punctuated equilibrium

Laurence Evans laurence.e at usa.net
Sat Oct 9 11:18:01 EST 1999


    "Soon after a major innovation, discovery of profoundly different
variations is easy. Later innovation is limited to modest
improvements on increasingly optimised designs" (Stuart Kauffman, At
Home in the Universe, quoted in Nash, 1995).


    As mentioned early in this chapter, the paleontologist Niles
Eldredge finds patterns of evolution in the fossil record that
geneticists cannot explain by standard evolutionary models. He sees
energy flow within ecosystems as the "actual organising
ingredients of ecosystems." Whereas geneticists believe that species
show a gradual change in response to environmental
change, the naturalist Eldredge claims that as the environment changes,
the species will track its required habitat and become
extinct with the loss of that habitat. He therefore sees much less flux
in a species form over time. Evolutionary stasis is evident
from the fossil record: species may remain unchanged for millions of
years once formed (Avers, 1989). With this pattern,
termed punctuated equilibrium, he found the rapid evolution of new
species before the period of stasis. Because of this process,
there is a glaring absence of transitional or intermediate forms in the
fossil record. Geneticists cannot explain this with their
models. Scientists need to explain this pattern in the fossil record if
a sound evolutionary model is to be found.
    Research has shown that punctuated equilibrium is quite common.
Hundreds of studies have been made, with the general
finding that species remain stable for millions of years and then change
so rapidly that no fossil record of the transition is to be
found. Even under the influence of environmental change, species
remained static over millions of years (Prothero, 1992). Upon
this observation, Prothero (1992) notes, "Mayr (1992) argues that it is
merely the integration of species as complex wholes, so
that small-scale changes are insufficient to upset the complex balance
of integrated genes. Others suggest that fundamental
developmental constraints play an important role in restricting the
possible avenues of change (Gould and Lewontin, 1979;
Kauffman, 1983). Still others suggest that there might be properties of
species that may not have been discovered yet by
geneticists and evolutionary biologists, properties which operate on
scales of millions of generations and years (Vrba and
Eldredge, 1984)."

    In contradiction to this are the textbook examples of the gradual
evolution of species. A classic example is the herring gull of
the Northern Hemisphere (Dennett, 1995). This species forms a broad ring
around the North Pole. Along its distribution its
form changes gradually, but it is one species. Where the geographically
extreme forms of the ring around the North Pole meets
in Europe, the effect of this gradual change over the continuum is
evident. They form two species, the herring (Larus argentatus)
and the lesser black-backed gull (Larus fuscus) that do not interbreed.
Their young from the first winter are indistinguishable
(Tuck & Heinzel, 1979).

    They term this graded spectrum of forms (phenotypes) of a species
over its geographical range a cline (Avers, 1989).
Neighbouring populations can and often do interbreed, while those at the
extremes cannot usually interbreed. Was an
intermediary population that linked the distribution to become extinct,
we could call the two extreme limits of distribution
separate species!

    How does the Lotka Volterra model account for these observations?
Let me explain. A part of the adaptive process resulting from
evolution through natural selection of closely- and long-associated
species is a decrease in the i-factor (cost of interaction = interactive
factor). The Lotka Volterra model shows this. A lowered i-factor results
simply as a product of the improved survival potential or fitness of
compatible or even interdependent interactors. We must apply the same
principle to intraspecific interactions that are subject to natural
selection. A major difference here is that the gene pool is common to
both interactors. As such, a decrease in the i-factor through natural
selection is economically based and does not lead to divergence.
Individuals have higher fitness if they can be more efficient while
still reproducing. With time this leads to greater efficiency and
fitness for the species as a whole.

    Improved economy is reflected in the fossil record. Mammals are
generally less robust than dinosaurs and dinosaurs
generally have a more efficient mode of locomotion than the earlier
reptiles. Ecologists can find the result of this reflected in
behavioural repertoires that reduce the energy expenditure or other
damage resulting from such intraspecific interactions. The
result of this process upon the whole species is that once speciation
has taken place, subsequent evolutionary rates will slow.
Economic factors come into play under the force of natural selection
acting to decrease the i-factor. Variations from some
economic norm are less fit and are therefore eliminated through natural
selection. The species becomes a real entity maintained
in a state of stasis because of natural selection, intraspecific
interactions and reproduction.

    Now we must enter the possibility of the genetic isolation of part
of the population. An immediate assumption has to be that
this founding population occupies a niche at least partly isolated
reproductively through some mechanism. Geographical isolation
is the most easily visualised example, but the possible ways this
separation could occur are as diverse as nature herself.

    Once such an isolated subpopulation is founded, that genetic
population is subject to natural selection decreasing the i-factor
of that population. Involved too, are mutation, genetic drift, the
founder effect and the adaptation to the new environment
through natural selection. The individuals of such an isolated genetic
unit may encounter conditions that they are unable to cope
with and become extinct. If a species has already evolved, this will
fall under the category of species sorting. Heavy forces of
natural selection, shaping the necessary adaptation of the new or
potential species, leads to the rapid rate of evolution at this
early phase of speciation. Adaptation through natural selection brings
the population into accord with its new environment. At
this early phase of speciation, the population is occupying a new niche.
Initially the struggle for survival and adaptation enables a
capable variant to persist despite its economic efficiency. At this
stage survival needs override social competition for economic
efficiency. As time proceeds, increased intraspecific interactions lead
to a decrease in the i-factor for the incipient species. As
the animal becomes adapted to its new environment, the effect of
intraspecific interactions increases in importance, so that after
an initial, very rapid evolution the form of the species stabilises.

    Are there any patterns in nature to support this result of the
model? The above model is very similar to the founder effect
and peripatric speciation proposed by Ernst Mayr in 1954 (Avers, 1989).
He requires the same conditions for his model:

    [1] small isolated populations at the periphery of the main
population;

    [2] genetic drift and founder individuals;

    [3] no gene flow between the isolated and main population;

    [4] subjection to different selective pressures in the new habitat;

    [5] changes in allele frequencies at many sites, even of previously
fixed features (Avers, 1989);

    The result predicted is a genetic revolution. Evolution through
natural selection leads to reduced intraspecific and interspecific
interactive costs. If two newly formed, but similar species meet, but
are reproductively incompatible for some reason, natural
selection will select mechanisms of divergence to reduce the
interactor's i-factors and improve their relative fitness.
Intraspecifically the same selective force leads to the stasis of the
species as variants are generally less efficient. The driving
force of this stasis is intraspecific interactions. Where two
interactors are very different, such as the bee and the flower, the
same process of natural selection upon the interactive cost of the
association may lead to interdependence through
coadaptation. Where associated species do not interact there can be no
coadaptation, but there can be holistic adaptation.
This occurs where an organism is adapting to biotic and abiotic factors
within its environment (e.g.. the shady conditions under a
forest canopy). Smith (1990) terms holistic adaptation indirect
mutualism, stating that "it could influence community
organisation." All that we are really identifying here is an
unrecognised aspect of natural selection and that is selection for and
the evolution of economic efficiency to improve relative fitness.

    In support of peripatric speciation and by coincidence the predicted
effect of the i-factor upon speciation, Mayr found:

    [1] the main population of species show little obvious variation in
major features;

    [2] small, isolated populations showed greater variation in
features.

    The MELV model does not require total reproductive isolation but
simply that:

    [1] hybrids of the two interacting populations have lower fitness
than either population;

    [2] each variety has a higher relative fitness in its own habitat
that in its neighbours.

    As explained by Darwin, individuals from either population will then
continually colonise the interactive zone, but neither
population can penetrate the other's. Natural selection favours all
means of divergence that reduce the interactive costs. The
MELV model explains the rapid evolution followed by species stasis,
while genetic models predict enhanced variance in new
populations (Avers, 1989). The MELV model supports the punctuated
equilibria model of Niles Eldredge and Steven Jay
Gould (1972) that is based on the fossil record. It shows an
ecologically based mechanism that overrides and interacts with
genetic processes and in the genetics-ecology debate,  restores the
organism as the unit of selection. Perpetuity and
compatibility represent an evolutionary and ecological process resulting
the diversity of forms and associations found in nature.
Its principle is so simple that it must be true. "


from:
http://members.xoom.com/ecotao/conc_model.htm




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