolutionary
acceleration. In fact, the early angiosperms (of the ANITA groups and
the unknown stem lineage which preceded them) may have slowly muddled
along for quite some time, until a major change occurred in the
hydraulic plumbing of angiosperm leaves. See the following recently
published paper: 

http://www3.interscience.wiley.com/journal/123193748/abstract 

              It seems to indicate that it
was a substantial increase in photosynthetic energy harvesting which led
to the accelerated diversification of one clade of angiosperms during
the Cretaceous.  More energy meant that angiosperms could splurge on
more seeds and bigger flowers (as well as making good-tasting,
nutritious, leaves more expendable). This tended to not only feed the
diversification of many insect and vertebrate taxa, but that would have
in turn increased the angiosperms' own diversification in a sort of
feedback loop which continued throughout much of the Cretaceous and on
into the Cenozoic (when the grasses continued with their own spectacular
diversification). All of this would be somewhat gradual in Darwin's
mind, but with varying rates of accleration when significant climatic
changes (in particular) occurred. 
           This all demonstrates that what we call
"gradualism" is a semantic minefield, and what one person calls
gradualism is different from what another might call gradualism, since
there a vast continuum between extreme gradualism (coelacanth fish or
horseshoe crabs) and the extremes of saltatory punctuation (polyploidy
which can on occasion eventually result in a new major taxon, especially
in plants). Darwin could not have understood the significance of genetic
"punctuations" in particular, because the science of genetics had not
yet developed. As I said before, it is just too easy to criticize Darwin
in retrospect. And we are still grappling with the details of his
"abominable mystery" to this day. Does the origin of angiosperms begin
in the Jurassic, further back in the Triassic, or even as far back as
the Permian as some have suggested? It's still a mystery. 
                    -------Ken
Kinman 
P.S. The major advance in the hydraulic plumbing of angiosperm leaves
seems to have been most pronounced in the 
monocot-Ceratophyllales-eudicot clade which arose about 145 million
years ago. This is Magnoliopsida clade 4 in the angiosperm
classification which I presented here last April (below are just the
pertinent details of Class Magnoliopsida):

  1 Class Magnoliopsida%% (basal dicots)          
         1 Amborellales 
           2 Hydatellales (incl. Archaefructaceae)  
           B Nymphaeales 
           3 Austrobaileyales 
           4 {{Liliopsida}} (= monocots)      
           B Ceratophyllales 
           C {{Rosopsida}} (= eudicots)
           5 Chloranthales 
           6 Piperales
           B Canellales 
           7 Laurales
           8 Magnoliales 

_a_ Class Liliopsida (monocots) 

_b_ Class Rosopsida (eudicots) 

       In the earlier clades of Magnoliopsida, as well as the unknown
members of the stem group which preceded the crown group of angiosperms
(a stem group which is thus still an "abominable mystery"), the
hydraulic plumbing of the leaves remained relatively plesiomorphic, well
after the origin of angiosperm flowers.  It remains to be seen whether
flower morphology or leave morphology was more important to the
overwhelming success of angiosperms over other terrestrial plant groups.
Each would have played a part, but which played a more important part
remains to be seen.





More information about the Taxacom mailing list