Terrestrial Palaeoecology and Global Change

320 Valentin A. Krassilov. Terrestrial Palaeoecology
pre-existing pattern of ecotypic differentiation. In fluctuant ecosystems, the morphological
disparity increases through developmental acceleration rendering a macromutaional phenotypic
effect (Goldschmidt, 1940). Subsequent splitting of macropolymorphic species may
result in superspecies configurations (Krassilov, 1989b).
This model is further discussed in relation to genetic assimilation (VIII.6) and biospheric
crises (IX.9.3).
VIII.6. Genetic assimilation
In the 1890’s-1920’s, many palaeontologists shared orthogenetic views later sunk into
oblivion due to the fierce opposition of neo-Darwinians. In essence, the orthogenetic
theory stated that closely related species represent successive stages of individual development
(Eimer, 1898). Evidence in favour of orthogenesis came from all branches of
palaeontological research (e.g., Hyatt, 1986; Cope, 1887; Plate, 1920). Classical examples
are furnished by the evolutionary successions of ammonoids, horses, humans, etc.
In the early land plants, the progressive webbing of syntelomic branchlets on the way to
leaf blades and a concurrent differentiation of vascular system are likewise orthogenetic,
fixed at different developmental stages over the arrays of co-existing forms.
Morphological modes of such developments were thoroughly investigated (Severtsov,
1939; De Beer, 1940; Rensch, 1954), yet the genetic mechanisms remained obscure
until, in the 1970’s, a correlation was revealed between the interpopulation variability in
developmental traits (the emergence time, growth rates, metamorphosis, germination,
etc.) and the differences in the genomic structures, in particular, the repetitive DNA
(Schroeter & Hewitt, 1974, John & King, 1977; Beverstock et al., 1976; Lubs et al.,
1977). Fast adaptive shifts were related to a repatternings of repetitive DNA (Bernardi
et al., 1976; Mukai & Cockerham, 1977) causing developmental heterochronies. Certain
genetic polymorphisms were interpreted as paedomorphic, a retention of juvenile isozymes
patterns in the adults (Maters & Holmes, 1975; Nair et al., 1977).
Geographic clines of polymorphic traits were shown to conform to their developmental
sequences. A correlation of developmental heterochronies at morphological and molecular
levels was extended from conspecific populations to closely related species in
various groups of plants and animals (Laird & McCarthy, 1969, Maher & Cox, 1973;
Narayan & Rees, 1977; Vladychenskaya & Kedrova, 1977; Buongiorno-Nardelli et al.,
1977, Macgregor & Andrews, 1977) thus supporting the orthogenetic model.
A denial of orthogenesis pertains to the difficulties it poses for the conventional mutation/selection
mechanism. The genetic differences between local ecotypes are conventionally
ascribed either to drift or selection, or else to recruitment of pre-adapted
genotypes from a polymorphous source. However, the differentiation is too rapid for
such mechanisms to play a substantial role, suggesting a more direct adaptive response
either through an effect on intercellular environment modifying the transcriptional activ-