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Subject: SW Focus Report - Evolution
Date: Tuesday, November 24, 1998 11:59 AM




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FOCUS REPORT: EVOLUTION

A Summary Group from SCIENCE-WEEK
-------------------------------------------------

ON EVOLUTIONARY ADAPTABILITY
Although developmental and evolutionary biology classically have
been distinct research disciplines, one of the consequences of
the emergence of molecular biology during the latter half of the
20th century is the apparent active merging of evolutionary and
developmental biology at several levels. Kirschner and Gerhart (2
installations, US) present a detailed analysis of the interaction
of development and evolution, the analysis amplified in a
contiguous paper by West-Eberhard (Smithsonian Tropical Research
Institute, US). The essential idea of Kirschner and Gerhart is
that flexible development, rather than being an alternative to
selection in the evolution of form, mediates the production of
selectable variation. They propose that environmentally sensitive
developmental flexibility, far from merely interfering with the
effects of genes, can ameliorate the deleterious results of
mutation and of environmentally induced variation, increasing the
viability of novel forms. The authors make the following points:
1) Evolvability (evolutionary adaptability) is an organism's
capacity to generate heritable *phenotypic variation. 2)
*Metazoan evolution is marked by great morphological and
physiological diversification, although the core genetic, cell
biological, and developmental processes are largely conserved. 3)
Metazoan diversification has entailed the evolution of various
*molecular regulatory processes controlling the time, place, and
conditions of use of the conserved core processes. 4) These
molecular regulatory processes, and certain of the core
processes, have special properties relevant to evolutionary
change, and these special properties reduce the interdependence
of components and confer robustness and flexibility on processes
during embryonic development and in adult physiology. 5) These
processes also confer evolvability on the organism by reducing
constraints on change and allowing the accumulation of nonlethal
variation. 6) Evolvability may have been generally selected in
the course of selection for robust flexible processes suitable
for complex development and physiology, and specifically selected
in evolutionary lines undergoing repeated *radiations. The
authors present detailed specific examples of various special
processes that may be relevant for evolvability, and they
conclude: "Today. we see the survivors of lineages that underwent
multiple radiations. These lineages have diversified by
maintaining a core of highly conserved processes and modifying
others. The core processes have unusual capacities to deconstrain
change in other processes and components. This has proven to be a
powerful strategy for the variation side of Darwin's variation
and selection principle of evolution."
QY: Marc Kirschner 
QY: Mary Jane West-Eberhard, Smithsonian Tropical Research
Institute, Unit 2511, APO AA 34020-9511
(Proc. Natl. Acad. Sci. US 21 Jul 98 95:8417,8420)
(Science-Week 21 Aug 98)
-------------------
Related Background:
... ... *phenotypic: The term "phenotype" refers to the total
appearance of an organism as determined by the interaction during
development between its genetic constitution (genotype) and the
environment. Thus, the concept of phenotype has always implied a
liaison between developmental and evolutionary biology. The idea
of Kirschner and Erhart is that the capacity for phenotypic
variation is itself an important evolving evolutionary element.
... ... *Metazoan: In general, the term "metazoa" refers to all
multicellular animals. Among important distinguishing
characteristics of metazoa are cell differentiation and
intercellular communication. For certain multicellular colonial
entities such as sponges, some biologists prefer the term
"parazoa".
... ... *molecular regulatory processes: An example of a
molecular regulatory process discussed by the authors is the
regulation of intracellular calcium ion concentration by the
calmodulin proteins, this regulation in turn activating or
deactivating physiological processes or other regulatory
processes.
... ... *radiations: In this context, the term "radiation" refers
to the spread of a group of biological entities into new
environments with consequent diversification.


ON GAIA AND NATURAL SELECTION
Considering a global system as complex as the planet Earth and
its resident biological entities, there are many possible
schemes, some more metaphorical than others, for organizing
observations and predicting future events. In recent years, The
*Gaia hypothesis first formulated by James E. Lovelock in 1979
has emerged as a possible conceptual framework for studying the
interaction of the environment of the surface of the Earth and
Earth's biota. The essential aspect of the Gaia hypothesis is the
consideration of feedback mechanisms whose consequence is self-
regulation of the environment-biota global system with an
emphasis on the importance of the biotic component in the
physical history of the planet. The Gaia hypothesis has its
severe critics, as evidenced in the following view of Tjeerd H.
Van Andel (*1994) (University of Cambridge, UK): "The conflict
between accepting what science teaches us and what the human
heart would like to believe is well illustrated by James
Lovelock's Gaia concept that places life in charge of the
functioning of our planet. It is a lovely thought, a tempting one
too, because it is a form of religion and the human soul requires
the comfort of a guided universe; it needs religion. Alas, it is
also unnecessary, because the world as it was, has evolved, and
now exists, is not explicable. It is merely very complex, and
life plays a role in it, but not the main one." This evaluation
notwithstanding, the effort to find a workable scheme to generate
understanding of environment-biota interactions continues.
... ... Timothy M. Lenton (University of East Anglia, UK)
presents an extensive review of the Gaia hypothesis and the place
of Darwinian natural selection in that scheme. The author makes
the following points: 1) Organisms alter their material
environment, and their environment constrains and naturally
selects organisms. This connection indicates feedback between
life and its environment. 2) The Gaia theory proposes that
organisms contribute to self-regulating feedback mechanisms that
have kept Earth's surface environment stable and habitable for
life, and the theory seeks to explain these mechanisms and how
they arise. 3) Natural selection, acting on faithful replication
of inherited variation, determines that the organisms that
dominate are the ones that leave the most descendants. Together,
natural selection and Gaia pose a puzzle: How can self-regulation
at the planetary level emerge from natural selection at the
individual level? 4) The author attempts to address this puzzle
by focusing on the feedbacks to biospheric growth and selective
pressures that can arise from environment-altering traits of the
biota. Land ecosystems and marine phytoplankton are discussed in
terms of several models. The author concludes: "The implications
may be far-reaching; simple principles suggest that environmental
regulation can emerge at levels from the individual to the
global. Natural selection is seen as an integral part of Gaia,
and Gaia theory also has something to offer evolutionary biology.
Gaian models suggest that we must consider the totality of
organisms and their material environment to fully understand
which traits come to persist and dominate."
QY: Timothy M. Lenton 
(Nature 30 Jul 98 394:439) (Science-Week 21 Aug 98)
-------------------
Related Background:
... ... *Gaia hypothesis: The theory was named for Gaia, the
Greek Earth-mother goddess.
... ... *1994: T. H. Van Andel, *New Views on an Old Planet: A
History of Global Change*, Cambridge University Press, 1994, p.
402.


EVOLUTION: ADAPTIVE RADIATION IN A HETEROGENEOUS ENVIRONMENT
The term "adaptive radiation" refers to the rapid evolution of
one or a few forms into many different species that occupy
different habitats within a new geographical area. The marsupial
radiation in Australia illustrates the process: when marsupials
were protected from competition with placental mammals by the
isolation of the continent, the process led to an entire array of
species with widely divergent functions, from herbivores to
carnivores. ... ... Rainey and Travisano (University of Oxford,
UK) report an investigation of the role of ecological opportunity
and competition in driving the genetic diversification associated
with adaptive radiation. The form studied was the common aerobic
bacterium Pseudomonas fluorescens, which evolves rapidly under
novel environmental conditions to generate a large repertoire of
mutants. As bacteria reproduce asexually, identical populations
can be established from a single genotype, and all subsequent
variation is therefore generated de novo by mutation. The authors
report that when provided with ecological opportunity, identical
populations of P. fluorescens diversify morphologically, but when
ecological opportunity is restricted, there is no such
divergence. In spatially structured environments, the evolution
of variant forms follows a predictable sequence, with competition
among the newly evolved niche-specialists apparently maintaining
this variation. The authors suggest their results demonstrate
that the elementary processes of mutation and selection alone are
sufficient to promote rapid proliferation of new designs, and
that their results support the theory that trade-offs in
competitive ability drive adaptive radiation.
QY: Paul B. Rainey 
(Nature 2 Jul 98 394:69) (Science-Week 31 Jul 98)


A MOLECULAR TIMESCALE FOR VERTEBRATE EVOLUTION
A timescale is necessary for estimating rates of molecular and
morphological change in organisms, and for interpreting patterns
of macroevolution and biogeography. Timescales have usually been
obtained from the fossil record, where the earliest represent-
atives of two lineages establish a minimum time of divergence of
these lineages. An alternative method involves the clock-like
accumulation of sequence differences in certain genes. Estimates
from single genes may have large statistical errors, but studies
of multiple genes provide a more reliable estimate of divergence
time. ... ... Kumar and Hedges (Pennsylvania State University,
US) report an analysis of 658 nuclear genes to provide divergence
time estimates for mammalian orders and major lineages of
vertebrates. The molecular times agree with most early
(Paleozoic) and late (Cenozoic) fossil-based times, but indicate
major gaps in the Mesozoic fossil record. The authors propose
that at least 5 lineages of placental mammals arose more than 100
million years ago, and most of the modern orders seem to have
diversified before the Cretaceous/Tertiary extinction of the
dinosaurs. The authors suggest their molecular timescale for
vertebrate evolution will be useful in calibrating local
molecular clocks and in estimating intra-ordinal divergence times
more reliably, especially in groups with poor fossil records.
QY: S. Blair Hedges (sbh1@psu.edu)
EMAIL
(Nature 30 Apr 98 392:917) (Science-Week 22 May 98)


DISCOVERY OF OLDEST KNOWN SEA-TURTLE FOSSIL
R. Hirayama (Teikyo Heisei University, JP) reports the discovery
of an exceptionally well-preserved skeleton of the oldest known
chelonioid (sea turtle). Reptiles constitute a primarily
terrestrial assemblage, but several groups returned to the marine
environment after the first appearance of reptiles in the
Paleozoic era. Successful diversification of the chelonioid sea
turtles, particularly during the Cretaceous period, was perhaps
one of the most important events in the history of turtles (and
marine reptiles). The fossil record of chelonioids before the
Late Cretaceous has been poorly documented. The skeleton reported
by the author is from the Early Cretaceous stage (about 110 mill-
ion years before the present) of eastern Brazil. This specimen
represents a new taxon, extending the history of chelonioids by
10 million years, and the author suggests it sheds new light on
the early evolution of the group: the details of the discovery
support the idea that the establishment of the salt-excreting
system, and the occupation of a marine habitat, may have preceded
the formation of rigid paddles in the history of chelonioids.
QY: Ren Hirayama (renhrym@ab.mbn.or.jp)
EMAIL
(Nature 16 Apr 98 392:705) (Science-Week 8 May 98)


ON CONFRONTING CREATIONIST FALLACIES
If there is any arena in which the interface between the
scientific community and society at large is of importance it is
the education of the public, and especially the education of the
young public. And perhaps no part of that arena in the US is of
more consequence to many citizens than the attempt by certain
groups to fashion the teaching of science according to
preconceived religious views. ... ... D. Wise (Franklin and
Marshall College, US) reviews the current controversy between
the scientific community and creationists and discusses a
strategy of intellectual confrontation. Among other things, Wise
enumerates the following creationist ideas from the Bible that
are totally irreconcilable with extant scientific data: 1) the
Earth came into existence before the sun and stars; 2) the land
plants came into existence before the Sun; 3) the first life
forms were plants; 4) fruit trees appeared before fish; 5) fish
appeared before terrestrial arthropods; 6) birds appeared before
land reptiles. Indeed, the acceptance of any of these ideas with
a restraint of consistency results in the tearing down of the
entire fabric of modern science (and the tearing down  of all
its applications, including modern medicine). Wise concludes,
"As scientists, we must emphasize repeatedly that the argument
against creationism is not against religion as such but rather
against a fringe group's attempt to force the Bible into the
public schools in the guise of a science textbook... The time
has come to stop fighting defensive skirmishes and start
challenging creationists to defend in toto what they call
science -- humorous absurdities and all."
QY: Donald U. Wise 
(American Scientist Mar/Apr 1998)


DISCOVERY OF 3-DIMENSIONAL ALGAE AND ANIMAL EMBRYO FOSSILS
A phosphorite is a sedimentary rock composed chiefly of phosphate
minerals. The Proterozoic era (also called the Algonkian) is the
geologic time between the Archean and the Paleozoic, with the
Archean beginning about 3.9 billion years ago and involving the
first appearance of sedimentary rocks and the first primitive
organisms at the bottom of the oceans. In paleontology, the term
"radiation" refers to a diverging and diversifying spread of
animals or plants into new environments with a resultant
production of new evolutionary forms, and the Ediacaran radiation
refers to an assemblage (until now the oldest) of soft-bodied
marine animals, the assemblage first discovered in the Ediacara
Hills in Australia. The algae comprise a large mixed group of
photosynthetic and essentially single-celled plants, and are
considered ancestral to modern green plants. Thalli are primitive
types of plant bodies not differentiated into stems, leaves, and
roots; the term also refers to the gametophyte generation (the
phase of the plant life cycle producing reproductive cells) of
some ferns and lichens. The term "cleavage stages" refers to the
early stages of embryo formation when the egg cell rapidly
divides into smaller and smaller cells. The "Bilateria" are a
major division of the animal kingdom comprising all forms with
bilateral symmetry, and the term "bilaterians" refers to the
first such forms appearing after the emergence of protozoa. The
term "phylogeny" refers to the evolutionary history of an
organism or group of organisms. ... ... Xiao et al (3 authors at
2 installations, US CN) report the discovery of phosphorites of
the late Neoproterozoic (570 +- 20 million years ago) in the
Doushantuo Formation, southern China, an apparent exceptional
record of multicellular life from just before the Ediacaran
radiation of macroscopic animals. Abundant thalli with cellular
structures preserved in 3-dimensional detail show that late-
Proterozoic algae already possessed many of the anatomical and
reproductive features seen in modern marine flora. Embryos
preserved in early cleavage stages indicate the divergence of
lineages leading to bilaterians may have occurred well before
their macroscopic traces or body fossils appear in the geological
record. The authors suggest that discovery of these fossils shows
that the early evolution of multicellular organisms is amenable
to direct paleontological inquiry, and that paleontological
observations, together with insights from molecular phylogeny and
developmental genetics, can facilitate a modern integration of
phylogeny, development, and paleontology that extends deeply into
evolutionary history to address the early evolution of
multicellular life. QY: Andrew H. Knoll, Botanical Museum,
Harvard University, 617-495-1000 (Nature 5 Feb 98)


PRECAMBRIAN SPONGES WITH CELLULAR STRUCTURES
The sponges (Porifera) are a phylum of primitive multi-cellular
animals (Metazoa), always attached at one point to a substrate,
usually without a definite symmetry, and usually marine. The
sponge body is a loose aggregation of cells with little
intercellular coordination, but the cells are specialized into
various types with various functions important to the viability
of the entire organism. There is also in sponges an internal
"skeleton" of chalk, silica, or protein, and "calcareous sponges"
are sponges containing a relatively large amount of calcium
carbonate. In this report, "spicules" are the hard internal
structures in sponges composed primarily of silica or calcium
salts, and a "monoaxonal" spicule is a spicule having essentially
only one dimension (e.g., needle-like). The term "epidermis"
refers to the outermost layer of cells in any multicellular
organism; "porocytes" are tubular cells that constitute the walls
of certain sponges; "amoebocytes" are freely moving cells (sponge
cells in this context) within a metazoan tissue; "sclerocytes"
are cells involved in the formation of the sponge skeleton;
"spongocoel" refers to the branching internal cavity of a sponge,
the cavity having a connection in one place or another to the
external aqueous environment; "flagella" are long and thin
cellular organelles that protrude from the surfaces of cells and
are specialized to produce locomotion. The Cambrian period
extended from 545 to 505 million years ago, and was the time
during which many multicellular organisms first arose, and the
Vendian period is the Precambrian metazoan fossil period. The
term "Cambrian explosion" refers to the apparent relatively
sudden appearance of an enormous number of living forms during
the Cambrian period. ... ... Li et al (3 authors at 2 install-
ations, TW CN) report the identification of sponge remains in the
Early Vendian Doushantuo phosphate deposit (cf. report #8, this
issue) in south China. The skeletons consist of siliceous
monoaxonal spicules, with preserved soft tissues including the
epidermis, porocytes, amoebocytes, sclerocytes, and spongocoel,
and among thousands of metazoan embryos a sponge larva having a
shoe-shaped morphology and dense peripheral flagella. The authors
suggest the data indicate the calcareous sponges may have an
extended history in the Late Precambrian, and that animals lived
40 to 50 million years before the Cambrian explosion. They
further suggest these Doushantuo rocks provide a potentially
inexhaustible resource for understanding the early evolution of
animal life. QY: Chia-Wei Li, National Tsing Hua Univ., Hsinchu,
Taiwan, China (Science 6 Feb 98)


A MODEL FOR THE EVOLUTION OF THE GENETIC CODE
Eukaryotic cells are cells with discrete organelles such as
nuclei, mitochondria, etc. As organelles, the mitochondria, which
are of prime importance in the oxygen metabolism of eukaryotes,
are a special class, since it is generally believed they probably
originated as primitive cells that established themselves in
symbiotic arrangements in the interiors of larger cells. The
mitochondria, in fact, carry their own DNA and thus their own
genetics. In mathematics, a Lie algebra (named after M.S. Lie
1842-1899) is a system of vector fields on a topological space in
which independent quantities are reduced to groupings whose
relationships are then subject to algebraic operations. In the
context of this report, the term "representation theory" refers
to the study of algebraic groups by the use of their represent-
ations, with the representation of a group given by a virtual
"mapping" of the group onto a group of another type. Pyrimidine
and purine are precursors of DNA nucleotide bases. A codon is the
elemental genetic coding unit, a triplet of 3 consecutive
nucleotides that define a specific amino acid. Some amino acids
are determined by more than one codon, leading to a degeneracy
(redundancy) in the genetic code. ... ... Bashford et al (3
authors at University of Tasmania, AU) present a model for the
structure and evolution of the eukaryotic and vertebrate
mitochondrial genetic codes based on the representation theory of
a Lie superalgebra, with a key role played by pyrimidine and
purine exchange symmetries in codon quartets. The authors suggest
that the group theoretical technique is able to give a succinct
account of many of the currently understood aspects of the
evolution of the genetic code and the observed degeneracy
structure of the codon:amino-acid correspondences, and that their
particular model is susceptible to quantitative verification.
QY: P.D. Jarvis 
(Proc. Natl. Acad. Sci. US 3 Feb 98)


DISCOVERY OF OLDEST KNOWN ANT FOSSILS
The ants are one of the most successful extant species, until now
believed to have first appeared about 40 million years ago. With
the wasps, bees, and termites, they are remarkable as invertebr-
ates exhibiting colonies that involve division of labor and a
complex social organization. The so-called "Turonian" geologic
stage (92 million years ago) corresponds approximately to the
Upper or Middle Cretaceous period. ... ... Agosti et al (3
authors at American Museum of Natural History New York, US) now
report fossil ants, including a new genus of Ponerinae,
approximately 50 million years older than the previous oldest
specimens. The discoveries were made in New Jersey (US) in amber
from the Turonian stage, and include 3 worker and 4 male ants.
The authors suggest the specimens have important implications for
dating the origin of ants, and extend the age of an extant ant
subfamily back about 50 million years.
QY: Donat Agosti  (Nature 29 Jan 98)


LAND-MASS MIGRATIONS OF LATE CRETACEOUS MAMMALS
We have already noted the Cretaceous period as the geological
period ranging approximately from 146 million years ago to 65
million years ago. During this period the geographical distrib-
ution of Earth's land masses was apparently substantially differ-
ent than it is today. Some 375 million years ago, two large
supercontinent called Laurasia and Gondwana comprised most or all
of the present continental land masses, Laurasia in the north and
Gondwana in the south. These two supercontinents merged into the
single supercontinent Pangaea about 250 million years ago, and
subsequent fragmentation of Pangaea began about 180 million years
ago and the fragmentation eventually produced the continental
land masses we know today. During this fragmentation there
existed the second phase of Gondwana, incorporating what is now
South America, Africa, Antarctica, Australia, and India. This
historical picture, which is the present general consensus, is
the result of a large number of geological and paleobiological
studies, and in paleobiology one important task is to reconstruct
the land mass migrations of the fauna and flora evolving through-
out these large-scale geographical alterations. Now Krause et al
(5 authors at 4 installations, US IN DE) report the occurrence of
a highly specialized and distinctive group of extinct mammals,
the Sudamericidae (Gondwanatheria), in the late Cretaceous of
Madagascar and India. The authors suggest these new fossil
records are the first evidence of gondwanatheres outside South
America, and the first indication of cosmopolitanism among late
Cretaceous Gondwanan mammals. They also suggest that Antarctica
may have served as an important Cretaceous biogeographic link
between South America and Indo-Madagascar.
QY: David W. Krause 
(Nature 4 Dec 97)


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