Terrestrial Palaeoecology and Global Change

Chapter 10. Conclusions
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When the mainstream evolutionary tendencies are reversed, we are justified in recognizing
a biospheric crisis, such as over the Permian/Triassic or the Cretaceous/Tertiary
boundaries. Both were preceded by an increase in geomagnetic instability and culminated
with a rise/expansion of continental crust and the voluminous intracratonic magmatism.
At both an upheaval of mantle material caused prominent geochemical anomalies
(including the iridium spikes) correlated, through the effects on biological production,
with an anomalous isotopic composition of organic carbon and the anomalously low
biological diversity. Geological developments affect biota through their impact on biotic
production rates mainly (e.g., through an input of iron from volcanic sources enhancing
the mobility of phosphorus in freshwater ecosystems: IX.1). Eutrophication spreads with
terrestrial runoff over estuaries to marine ecosystems (as recorded by the algal blooms
over the Permian/Triassic boundary). The crisis thus starts deep in the earth’s interior
and surfaces with the biotic turnovers.
With normalizing selection suspended after the collapse of long-standing climaxes,
the surviving “disaster” populations reveal the full ranges of their genetic potentials, with
the extremes that are otherwise exterminated. Such extremes are qualified as macromutations.
In particular, a demand on reproductive rates induces an acceleration of developmental
rates resulting in a paedomorphic variation. The same conditions favour an
openness of genetic systems to introduction of exogenous genetic material. Horizontal
gene transfer may gain in significance in post-crisis communities (on evidence of morphological
characters spreading at certain time-levels across the diverging phylogenetic
lineages: VIII.1.4, Fig. 113). With stabilization, the macropolymorphous populations might
have served as the foci of subsequent adaptive radiations.
Thus evolutionary developments at the population and genomic levels are top-down
regulated by ecosystem evolution. Speciation is closely related to the biospheric cycles
through their impact on population strategies. A coarse-grain strategy highly sensitive to
environmental heterogeneities (“grains”) prevails under stable conditions giving way to
an indiscriminate fine-grain strategy with erratic disturbances (VIII.3.4). The down to
grain adaptation is accompanied by a genetic divergence of local ecotypes. With a switch
to the fine-grain strategy, the pattern of the coarse-grain diversity is swept out. Genetic
mixing through introgression of the down-to-grain adapted genotypes generates a new
system of genetic variation, a speciation event.
The genome responds to environmental demands by a change of differential gene
transcription rates that are memorized (assimilated) through the respective changes in
these gene replication rates. Schematically, the more transcriptionally active genes fall in
the fast replicating category, whereas an inactivated part of the genome is slow replicating
and can be potentially lost through underreplication. The respective regulatory systems
are adjusted to the altered gene activities through a deletion/amplification of their
repetitive elements providing for an earlier switch on of the environmentally induced
genes, inflicting a directional evolution of the genome.