Biology Reference
In-Depth Information
complex relationships between species in communities. The fact that non-
equilibrium conditions are much more common than equilibrium ones,
as shown in the preceding chapters, makes it even less likely that such
laws can ever be found: most nonequilibrial patterns are largely or entirely
unpredictable and can therefore not be modelled by simple equations.
Both the approaches of Kauffman (
1993
) and Wolfram (
2002
) show
that species seldom (if ever) reach global adaptive optima. Since local
optima are abundant and perhaps almost infinite in number, an over-
whelming majority will remain unoccupied. In other words, there are a
vast number of empty niches. It is highly unlikely that species, even if they
are closely related, will occupy the same local optimum, because the
processes that have led them to this optimum are largely random, i.e.,
most species will have little chance for interactions, whether positive or
negative. Empirical evidence lends strong support to these conclusions
(for examples see pp. 72-76). Results using cellular automata put these
findings and, with them, the prevalence of nonequilibrium in ecological
systems, in a convincing theoretical framework.
As discussed on pp. 152-165, many studies have attempted to find the
causes of latitudinal gradients in species diversity. Rohde (
1992
,
1998a
)
gave a nonequilibrium explanation, that is, he suggested that the primary
cause, supplemented by some secondary ones, is the accelerated speed of
evolution at higher temperatures, resulting from greater mutation rates,
shorter generation times, and greater speed of selection at higher tem-
peratures in largely empty niche space. More generally, the hypothesis
suggests that species diversity in ecosystems is determined by ''effective
evolutionary time'', i.e., the above factors and the time under which
systems have existed under more or less constant conditions. The hypoth-
esis has been supported by a considerable number of recent studies (for
details see pp. 158-165). In the framework of NKS, the hypothesis makes
sense as well: if additions of more and more ''programs'' (species) in an
automaton occur faster at higher temperatures, as indeed shown in several
recent studies using DNA (e.g., Wright et al.
2003
; Martin and McKay
2004
), more species will have evolved at higher than at lower tempera-
tures. Even if addition of species is random, some of them may be more
complex than those already present, because even quite simple programs
may easily lead to complexity (for an example see Figure
11.2
). This
would explain the observations that some tropical bird species are more
colorful and have more intricate song patterns than birds from colder
environments, and that many tropical plant species have very conspicuous
and colorful
flowers. These phenomena do not necessarily result
