“The evolution of species
is pushed—or sucked—in the direction taken by succession,
in what has been called increasing maturity... evolution should conform
to the same trend manifest in succession. Succession is in progress
everywhere and evolution follows, encased in succession's frame.”
Ramón Margalef
Evolution emerged, providing a new way to degrade energy. If not immortal,
the new way was at least continuous and expanding, making life's complex
systems distinct. Early organisms poisoned themselves by using gradients
too much. Depleting their energy sources and the materials needed to
build cycling systems, they were saved because they begot slightly different
forms that prospered on a variety of gradients. Thermodynamics is necessary,
but not sufficient, to understand evolution.
The title of G. Evelyn Hutchinson's book The
Ecological Theater and the Evolutionary Play (1965) suggests that evolution occurs
within ecosystems. Hutchinson's emphasis on ecology reflects his understanding
of the importance of energy in evolution. In fact, ecology and evolution
show similar directional tendencies. As the epigraph for this chapter
notes, "The evolution
of species is pushed—or sucked—in the direction taken by
succession.” The selective processes taking place in ecosystems
are the same ones shaping evolution. The main difference is that in ecosystems
short-term factors and a fixed gene pool are more important, while evolution
is a long-term process working with a variable gene pool.
These tendencies reflect energy's shaping influence in complex systems,
whether over the more limited time frame of ecological processes, or
over the vast stretches of evolutionary time. An ecosystem may take
hundreds of years to develop; biological evolution began over 3.5 billion
years ago. But in both cases we see the phasing out of rapid growth,
as an initial start-up phase gives rise to increased cycling and diversity.
The evidence for increasing diversity over evolutionary time is well
established, and if we agree that the biosphere has expanded over time,
it is clear that cycling also has increased. Since organisms are complex
open systems imbibing carbon, energy, and electrons from their surroundings,
to which they add degraded materials and heat as they grow, they perforce
increase rates of the cycling of elements such as carbon, nitrogen,
and phosphorus needed by life.
Systems that capture more energy and efficiently convert that energy
into offspring are less apt to be eliminated in the selective process.
Natural selection favors systems adept at managing thermodynamic flows.
The most effective systems, be they organisms, ecosystems, or biospheres,
appear to increase their diversity until they achieve local optima of
energy flow. The principle of capture and degradation can be seen in
the increase in taxa over time. The increase in taxa (kingdoms, orders,
families, and species) provides vivid testimony to the claim that evolution
has developed increasingly novel and long-lasting pathways for dissipation
since life started. Because it began with zero, the number of species
on the Earth has increased over geologic time. To reiterate, from the
viewpoint of the biosphere as a single complex system each new species
represents a new bioenergetic bifurcation; each new species is like a
new leaf on the tree of life, a new pathway for energy capture, storage,
and degradation. Presently humanity is severely pruning the tree of life.
But perhaps it will grow back, as before, with renewed vigor.
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Part
III: The living
11.
Thermodynamics and Life
12.
Brimstone Beginnings
13.
Blue Planet Blues
14.
Regress under Stress
15.
The Secret of Trees
16.
Into the Cool
17.
Trends in Evolution
The increase in the number of genera over geologic time. This graph uses data
from John Sepkoski's compilation of more than 37,000 genera of marine fossil
animals preserved from the Cambrian to the Present. Over this very long evolutionary
time frame, more pathways emerged for degrading available energy gradients. This
increase in species was punctuated by sudden declines in genera during the Permian,
during the Triassic, and at the Cretaceous-Tertiary boundary. After such events
the number of genera recovered as new species evolved to fill empty energy niches.
The recovery of species after such punctuated events is similar to the increase
in species after an ecological perturbation and the following increase in species
during succession. Myr = millions of years before today. (Data from
Sepkoski 2002.)
The evolution of the respiration intensity of large animals over geologic time.
Alexander Zotin developed the concept of size-independent respiration intensity,
qO2, or oxygen utilization per unit weight. This value, weight-normalized metabolism,
increases over evolutionary time. Not only were there more species over time,
but also those that survived degrade available gradients more intensely. A sparrow
or a hummingbird has a higher weight-specific metabolism than worms, mollusks,
and reptiles. (After Zotin 1984.)
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