Terrestrial Palaeoecology and Global Change

318 Valentin A. Krassilov. Terrestrial Palaeoecology
A ubiquity of neutral variation inferred form the assessments of mutation load in
natural populations (Kimura, 1983) inflamed a burst of non-selectional speciation models.
As a compromise, the maintenance of neutral loci was ascribed to their linkage with
the selected ones. This so-called “hitchhiking model” is not universal, however, while a
varying selection pressure over a broad range of environmental situations provides a
viable alternative (Nevo et al., 1984, 1997).
The foregoing discussion implies that the adaptive strategies of biological species
are defined primarily by their roles in the development of biotic communities over the
pioneer to climax sequences and, in parallel, over the transition from non-coherent to
coherent evolution. The contrasting adaptive strategies also pertain to the colonizing
versus residential habits (explorers/patients/violents: Ramensky, 1953), niche breadths
(generalists/specialists), environmental grain sensitivity (fine/coarse grain strategists:
Levins, 1968) and demographic options (Leto/Niobe strategists: Krassilov, 1992a,
1995a). All these can be considered as aspects of the so-called K/r strategies (McArthur
& Wilson, 1967), yet an indiscriminate approach does not help understanding the complexity
of ecosystem evolution.
In the course of ecosystem development, the constituent populations are channelled
towards one strategic option or another. Periods of coherent evolution promote the coarsegrain
environmental strategy in conjunction with the Leto strategy of parental care/low
reproduction rates. The prevailing mode of morphological change is the anaboly (Severtsov,
1939), an adaptive modification at the terminal stages of individual development. Conversely,
in disturbed ecosystems, population strategies switch to a fine-grain mode, with
prolific reproduction compensating for the high mortality rates (the Niobe strategy). The
associated ontogenic mode is an accelerated development resulting in a condensation of
developmental stages and paedomorphic transformations.
In this model, a neutral polymorphism at an increasing number of loci would appear in
extreme situations of either a suspended stabilizing selection in undersaturated communities
or a restriction of ecological niches and consequential loss of function in oversaturated
communities. Co-adaptation introduces the interaction niches that come to dominate
the ecospace differentiation decoupling the partners from certain environmental
limitations that are thereby neutralized (VIII.1.3), entailing a loss of adaptive meaning
for variation at the respective loci. On the other hand, a neutral variation may assume a
selective value with a new dimension added to selective environment. Thus sexual selection
imposes a symbolic meaning upon individual distinctions of little if any adaptive
significance in respect to environmental variables. Human personality is mainly based on
variation of the kind.
Since speciation takes too long to be studied experimentally, we have to rely on palaeontological
data for verification of theoretical models. The commonest situation in the
fossil record is an evolutionary appearance of a species simultaneous over the range of
its ancestral species (e.g. of a schizaeaceous fern Anemia dicksoniana replacing A.
(Ruffordia) goeppertii over the Northern Hemisphere at the Albian/Cenomania bound-