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or a pushed-back version (by altered timing of development) of epige-
netic states in ontogeny that were present in each of the adult forms of
proceeding (in evolution) species (Gould 1977). Although in its original
form recapitulation posited that adult features of ancestral species were
formed in descendant embryos, it is now clear that previous adult states
only resemble the developmental stages in the most evolved species.
This extreme recapitulation view has been widely debated with a closely
related view called
“
increasing or progressive differentiation
”
proposed
by Von Baer (Gould 1977). Von Baer
'
s ideas are more in keeping with
Waddington
s ideas of development. Here, descendent species move
through the same differentiation path as their less evolved ancestors but
have altered their differentiation by adding a
'
final (newly evolved)
component not present in the ancestor. In modern thinking, Von Baer
s
increasing differentiation could be interpreted to include deleting or
modifying components as well. In this way, newly evolved complex
organisms do not completely abandon earlier (in evolution) develop-
mental paths to be able to take new forms
de novo
, but as in the analogy
to cultural evolution continue to add or to modify developmental
modules to increase or alter complexity. This view has been also
criticized because it would make development of extant species exceed-
ingly long if all previously evolved developmental programs were
retained (Gould 1977). That this is not necessary can be easily seen if
we remind ourselves that any or all ancestral developmental programs
are undoubtedly retained as the
'
path to the descendent adult
forms, not to inform us that ancestors and descendents are derivatively
related as have many inferred.
By analogy to the concept of increasing differentiation, cells
“
best
”
first form
a dome in the leaf meristem of
Pilularia
, which extends to produce next a
long cylinder- or needle-like leaf. After forming a Pilularia-like
first leaf,
the next leaf then
flattens and expands in one dimension to form a blade
that bifurcates into a double-lobed second leaf in
Regnellidium
. The
dome-shaped meristem of Marsilia sequentially produces both of these
structures and then continues by increased differentiation to bifurcate
the two lobes to form a third quadrifoliate leaf (Pryer and Hearn 2008). As
in Von Baer
rst
morph through each of the previous steps, which could represent steps
resulting from slower (heterochronic) progression of developmental or
epigenetic states of the genome. Very interestingly, the heterochrony of
Marsilia is environment dependent and the
'
s view, the most evolved,
final complex structures
final quadrifoliate leaf
cannot form under hyponutrient conditions (Pryer and Hearn 2008).
The mechanism and function of this connection has never been investi-
gated. These leaf morphology transitions can be viewed as resulting from
