Terrestrial Palaeoecology and Global Change

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Chapter 8. Ecosystem evolution 321 ities of the genes involved (Schwartz, 1971) or mediated through a sensitivity of noncoding DNA to environmental induction (e.g., Romanov & Vanyushin, 1977). An increase in the content of repetitive DNA species correlated with the trophically induced heritable modifications in Linum usitatissimum (Durrant, 1962; Callis, 1973 and elsewhere) testifies to the validity of the latter mechanism. These mechanisms give some substance to the slightly ambiguous theory of genetic assimilation (Waddington, 1962), obviously related to the phenomena previously known as the Baldwin effect (Baldwin, 1902) or the Dauermodifikationen (Jolos, 1913). In its original version, the theory postulates a higher probability of fixation for genetic changes channelling the development in the direction of an adaptive phenotypic modification, also interpreted as a higher frequency of advantageous mutations (Hall, 1990). The theory thus admits that mutations are not occasional but are instead induced by environmental effects on development. This is fully justified by the systemic nature of the genome provided with the molecular repair mechanisms that screen most of occasional changes detrimental to the programmed development. Since genome is a system of developmental memory, the environmentally induced changes of development should be memorized in one way or another. In both the brain and genomic memories, the incoming information is processed and stored in a retrievable form. Structural changes, involving nuclear DNA, occur over the memorizing processes at both the neuronal and genomic levels (Wallace, 1974; Kazakhashwili, 1974; Black et al., 1987). New experience is assimilated by learning the genomic form of which is the genetic assimilation. It may seem that genomic structures are far less responsive to direct impact of new experience than the neuronal configurations are. Yet learning plays a restricted role also in respect to subconscious neuronal memory unless it is elevated to the conscious level. So the difference may be of a degree only. In the same way as brain functioning is not explicable at the level of individual neurones, the genetic memory operates at the level of the total genome rather than a particular nucleotide sequence. The holistic nature of genomic memory reveals itself in the retrieval of morphological images over a succession of genetic changes. The phenomenon of retroconvergence is a spectacular example. Retroconvergence is a similarity between composite organs and incorporated basic structures (Krassilov, 1995c). Thus, the composite strobili of conifers, originating from aggregations of simple cones, are convergently similar to the latter in their gross morphology. The same applies to seeds and seed-bearing cupules (Fig. 128). Likewise, in compound leaves, fusion of leaflets results in a composite leaf that looks exactly like the leaflets. There can be several rounds of retroconvergence. The phenomenon has been studied in detail in a Triassic peltasperm, Lepidopteris (Scytophyllum), in which the marginal fusion of leaf segments produces a lobed leaf blade, with the original pinnate venation retained in the lobes (Fig. 129). At an advanced stage, the lobes are no longer discernible but the conservative venation betrays the composite nature of the blade. Eventually, the venation is also modified in the direction of the initial dichotomous pat-
322 Valentin A. Krassilov. Terrestrial Palaeoecology a c d b Fig. 128. An example of retroconvergence of seed/cupule structures: (a, b) Moresnetia, horned cupules of a Devonian progymnosperm (courtesy of A.R. Ananiev), (c) Lazarospermum, a Late Permian seed (Krassilov, 1999b) and (d) Eoantha, horned cupule of an Early Cretaceous proangiosperm (Krassilov & Bugdaeva, 1999).
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Terrestrial Palaeoecology and Global Change

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