ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

March 24, 2000 -- Vol. 4 Number 12

-----------------------------------------------

It can never be satisfied, the mind, never.
-- Wallace Stevens (1879-1955)

-----------------------------------------------

Contents of This Issue:

1. Astrophysics:
Observations of Galaxy Formation in Action
------------------------------------------
New evidence concerning violent galaxy-galaxy interactions that
produce "tidal dwarf galaxies" indicates that the renewed star-
formation process in such dwarf galaxies mimics the star-
formation process in normal spiral galaxies. (Includes related
background material.)

2. Astronomy:
New Evidence on the Threat of Near-Earth Asteroid Impact
--------------------------------------------------------
Although new evidence suggests the number of larger near-Earth
asteroids is approximately half that previously estimated, the
number of smaller near-Earth asteroids is approximately 100 times
larger, and there may be a threat of at least 1 major impact
during the next century. (Includes related background material.)

3. Applied Biology:
Functional Human Corneal Equivalents Constructed from Cell Lines
----------------------------------------------------------------
A new technique allows the development of a morphological and
functional equivalent of the human cornea from living cells, and
has promise of wider application in transplantation tissue
engineering.

4. Molecular Biology:
On the Rat as a Model System in Functional Genomics
---------------------------------------------------
Functional genomics is the identification of the biological role
of various sequences in a genome, and of great importance for
human functional genomics will be the choice of suitable animal
models and the complete sequencing of the genomes of those
animals. The rat is a leading candidate for such an animal model.

5. Molecular Biology
First Myoglobin-Like Heme-Containing Protein in Archaea
-------------------------------------------------------
New evidence indicates the presence of myoglobin-like heme
proteins in Archaea, the apparent oldest kingdom of living
systems, with the heme-proteins acting as oxygen sensors in
aerotaxis. (Includes related background material.)

6. Medical Biology:
On Narcolepsy
-------------
Evidence indicates specific neural degeneration may be involved
in narcolepsy, a disorder which affects approximately 300,000
people in the US and millions of people worldwide.

In Focus: On Genes, Organisms, and Species

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

1. ASTROPHYSICS:
OBSERVATIONS OF GALAXY FORMATION IN ACTION
     The most important process underlying the birth of galaxies
and stars is the gravitational condensation of gases and
particulate matter. This gravitational condensation occurs over
extremely long time spans, by human standards -- hundreds of
millions of years. We cannot record these events for any single
system, but we can observe various systems in various stages, and
we can devise mathematical models of the events and test
predictions of the models against observations.
     We know the dimensions of gaseous entities -- they are atoms
and molecules. But what of the particulate matter, the "dust" of
the interstellar medium? Roughly 1 per cent of the interstellar
medium is dust, the rest hydrogen gas (80%) and helium gas (20%).
The dust consists of particles the size of the particles in
cigarette smoke, the particles believed to be of carbon, iron,
and silicates, mixed with or coated with frozen water. From
estimates of the density, the average distance between dust
grains is approximately 150 meters, so that we are considering
extremely sparse populations of particles.
     In addition, the interstellar medium is not uniformly
distributed through space, but contains regions of high-density
and low-density clouds. The low-density clouds are essentially
expanded distributions and "hotter", while the high-density
clouds are essentially contracted distributions and "cooler". The
cool, high-density clouds contain only approximately 10 to 1000
atoms per cubic centimeter, which is less than any vacuum we can
achieve on Earth in the laboratory, so "high-density" has a
special meaning here.
     Here is the outline of the history of a galaxy:
     The galaxy forms from the gravitational condensation of
massive amounts of hydrogen and helium in the cool, high-density
interstellar clouds. First, a galactic sphere of gas is formed.
This gravitational condensation sphere flattens and becomes a
rotating disk of gas. In the disk, perturbations produce internal
clusterings of gas molecules. These clusterings lead to proto-
stars in clusters. The protostars, when they reach a certain
density and total mass, ignite into ordinary stars. Each cluster
of true stars slowly dissipates, with the individual stars
remaining associated by gravitational forces with the galaxy as a
whole.
     Detailed images from the Hubble Space Telescope have
revealed that galaxies frequently interacted in the past, and it
has been suggested that such interactions were important in
creating many of the bright galaxies which are currently
observed.
     Determining the details of galaxy formation and interaction
is currently one of the central problems in astrophysics, and the
past decade has seen important advances in understanding the
dynamics of these processes.
... ... J. Braine et al (4 authors at 4 installations, FR ES TW)
present new evidence concerning violent galaxy-galaxy
interactions, the authors making the following points:
     1) The authors point out that in many gravitational
interactions between galaxies, gas and stars that have been torn
from the precursor galaxies can collect in tidal "tails". Star
formation begins anew in some of these regions, producing what
are called "tidal dwarf galaxies". Observations of these new
galaxies provides insight into processes relevant to galaxy
formation more generally, because the timescale of interaction is
usually well-defined. But tracking the star formation process has
been difficult because the tidal dwarf galaxies with young stars
have shown no evidence of the molecular gas out of which these
young stars are presumed to have formed.
     2) The authors now report the discovery of molecular
hydrogen (traced by correlative carbon monoxide emission) in two
tidal dwarf galaxies. In both cases, the concentration of
molecular gas peaks at the same location as the maximum in
atomic-hydrogen density, unlike the situation in most gas-rich
galaxies. The authors infer from this that the molecular gas
formed from the atomic hydrogen, rather than being torn in
molecular form from the interacting galaxies. The authors suggest
that star formation in the tidal dwarf galaxies therefore appears
to mimic the process in normal spiral galaxies such as our own.
-----------
J. Braine et al: Formation of molecular gas in the tidal debris
of violent galaxy-galaxy interactions.
(Nature 24 Feb 00 403:867)
QY: Jonathan Braine [Jonathan.Braine@observ.u-bordeaux.fr]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 24Mar00
[For more information: http://scienceweek.com/swfr.htm]
-------------------
Related Background:
COSMOLOGY: ON THE EPOCH OF GALAXY FORMATION
One of the central problems in cosmology is to understand the
entire history of galaxies, from their first gravitational
collapse in the early Universe to the rich variety and detail
observed in the present-day Universe. ... ... Charles C. Steidel
(California Institute of Technology, US) presents a review of
current research concerning galaxy formation, the author making
the following points:
     1) There are essentially two approaches to the problem of
the evolution of galaxies. The first approach is the classical
approach in which observations of the chemical content,
kinematics, and spatial distribution of stars in our Galaxy and
in nearby galaxies are used to infer the sequence of events that
produced what we see today. The second approach to the evolution
of galaxies uses samples of galaxies at progressively larger
distances as snapshots of the increasingly remote past. One
advantage of the second approach is that it may be possible to
observe directly the evolution of galaxy populations and actually
catch incipient galaxies in the process of formation. Relevant to
the second approach is the fact that it is now possible to
observe galaxies over greater than 90 percent of the age of the
Universe, as a result of new telescopes, new instruments, and new
techniques.
     2) In the 1960s and 1970s, there emerged the view that large
galaxies formed through a rapid gravitational collapse that would
have resulted in huge bursts of star formation at early epochs,
leaving behind galaxies that had rapidly exhausted their fuel to
age quietly until the present, such galaxies appearing now as
large collections of old *red stars, e.g., *elliptical galaxies
and the central bulges of *spiral galaxies. This formation
scenario is often referred to as "monolithic collapse".
     3) The modern theoretical framework for understanding galaxy
formation can be said to date back only approximately 20 years.
The modern approach, rather than working backwards from our
present understanding of stellar evolution and stellar dynamics,
works forward from prescribed initial conditions by using the
physics of structure formation. The modern approach attempts to
understand galaxy formation as a natural consequence of the
growth of mass fluctuations resulting from gravitational
instability.
     4) Theoretical arguments and numerous observations suggest
that initial conditions imposed in the early Universe, together
with gravitational instability, would result in a universe in
which the smallest mass fluctuations collapse first and merge
into progressively larger structures as time goes on. The
theoretical models currently favored are all variations on this
general framework, which is often called "hierarchical structure
formation". The basic premise is that *dark matter dominates the
overall mass density, interacts only gravitationally with normal
matter, but ultimately dictates the underlying structure of all
matter.
     5) Recent observational progress has largely involved making
the early stages of the Universe less mysterious by filling in
some of the glaring gaps in empirical information concerning the
earliest observable galaxies. In general, what has emerged over
the last 3 years is an outline of overall energy production by
star formation for nearly the entire history of the Universe, the
ability to discern the detailed morphologies (shapes and sizes)
of the distant galaxies, and the demonstration of efficient
observational techniques that make feasible large-scale surveys
of galaxies in the distant Universe.
     6) The author concludes: "The health of general theoretical
ideas about galaxy formation is quite robust at present, although
the details are still very immature... The uncovering of the
universe of galaxies beyond the epoch that has been explored
already, and more detailed views of the currently accessible
epochs, will be made possible by the Next Generation Space
Telescope, a large infrared-optimized telescope planned for 2007
or 2008. The next decade promises to bring a literal avalanche of
data relevant to the question of galaxy formation. The challenge
will be to make sense of it all."
-----------
Editor's note: In addition to the background material below, see
the SW Focus Report "Astrophysics: Evolution of Galaxies"
available at URL [http://www.scienceweek.com/swfr039.htm].
-----------
Charles C. Steidel: Observing the epoch of galaxy formation.
(Proc. Natl. Acad. Sci. US 13 Apr 99 96:4232)
QY: Charles C. Steidel [ccs@astro.caltech.edu]
-----------
Text Notes:
... ... *red stars: In general, red stars have surface
temperatures between 2500 and 5000 degrees kelvin. Two types of
red stars are recognized: red dwarfs (mass 0.8 to 0.08 solar-
mass) and red giants (diameter 10 to 100 times the Sun). Red
dwarfs are the most common type of star and apparently the
longest lived. Red giants are apparently old stars that have
exhausted their core hydrogen fuel.
... ... *elliptical galaxies: These are galaxies that have no
disc component, the shape varying from almost circular to narrow
ellipses. The stars within elliptical galaxies are predominantly
old stars. Elliptical galaxies display the greatest variation in
mass, ranging down to extreme dwarf galaxies (approximately
10^(6) solar-mass).
... ... *spiral galaxies: A type of galaxy with bright arms of
stars, gas, and dust that extend in a spiral pattern from a
central hub. The arms of the spiral apparently contain young
stars, while the hub contains old stars.
... ... *dark matter: In general, in this context, the term "dark
matter" refers to material whose presence can be inferred from
its effects on the motions of stars and galaxies, but which
cannot be seen directly because it emits little or no radiation.
It is believed that at least 90 percent of the mass in the
Universe exists as some form or dark matter.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Jun99
-------------------
Related Background:
ON THE EVOLUTION OF GALAXIES IN A CHANGING UNIVERSE
In a detailed review of research in this century concerning the
evolution of galaxies, Alan Dressler (Carnegie Observatories
Pasadena, US) makes the following points: 1) When the century
opened, most astronomers assumed the Universe was eternal and
basically changeless, its general structure immutable. In 1915,
with the publication of Einstein's general theory of relativity,
there were implications of the cosmic role of gravity, but these
implications were for the most part ignored. Indeed, even after
1924 and the proof by *Edwin Hubble (1889-1953) that the spiral
nebulae are other galaxies at vast distances, astronomers were
slow to recognize the implications of the new observations. 2)
The first big step in changing the view of the cosmos was the
construction by *George Ellery Hale (1868-1938) of the 100-inch
Hooker reflector on Mount Wilson (US), a project completed in
1918. This was the telescope used by Hubble and his colleagues to
reveal the large-scale organization of the Universe into
galaxies, the vast size of the Universe, and the expansion of the
Universe. *George Lemaitre (1894-1966) soon proposed that the
expansion of the Universe implied a dense explosive birth of the
Universe at a specific finite time in the past (the event that
came to be called the Big Bang). 3) An even greater telescope was
needed, and again George Hale led the way in the building of the
200-inch reflector on Palomar Mountain (US), that instrument
finally completed in 1948. The 200-inch telescope produced the
first observations of galaxy evolution -- the first evidence that
galaxies observed at high *redshift are unlike galaxies closer to
us in time. An even greater accomplishment of the 200-inch
telescope was a series of observations concerning the spectra of
*quasars, and the evidence for their immense distances and
luminosities. Theorists eventually proposed that quasars were
*black holes of 100 million solar-masses or more. It is now clear
that most galaxies with a central bulge, including our own
Galaxy, harbor massive black holes at their cores. 4) In 1961,
Allan Sandage published a landmark paper outlining the
possibility of testing cosmological models with the 200-inch
telescope, and over the next two decades Sandage devoted himself
to this project. Unfortunately, the underlying premise of the
project -- that the brightest galaxy in every galactic cluster
has about the same true luminosity -- was demonstrated by
*Beatrice Tinsley (1941-1981) to be untenable. 5) In the fall of
1977, at a Yale University (US) conference on the evolution of
galaxies, Harvey Butcher and Augustus Oemler presented their
evidence for relatively young star-forming galaxies. This
evidence, which implied strong galaxy evolution during relatively
recent cosmic time, met with controversy and skepticism. 6) In
the 1980s, observations by various groups proved that Butcher and
Oemler were correct, and it was now understood that these
relatively young galaxies were often producing new stars in huge
bursts. These bursting galaxies are evidently spirals with a more
disheveled appearance than is common today, and in their twisted
and distorted disks huge numbers of stars were recently born. 7)
During the past 2 years, among the most interesting results of
various observations with various instruments is the formulation
of the so-called Madau diagram (popularized by Piero Madau) that
plots the Universe-wide rate of star formation from early times
to today, spanning almost the whole history of the cosmos. The
rate of star formation apparently rose rapidly in the first few
billion years, the peak rate at about 5 or 6 billion years later
at redshifts of 1 to 2. (Our Sun apparently formed at a time
corresponding approximately to redshift = 0.5.) The author
concludes: "Our generations are fortunate to live to see one of
the great mysteries of where we came from in process of being
solved."
QY: Alan Dressler, Carnegie Observatories, Pasadena, Ca. US.
(Sky & Telescope October 1998) (Science-Week 11 Sep 98)
-------------------
Related Background:
... ... *Edwin Hubble (1889-1953): Hubble first studied law
before switching to astronomy at the age of 25. He began his work
at the Mount Wilson Observatory with the 100-inch telescope at
the age of 30. In 1941, at the age of 52, he tried to join the US
Army to fight the Nazis, but he was persuaded that he could do
more in war-related research. 
... ... *George Ellery Hale (1868-1938): Hale is best known for
his work building large-telescopes (and for obtaining the funds
for the Yerkes Observatory, named after the street-car magnate
Charles Tyson Yerkes), but already at the age of 21 he invented
the spectroheliograph, a device that made it possible to
photograph the light of a single spectral line of the sun, and he
made several ground-breaking observations with this instrument.
... ... *George Lemaitre (1894-1966): Lemaitre began his
professional life as a civil engineer, then at 21 he switched to
physics and mathematics. He also became a Roman Catholic priest
at the age of 22. After obtaining his PhD at the Massachusetts
Institute of Technology in 1927, he settled in Belgium as a
professor of astrophysics at the University of Louvain. At the
time of his death, he was president of the Pontifical Academy of
Sciences at Rome. Lemaitre's theoretical ideas concerning the
origin of the Universe were published in 1927, when he was 31,
but the paper was largely unnoticed until the astrophysicist
Arthur Eddington (1882-1944) called attention to it much later.
... ... *redshift: Redshift (symbol: z) is a lengthening of the
wavelengths of electromagnetic radiation from a source caused
either by the movement of the source (Doppler effect) or by the
expansion of the universe (cosmological redshift). Redshift is
defined as the change in wavelength of a particular spectral line
divided by the unshifted wavelength of that line. Large redshifts
imply large radial velocities (which imply large distances,
according to current cosmological theory), but at redshifts
greater than about 0.2 there is a relativistic divergence from a
linear relation. A redshift of 4.0 corresponds to an object
receding with a radial velocity 92% that of the velocity of
light. The largest astrophysical redshifts so far observed are of
the order of z = 4.9.
... ... *quasars: (quasi-stellar objects). Extremely luminous
sources radiating energy over the entire spectrum from x-rays to
radio waves, and which are apparently the oldest and most distant
objects in the universe.
... ... *black holes: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable. Another view of a black hole
is that it is a mass that has collapsed to such a small volume
that its gravity prevents the escape of all radiation. Space and
time essentially have no meaning in a black hole. The boundary of
the black hole is called the "event horizon", because any event
within the boundary is invisible outside, the invisibility
resulting from the fact that no radiation can escape to be
detected. The radius of the black hole depends upon how much
matter has fallen into the region; it is called the "Schwarzchild
radius", and it is usually a few kilometers. However, massive
black holes are possible and are thought to be the source of
quasars. If quasars indeed involve black holes, the radiation is
from material just outside the black hole, and not from anything
within it. Nothing inside a black hole can get out of it.
... ... *Beatrice Tinsley (1941-1981): During her short life,
Tinsley managed to be a force in astronomy from her first entry
into the field. At the age of 25, an unknown graduate student at
the University of Texas, she rose before an audience about to
hear Allan Sandage and publicly challenged his idea that giant
elliptical galaxies exhibited luminosities constant enough to be
used as "standard candles" to estimate distances. She proved her
point by the age of 36, and the variability of galaxy
luminosities became the consensus view. It was Tinsley who co-
hosted the 1977 Yale conference that set the course of galaxy-
evolution studies. She died 4 years later of cancer. Near the
end, she wrote the following: "Let me be like Bach, creating
fugues; till suddenly the pen will move no more."
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 11Sep98
-------------------
Related Background:
ON THE AGGREGATION OF YOUNG GALAXIES IN A DARK-MATTER UNIVERSE
The term "semi-analytic modeling" refers to a quantitative
modeling procedure in which empirical data are used in places to
fix the values of parameters or functions, rather than deriving
these from theoretical principles. N-body simulations, which
usually require extraordinary computational resources, are
simulations involving calculations of interactions of a large
population of entities. "Dark matter", which is thought to
comprise as much as 90% or more of the mass of the universe, is
undetectable except by gravitational effects, and "cold dark
matter" refers to dark matter particles created with low velocity
dispersions in the early universe. At the present time, computer
simulations and empirical observations of galaxy clustering favor
the idea that most dark matter in the universe is cold dark
matter. ... ... Governato et al (7 authors at 3 installations, UK
DK US) report the use of a combination of theoretical techniques
(semianalytic modeling and n-body simulations) to show that large
concentrations of young galaxies (i.e., galaxies in existence
when the universe was one-tenth of its current age) should be
quite common in a universe dominated by cold dark matter, and
that such galaxy concentrations are the progenitors of the rich
galaxy clusters seen today. The authors suggest these clustering
properties of primeval galaxies will be compared with data
collected in the near future, and that the comparison will be a
test of our current understanding of galaxy formation within the
framework of a universe dominated by cold dark matter.
QY: C.S. Frenk [c.s.frenk@durham.ac.uk]
(Nature 26 Mar 98) (Science-Week 10 Apr 98)

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

2. ASTRONOMY:
NEW EVIDENCE ON THE THREAT OF NEAR-EARTH ASTEROID IMPACTS
So-called "near-Earth asteroids" are small rock bodies (diameters
less than 10 kilometers) with orbits that approach the orbit of
the Earth. It has been estimated that most such asteroids have a
chance of approximately 0.5 percent of colliding with the Earth
in the next million years. The total number of such bodies with
diameters greater than 1 kilometer has been estimated to be in
the range of 1000 to 2000, which translates to an approximately 1
percent chance of a catastrophic collision with the Earth in the
next millennium. These numbers, however, are poorly constrained
because of the limitations of photographic plates used in
previous searches.
... ... D. Rabinowitz et al (4 authors at 2 installations, US)
now report an analysis of a survey for near-Earth asteroids that
used improved detection technologies, including the use of a
large-format charge-coupled device (CCD). The authors report that
the total number of asteroids with diameters greater than 1
kilometer is approximately half the earlier estimates, and that
"at the current rate of discovery of near-Earth asteroids, 90
percent will probably have been detected within the next 20
years."
... ... In a commentary on the above report, David Jewitt
(University of Hawaii, US) states as follows: "The focus on near-
Earth asteroids larger than 1 kilometer ignores the threat from
smaller but much more numerous objects. The Earth's atmosphere
offers little protection against objects larger than 100 meters
in diameter. These smaller objects outnumber near-Earth objects
larger than 1 kilometer by a factor of 100, so they are much more
likely to strike in our lifetimes. There is a 1 percent chance
that the Earth will be struck by a 300-meter near-Earth object in
the next century. Such an impact would deliver a withering 1000-
megaton explosion and cause perhaps 100,000 deaths. If the impact
occurred in or near a densely populated region -- the eastern
seaboard of the US, for example, or Western Europe or coastal
Asia -- the fatalities could rise into the tens of millions.
Neither can we take refuge in the fact that 70 percent of the
Earth is covered by oceans. Impact-induced tsunamis could wipe
out coastal cities over a wide area." The author suggests an
extensive survey to completely identify the population of small
potentially threatening near-Earth objects, with such a project
including a large (6 to 8 meter) telescope with a wide field of
view tiled with charge-coupled device optical detectors and
connected to a massive computer array capable of meeting data
processing demands. The author suggests such a telescope could be
constructed for US$100 million, half the price of a Jumbo jet.
The author points out that so far there is no sign of such a
project being funded by governments or their agencies. The author
concludes: "Perhaps astronomers can attract the interest of
private donors in the search for threatening near-Earth objects.
If not, it seems we will have to face the asteroidal impact
hazard with our eyes wide shut."
-----------
D. Rabinowitz et al: A reduced estimate of the number of
kilometer-sized near-Earth asteroids.
(Nature 13 Jan 00 403: 165)
QY: David Rabinowitz [david.rabinowitz@yale.edu]
-----------
David Jewitt: Eyes wide shut.
(Nature 13 Jan 00 403:145)
QY: David Jewitt [jewitt@ifa.hawaii.edu]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 24Mar00
[For more information: http://scienceweek.com/swfr.htm]
-------------------
Related Background:
ASTRONOMY: ON THE EFFECTS OF ASTEROID-EARTH IMPACTS
During the past several years, there has been much media
attention devoted to the prospect of an asteroid impacting Earth.
Such impacts were apparently more common several billion years
ago than at present, but impacts are definitely possible at any
time, and the US National Aeronautics and Space Agency (NASA) has
in place a program to detect all near-Earth asteroids larger than
approximately 1 kilometer in radius. But if we do detect a large
asteroid on a collision-course with Earth, it is not yet clear
what we can do about it with our present technology except
perhaps engineer a nuclear missile hit to deflect it.
... ... Jack J. Lissauer (NASA Ames Research Center, US) presents
the following considerations concerning the expected effects of
asteroid impacts on Earth and life on Earth:
     1) The largest mass extinction of the past 200 million years
occurred 65 million years ago, when approximately half of the
genera of multicellular organisms on Earth, including all of the
dinosaurs, suddenly died off. The geological record indicates
that a layer of impact-produced minerals and the element iridium
(an element rare in the crust of the Earth but more abundant in
primitive meteorites) was deposited at the time the dinosaurs
vanished -- the so-called Cretaceous/Tertiary or K/T boundary. In
addition to this, the largest known crater on Earth to be dated
at less than 1 billion years old was apparently formed at this
time. Taken together, these data imply that the K/T mass
extinction was caused by the impact into the Yucatan peninsula of
an asteroid or comet of approximately 10 kilometers in radius.
[*Note #1].
     2) The author presents the following tabulation of the
effects of impacts of objects of various sizes on the Earth and
on life on Earth:
... ... a) Super colossal object (radius > 2000 kilometers):
Melts the planet; drives off all volatiles and wipes out life on
the planet.
... ... b) Colossal object (radius > 700 kilometers): Melts the
crust; wipes out life on planet.
... ... c) Huge object (radius > 200 kilometers): Vaporizes
oceans; life may survive below the surface.
... ... d) Extra large object (radius > 70 kilometers): Vaporizes
upper 100 meters of oceans; pressure-cooks photic zone; may wipe
out photosynthesis.
... ... e) Large object (radius > 30 kilometers): Heats
atmosphere and surface to approximately 1000 degrees kelvin;
continents cauterized.
... ... f) Medium object (radius > 10 kilometers): Fires, dust,
darkness; atmosphere/ocean chemical changes; large temperature
swings; half of living species extinct.
... ... g) Small object (radius > 1 kilometer): Global dusty
atmosphere for months; photosynthesis interrupted; individual
deaths but few species become extinct; civilization threatened.
... ... h) Very small object (radius > 100 meters): Major local
effect; minor hemispheric effects; dusty atmosphere; only minor
global effects on life.
-----------
Jack L. Lissauer: How common are habitable planets?
(Nature 2 Dec 99 402supp:C11)
QY: Jack L. Lissauer [lissauer@ringside.arc.nasa.gov]
-----------
Text Notes:
... ... *Note #1: Some authors have stated the Yucatan impactor
had a diameter of 10 kilometers rather than a radius of 10
kilometers.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 14Jan00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON THE IMPACT HAZARDS OF ASTEROIDS
G. Verschuur (University of Memphis, US) reviews the
probabilities and consequences of asteroid collisions with Earth.
Our civilization has just passed through an extraordinary era of
scientific discovery that has brought with it the awareness that
planet Earth is profoundly vulnerable to devastating cosmic
collisions. In recent years, the evidence that mass extinctions
of life on Earth can be attributed to the consequences of comet
or asteroid impacts has become overwhelming. Most famous among
such catastrophes is the Cretaceous-Tertiary impact that
apparently marked the demise of the dinosaurs about 65 million
years ago. The attention of many planetary scientists has turned
to the problem of assessing the likelihood that our civilization
may be threatened by a rogue comet or asteroid in the near
future. Asteroid hunters estimate that 9000 objects of dimensions
0.5 kilometers or larger are in near-Earth orbits, and that of
these only 350 have been identified to date. A small-to-medium
size 200 meter object smashing into a 5-km deep ocean at 50 km
per second would raise a splash 35 kilometers high in 40 seconds
and produce tsunamis that would inundate lands bordering the
ocean. Calculations indicate that the impact anywhere on Earth of
even a medium-size asteroid 0.2 to 1.0 km would be catastrophic.
The author suggest that we have been lucky to avoid a recent 
catastrophic collision with a comet or asteroid, and that
although it may not happen in the next year or the next century,
eventually the Earth *will* be hit by a sizable piece of cosmic
debris. QY: Gerrit L. Verschuur, Univ. of Memphis 901-678-2169.
(Sky & Telescope June 1998) (Science-Week 1 May 98)
[For more information: http://scienceweek.com/swfr.htm]

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

3. APPLIED BIOLOGY:
FUNCTIONAL HUMAN CORNEAL EQUIVALENTS CONSTRUCTED FROM CELL LINES
The cornea is the transparent front portion of the eyeball, its
area occupying approximately one-sixth the circumference of the
globe of the eye, the structure consisting of 3 major cellular
layers: a) an outermost *stratified squamous epithelium; b) a
*stroma with *keratocytes; and c) an innermost monolayer of
specialized *endothelial cells. Because of its structure, the
cornea serves as both a barrier to the outside environment and a
major element in the optical pathway of the eye. Since the cornea
is without blood vessels and is effectively isolated from the
immune system (immunologically "privileged"), it is an excellent
candidate for transplantation tissue engineering. At the present
time, corneal transplants are the most common organ transplant
operation and the most successful type of transplant, since
rejection of the transplant occurs only rarely. In general, in a
transplant procedure, the defective cornea is removed and a donor
cornea of similar diameter is sewn in. The current shortage of
donated corneas has been partially overcome by the development of
artificial corneas made of plastic, but there have been no
reports of successfully reconstructed artificial corneas that
mimic the anatomy and physiology of the human cornea.
... ... M. Griffith et al (9 authors at 4 installations, CA US)
now report a study to develop a morphological and functional
equivalent of the human cornea. The authors report the
construction of human corneal equivalents comprising the 3 main
cellular layers of the cornea, with each cellular layer
fabricated from genetically engineered *immortalized human
corneal cells that were screened for use on the basis of
morphological, biochemical, and electrophysiological similarity
to their natural counterparts. The authors report the resulting
corneal equivalents mimicked human corneas in key physical and
physiological functions, including morphology, biochemical
*marker expression, transparency, ion and fluid transport, and
gene expression. The authors suggest that morphological and
functional equivalents to human corneas that can be produced in
vitro have immediate applications in toxicity and drug efficacy
testing, and form the basis for future development of implantable
tissues. The authors conclude: "This technology provides a strong
basis for the development of temporary or permanent cornea
replacements with low rejection rates. Future research could lead
to readily available, complex engineered tissues that reproduce
their natural human counterparts and are suitable for implants,
transplants, and biomedical research."
-----------
M. Griffith et al: Functional human corneal equivalents
constructed from cell lines.
(Science 10 Dec 99 286:2169)
QY: May Griffith [mgriffith@ogh.on.ca]
-----------
Text Notes:
... ... *stratified squamous epithelium: In animals, epithelial
cells (epithelium) compose the cell layers that form the
interface between a tissue and the external environment, for
example, the cells of the skin, the lining of the intestinal
tract, and the lung airway passages. The cells of the
epithelium are for the most part closely packed cells with little
extracellular material between adjacent cells, the cells arranged
in continuous sheets in either single or multiple layers. The
cells may be flat, cubelike, columnar, or a combination of
shapes, and "squamous" cells are flattened and scalelike. In this
context, "stratified squamous epithelium" refers to a structure
consisting of distinctly layered epithelial cells (layers varying
in size and shape of cells), the top layer of which are squamous
cells.
... ... *stroma: The framework, usually of connective tissue, of
an organ, gland, or other structure.
... ... *keratocytes: Connective tissue cells of the stroma of
the cornea capable of forming collagen fibers (fibroblastic
corneal stromal cells).
... ... *endothelial cells: Flat cells forming a layer lining
blood vessels, lymphatic vessels, the heart, etc.)
... ... *immortalized: In this context, the term "immortalized"
refers to cultured cells genetically engineered to exhibit
infinite lifespan (infinite replication).
... ... *marker expression: In general, the genetic expression of
specific chemical moieties in a macromolecule such as a protein.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Mar00
[For more information: http://scienceweek.com/swfr.htm]

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

4. MOLECULAR BIOLOGY:
ON THE RAT AS A MODEL SYSTEM IN FUNCTIONAL GENOMICS
In a few years, the complete sequence of nucleotide bases in the
human genome will be determined. But that is only the first phase
of the new human biology of the 21st century: after specification
of the complete sequence will come years of arduous work
identifying specific new genes and relating the proteins encoded
by these genes to specific sets of events in health and disease.
This second phase of genome research, the "functional genomics
era", will be carried out, for the most part, on animals, using
animal models to formulate hypotheses concerning the role played
by various parts of the human genome in human biology. The basis
for the use of animal models in this undertaking is the strong
apparent similarity of the genomes and physiology of certain
mammalian species to the human genome and human physiology. The
mouse, a small mammal easily maintained and easily bred, is
currently the favorite animal model used in mammalian genetics
research, but there are a number of candidates for future
functional genomics, and researchers, as always, will need to
make important strategic decisions concerning the focus of their
laboratory work. Intrinsic to the use of an animal model in
functional genomics is the complete sequencing of the genome of
that animal.
... ... Howard J. Jacob (Medical College of Wisconsin, US)
presents a review of the role the rat will play in annotating the
human genome in the functional genomics era. The author makes the
following points:
     1) The author points out that the laboratory rat, _Rattus
norvegicus_, was the first mammalian species domesticated for
scientific research, with work dating back to before 1850. From
this beginning, the rat has become the most widely studied
experimental animal model for biomedical research. Since 1966,
nearly 500,000 research articles reporting the use of rats have
been published, with most of these articles focused on evaluating
the biology and/or pathobiology of the rat. In contrast to its
central role in the study of behavior, biochemistry,
neurobiology, physiology, and pharmacology, the rat has lagged
far behind the mouse as a genetic model organism.
     2) The author points out that although research on rat
genetics and mouse genetics historically had a parallel
beginning, the mouse soon became the model of choice for
mammalian geneticists, whereas the rat became the model of choice
for physiologists, nutritionists, and other biomedical
researchers. Geneticists preferred the mouse because of its
smaller size, which simplified housing requirements, and the
availability of many coat-color and other mutants exhibiting
Mendelian patterns of inheritance. In contrast, physiologists and
other biomedical researchers favored the rat because its larger
size facilitated experimental interventions, and over time a
large number of rat models were used to develop disease models by
selective breeding that fixed natural disease *alleles in
particular strains or colonies.
     3) The author concludes: "The rat offers many advantages for
identification of gene functions that relate to common human
diseases, because of the existing body of knowledge of
physiological mechanisms, the availability of models that mimic
these diseases, the ease of breeding, and the ability to generate
new and better models that match subsets of patients at both the
*phenotypic and genomic levels... Once genes and their functions
are identified in rats, pathophysiologic mechanisms can be
elucidated, and human genetic counterparts can be more easily
identified."
-----------
Howard J. Jacob: Functional genomics and rat models.
(Genome Research November 1999 9:1013)
QY: Howard J. Jacob, Laboratory for Genetic Research, The Medical
College of Wisconsin, Milwaukee, WI 53226 US)
-----------
Text Notes:
... ... *alleles: An allele is one of two or more forms of a
given gene that control a particular characteristic, with the
alternative forms occupying corresponding loci on homologous
chromosomes.
... ... *phenotypic and genomic levels: In general, 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.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Mar00
[For more information: http://scienceweek.com/swfr.htm]

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

5. MOLECULAR BIOLOGY
FIRST MYOGLOBIN-LIKE HEME-CONTAINING PROTEIN IN ARCHAEA
     All biological cells require energy, both to drive the
chemical reactions involved in building cell components, and to
power the activities of these components. In many biological
systems, the major source of energy involves the utilization of
molecular oxygen in specific chemical processes, with such
utilization dependent on the recognition and uptake of oxygen by
specific biochemical entities.
     In general, the term "heme" (ferroprotoporphyrin) refers to
any iron-porphyrin complex irrespective of the valence state of
the iron atom, and the term "heme protein" (hemoprotein) refers
to any protein to which an iron-porphyrin compound is linked in a
stoichiometric manner [*Note #1]. The heme proteins include
hemoglobin, myoglobin, various cytochromes, catalase, and some
peroxidases. The term "hemoglobin" refers to any of a group of
red, iron-containing, oxygen-carrying pigments of the blood of
vertebrates and some invertebrates; the substance also occurs in
the root nodules of leguminous plants. All vertebrate (including
human) hemoglobins consist of two pairs of associated protein
polypeptide chains ("*globins"), each polypeptide chain carrying
a heme *prosthetic group in non-covalent bonding, the iron atom
of which is in the ferrous state and forms a coordination complex
with the pyrrole nitrogens. Myoglobin is a similar macromolecule,
occurring in muscle fibers, and consisting of a single
polypeptide chain of 153 amino-acid residues to which a single
ferroheme prosthetic group is non-covalently bound. (In a
vertebrate physiological context, in muscle fibers, hemoglobin
essentially transports oxygen via the blood to cellular
myoglobin, and myoglobin then stores the oxygen for subsequent
release to intracellular enzymes when the local oxygen supply
becomes limiting.)
     The archaebacteria (also called the Archaea) are considered
to be ancient compared to other kingdoms, and are possibly the
most ancient life forms. They typically exist in extreme
environments, and include the methane-producing bacteria
(methanogens), the "salt-loving" bacteria (halophilic bacteria),
and the sulfur-acid tolerant thermoacidophilic bacteria. There is
presently a controversy concerning whether the Archaea should be
classified as a kingdom separate from the Bacteria. If such a
distinction is made, then the three primary kingdoms of life
forms are Archaea, Bacteria, and Eukarya, the first two
(prokaryotes) without internal membrane-bound organelles such as
a nucleus, while Eukarya (eukaryotes) do have such internal
organelles.
     The term "aerotaxis" refers to the movement of an organism,
especially bacteria, with reference to a directed gradient of
oxygen or air.
     Comparisons of the amino acid sequences of globins from
Bacteria and Eukarya suggests that they share an early common
ancestor, but until now no members of the globin family have been
found in Archaea. Also, recent studies of biological signaling in
the Bacteria and Eukarya have revealed a new class of heme-
containing proteins that serve as oxygen sensors, but no heme-
based sensor has been described in the Archaea.
... ... S. Hou et al (8 authors at 3 installations, US) now
report the first myoglobin-like heme-containing protein in the
Archaea, and the first heme-based aerotactic transducer in the
Bacteria. The authors report these proteins exhibit spectral
properties similar to those of myoglobin and trigger aerotactic
responses.
-----------
S. Hou et al: Myoglobin-like aerotaxis transducers in Archaea and
Bacteria.
(Nature 3 Feb 00 403:540)
QY: Maqsudul Alam [alam@hawaii.edu]
-----------
Text Notes:
... ... *Note #1: The porphyrins are cyclic organic molecules
that form the basis for many important biochemical compounds.
They form complexes with metal ions, iron in the case
of hemoglobin and magnesium in the case of chlorophylls, and this
property is essential to their function.
... ... *globins: The term "globin" refers to the globular
protein that complexes with the heme group in oxygen-carrying
proteins such as hemoglobin and myoglobin. (This term must be
distinguished from "globulins", which refers to a class of
globular proteins of importance in the immune system).
... ... *prosthetic group: The non-protein group of a *conjugated
protein. It may be another organic molecule or an inorganic metal
ion.
... ... *conjugated protein: A protein to which a non-protein
component (prosthetic group) is attached. This should not be
confused with "conjugated" as a term referring to alternating
single and double bonds in a chemical structure.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Mar00
[For more information: http://scienceweek.com/swfr.htm]
-------------------
Related Background:
THE EVOLUTION OF HEMOGLOBIN
Heme is an iron-containing porphyrin acting as the prosthetic
group of several biological pigments, including hemoglobin,
*myoglobin, and several *cytochromes. Hemoglobin is the chief
transport protein involved in carrying oxygen within red blood
cells (erythrocytes). The heme *prosthetic group is similar in a
variety of hemoglobins, but the globular *apoenzyme has
fluctuated considerably in the course of evolution. Hemoglobin is
a tetrameric molecule, consisting of two pairs of nonidentical
polypeptides associated in a quaternary structure, each of the 4
heme groups containing an iron atom that binds oxygen reversibly.
The molecule occurs intracellularly in vertebrate erythrocytes,
but in invertebrates hemoglobin is usually found in simple
solution in the blood. It is also found in certain *nitrogen-
fixating plants. A variety of hemoglobin types are especially
evident in mammals, and in some cases specific polypeptide chains
are different for the fetus than for the adult. The molecule
produces a red color when oxygenated and a bluish-red color when
deoxygenated. The ability of the hemoglobin molecule to pick up
and unload oxygen depends on its shape in solution, and this
shape varies *allosterically with local pH, which in turn is a
function of the partial pressure of blood carbon dioxide. Carbon
dioxide combines with hemoglobin at its amino groups. Carbon
monoxide (CO) is highly poisonous because it binds to hemoglobin
irreversibly, causing suffocation at the cellular level. Human
hemoglobin consists of 574 amino acids, and has a globular shape
with a diameter of approximately 6 nanometers. The hemoglobin
concentration in normal human erythrocytes is extremely high: in
the erythrocyte, the hemoglobin molecules are only 1 nanometer
apart, but they apparently can rotate and flow past one another
without hindrance. ... ... Ross Hardison (Pennsylvania State
University, US) presents a review of current research concerning
the molecular biology and evolution of hemoglobin, the author
making the following points concerning the evolution of the
molecule:
     1) The 3-dimensional structure of hemoglobin -- its shape,
folds, pockets, and surfaces -- have been well conserved over the
evolutionary history of the protein. But some of the most rapid
and dramatic changes in hemoglobin proteins have occurred in the
ways these molecules are regulated -- the when and how of their
manufacture inside the cell.
     2) The active core of hemoglobin, the porphyrin ring, which
is responsible for the basic chemistry of the molecule in which
it is located, eventually came to be embedded, via evolution, in
larger organic structures. The structure of these organic
molecules imparts to the molecule its specific function,
determining, for example, whether that basic chemistry was used
in connection with respiratory reaction chains, oxygen transport,
or oxygen sequestration. Thus, structural changes in these
organic molecules have been translated into functional changes.
     3) Structurally similar molecules can become further
differentiated functionally by being expressed at different times
in the development of an organism, as is the case for fetal and
adult *globins, or under different circumstances, such as the
scarcity or abundance of oxygen. These distinctions are not
attributable to differences in the overall structures of the
proteins themselves, but to the differences in *expression
profiles achieved through differences in regulatory segments of
genes. It has become apparent that in many cases regulatory
regions of genes are changing more rapidly via evolution than are
the structures of proteins themselves.
     4) This idea is relatively new in the study of molecular
evolution, which has concentrated on comparisons between protein
structures to determine evolutionary relationships between
species. But molecular evolutionists are beginning to recognize
the value of looking at relations between regulatory regions.
-----------
Ross Hardison: The evolution of hemoglobin.
(American Scientist Mar-Apr 1999 87:126)
QY: Ross Hardison [rch@psu.edu]
-----------
Text Notes:
... ... *myoglobin: See main report.
... ... *cytochromes: The cytochromes are a system of electron-
transfer proteins with iron- or copper-porphyrin as a prosthetic
group. They are found in both animal and plant cells.
... ... *prosthetic group: See main report.
... ... *apoenzyme: In general, this refers to the protein
component of an enzyme-cofactor complex (holoenzyme). The
apoenzyme is usually catalytically inactive by itself.
... ... *nitrogen-fixating plants: In general, the incorporation
of atmospheric nitrogen to form nitrogenous organic compounds.
... ... *allosterically: In general, an "allosteric" molecule is
a molecule whose 3-dimensional conformation alters in response to
a change in its environment, the alteration resulting in a change
in molecular function. So, for example, the hemoglobin molecule
is allosteric under different blood pH values.
... ... *globins: See main report.
... ... *expression profiles: The "expression profile" of a
genome or a group of genes in a genome refers to the set of genes
that have been "switched on" to express their proteins. Various
types of cells have different expression profiles, and within a
single type of cell, the expression profile may change during the
life history of the cell.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 28May99
[For more information: http://scienceweek.com/swfr.htm]

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

6. MEDICAL BIOLOGY:
ON NARCOLEPSY
Narcolepsy is a relatively rare syndrome characterized by one or
all of the following symptoms: a) sudden loss of muscle tone
(cataplexy); b) paralysis immediately before or after sleep
(sleep paralysis); c) vivid auditory or visual illusions or
hallucinations immediately before or after sleep (hypnogogic
phenomena). Symptoms usually begin in young adults and persist
throughout life. Sleep occurs at any time, with sleep episodes
varying from few to many in a single day, and each episode
lasting minutes or hours. During an episode, onset of rapid eye
movement sleep (*REM sleep) is almost instantaneous, unlike
normal sleep in which the REM phase usually occurs after 60 to 90
minutes. In narcolepsy with cataplexy, the loss of muscle tone is
usually total postural muscle tone loss, with body collapse
lasting minutes while the individual remains paralyzed.
Concerning the incidence of narcolepsy, the syndrome affects
between 1 in 1000 to 1 in 2000 people in the US. Worldwide rates
range from 1 in 600 in Japan to 1 in 500,000 in Israel, and both
genetic factors and environmental factors have been suggested as
explanations for the variation. The overall incidence of
narcolepsy in the US is approximately 10 times that of
amyotrophic lateral sclerosis (Lou Gehrig's disease), half that
of multiple sclerosis, five times that of cystic fibrosis, and
approximately one-fourth that of Parkinson's disease.
... ... Jerome M. Siegel presents a review of current research on
narcolepsy, the author making the following points:
     1) The author suggests that brain and spinal cord circuits
that normally inhibit movement during sleep are triggered
inappropriately during cataplexy attacks in narcolepsy, causing a
loss of tone in the muscles that maintain posture.
     2) From histological evidence obtained in narcoleptic dogs,
the author suggests that degeneration of cells in the forebrain
eliminates inhibitory signals that are important for regulating
the activity of cells in the amygdala, a brain structure involved
in emotional responses. The loss of these inhibitory signals
causes increased activity in amygdala connections to a brainstem
structure (pons), and this in turn ultimately removes a source of
excitation from spinal motor neurons that control muscles. At the
same time, the cell loss in the amygdala activates circuits that
stimulate midbrain neurons which inhibit spinal motor neurons.
The result of the simultaneous loss of excitation and onset of
inhibition in spinal motor neurons is a sudden loss of muscle
tone, which causes the narcoleptic to fall.
     3) The author suggests that recent findings in narcoleptic
dogs indicates that mutations in specific neurotransmitter
receptors (hypocretin/orexin receptors) in the *lateral
hypothalamus can also cause cataplexy and other symptoms of
narcolepsy. The mutations may act by removing excitatory inputs
to cells that maintain muscle tone and arousal, and by triggering
the nerve cell degeneration seen in the forebrain.
-----------
Jerome M. Siegel: Narcolepsy
(Scientific American January 2000)
QY: Jerome M. Siegel, Univ. of Calif. Los Angeles 310-825-4321.
-----------
Text Notes:
... ... *REM sleep: It has been known since the work of Kleitman
and Aserinsky in 1953 that sleep consists of two quite different
components: a) non-rapid eye movement (non-REM) sleep ("slow-wave
sleep"), characterized by an apparent neurological quiescence;
and b) rapid eye movement (REM) sleep, which is the sleep during
most of the sleep period, and which is characterized by the high-
frequency low-voltage electroencephalographic activity that is
also the electrical signature of the waking brain. REM sleep is
also called "paradoxical sleep" and is often associated with
elaborate dreams.
... ... *lateral hypothalamus: The hypothalamus is a deep brain
structure with various clusters of nerve cells ("nuclei")
controlling several important homeostatic functions such as
temperature regulation and food intake, and in addition the sex
drive, aggressive emotions, psychosomatic effects, etc. The
hypothalamus essentially integrates the activity of the autonomic
nervous system, and it acts as an intermediary between the
endocrine (hormone) system and the nervous system, with various
hypothalamic neuron types secreting hormones themselves. (In
general, the term "hormones" refers to chemical messengers which
are distributed systemically via the bloodstream.)
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 24Mar00
[For more information: http://scienceweek.com/swfr.htm]

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

IN FOCUS: ON GENES, ORGANISMS, AND SPECIES
"The terms 'gene', 'organism', and 'species' have been used in a
wide variety of ways in a wide variety of contexts. Anyone who
attempts merely to map this diversity is presented with a massive
and probably pointless task... Genes are the entities which are
passed on in reproduction and which control the ontogenetic
development of the organism. Organisms are the complex systems
which anatomists, physiologists, embryologists, histologists, and
others analyze into their component parts. Species have been
treated traditionally as the basic units of classification, the
natural kinds of the living world, comparable to the physical
elements. But these entities also function in the evolutionary
process. Evolution consists in two processes (mutation and
selection) which eventuate in a third (evolution). Genes provide
the heritable variation required by the evolutionary process.
Traditionally, organisms have been viewed as the primary focus of
selection, although considerable disagreement currently exists
over the levels at which selection takes place. Some biologists
maintain that selection occurs exclusively at the level of genes;
others that supragenic, even supraorganismic units can also be
selected. As one might gather from the title of Darwin's book,
species are the things which are supposed to evolve. Whether the
relatively large units recognized by taxonomists as species
evolve or whether much less extensive units such as populations
are the effective units of evolution is an open question."
-----------
David L. Hull: A matter of individuality.
(Philosophy of Science 1978 45:335)

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

NOTICES  
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

CHANGE OF EMAIL ADDRESS: If at any time you need to change the
Email address at which you receive SW, please send the
information to [request@scienceweek.com], and the change will be
made and confirmed the same day.  

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

SCIENCE-WEEK SUBSCRIPTIONS: The subscription rate for ScienceWeek
(52 issues per year delivered via Email only) is US$20 for one
year. Subscriptions can be obtained with a credit card
(Visa, MC, Amex) at a secure website form accessed at:
http://scienceweek.com/subinfo.htm

Information concerning other methods of payment is available at
the above URL, or via Email at swsub@scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

The first issue of SCIENCE-WEEK appeared May 1, 1997, and it has
been published regularly each week since that date. SW is
designed to cross existing conceptual and linguistic barriers
between the scientific disciplines. In general, the biology is
written for physicists and chemists, and the physics and
chemistry are written for biologists, with an attempt to retain
some exactitude in the particular science involved in the news.
These are the aims. Undoubtedly, we are not always successful,
and for that we apologize. In any case, what we hope is that our
readers are reading out of their fields more than in their
fields, since that is the essence of this publication.

We welcome comments, suggestions, and criticisms from our
subscribers. Public letters relevant to any report are also
welcome. Editorial contact: [editors@scienceweek.com].

Editor/Publisher: Dan Agin
Managing Editor: Claire Haller
Associate Editor: Joan Oliner

Copyright (c) 1997-1999 SCIENCE-WEEK/Spectrum Press Inc.
All Rights Reserved

---------------------------------------------
This publication is protected by U.S. and International Copyright
Laws, and no display, transmission, or duplication in any medium,
including BBS, Internet Email, website duplication, fax, or print
is permitted without the explicit consent of the holder of the
copyright. SCIENCE-WEEK is published by Spectrum Press Inc.,
3023 N. Clark Street #109, Chicago, 60657-5205 IL, USA.
---------------------------------------------