What Is Life? The Physical Aspect of the Living Cell is a 1944 science book written for the lay reader by physicist Erwin Schrödinger. The book was based on a course of public lectures delivered by Schrödinger in February 1943, under the auspices of the Dublin Institute for Advanced Studies where he was Director of Theoretical Physics, at Trinity College, Dublin.
The lectures attracted an audience of about 400, who were warned "that
the subject-matter was a difficult one and that the lectures could not
be termed popular, even though the physicist’s most dreaded weapon,
mathematical deduction, would hardly be utilized."
Schrödinger's lecture focused on one important question: "how can the
events in space and time which take place within the spatial boundary of
a living organism be accounted for by physics and chemistry?"
In the book, Schrödinger introduced the idea of an "aperiodic
crystal" that contained genetic information in its configuration of
covalent chemical bonds.
In the 1950s, this idea stimulated enthusiasm for discovering the
genetic molecule. Although the existence of some form of hereditary
information had been hypothesized since 1869, its role in reproduction
and its helical shape were still unknown at the time of Schrödinger's
lecture. In retrospect, Schrödinger's aperiodic crystal can be viewed
as a well-reasoned theoretical prediction of what biologists should have
been looking for during their search for genetic material.[citation needed] Both James D. Watson, and Francis Crick, who jointly proposed the double helix structure of DNA based on, amongst other theoretical insights, X-ray diffraction experiments by Rosalind Franklin, credited Schrödinger's book with presenting an early theoretical description of how the storage of genetic information would work, and each independently acknowledged the book as a source of inspiration for their initial researches.
Background
The book is based on lectures delivered under the auspices of the Dublin Institute for Advanced Studies, at Trinity College, Dublin,
in February 1943 and published in 1944. At that time, although DNA was
known to be a constituent of cell nuclei, the concept of a "heredity
molecule" was strictly theoretical, with various candidates. One of the
most successful branches of physics at this time was statistical physics, and quantum mechanics,
a theory which is also very statistical in its nature. Schrödinger
himself is one of the founding fathers of quantum mechanics.
Max Delbrück's thinking about the physical basis of life was an important influence on Schrödinger. However, long before the publication of What is Life?, geneticist and 1946 Nobel-prize winner H. J. Muller had in his 1922 article "Variation due to Change in the Individual Gene"
already laid out all the basic properties of the "heredity molecule"
(then not yet known to be DNA) that Schrödinger was to re-derive in 1944
"from first principles" in What is Life? (including the
"aperiodicity" of the molecule), properties which Muller specified and
refined additionally in his 1929 article "The Gene As The Basis of Life" and during the 1930s. Moreover, H. J. Muller himself wrote in a 1960 letter to a journalist regarding What Is Life?
that whatever the book got right about the "hereditary molecule" had
already been published before 1944 and that Schrödinger's were only the
wrong speculations; Muller also named two famous geneticists (including
Delbrück) who knew every relevant pre-1944 publication and had been in
contact with Schrödinger before 1944. But DNA as the molecule of
heredity became foremost only after Oswald Avery's
bacterial-transformation experiments published in 1944; before those
experiments, proteins were considered the most likely candidates. DNA
was confirmed as the molecule in question by the
Hershey–Chase experiment conducted in 1952.
Content
In
chapter I, Schrödinger explains that most physical laws on a large scale
are due to chaos on a small scale. He calls this principle
"order-from-disorder." As an example he mentions diffusion,
which can be modeled as a highly ordered process, but which is caused
by random movement of atoms or molecules. If the number of atoms is
reduced, the behaviour of a system becomes more and more random. He
states that life greatly depends on order and that a naïve physicist may
assume that the master code of a living organism has to consist of a
large number of atoms.
In chapter II and III, he summarizes what was known at this time
about the hereditary mechanism. Most importantly, he elaborates the
important role mutations play in evolution.
He concludes that the carrier of hereditary information has to be both
small in size and permanent in time, contradicting the naïve physicist's
expectation. This contradiction cannot be resolved by classical physics.
In chapter IV, Schrödinger presents molecules,
which are indeed stable even if they consist of only a few atoms, as
the solution. Even though molecules were known before, their stability
could not be explained by classical physics, but is due to the discrete
nature of quantum mechanics. Furthermore, mutations are directly linked to quantum leaps.
He continues to explain, in chapter V, that true solids, which are also permanent, are crystals.
The stability of molecules and crystals is due to the same principles
and a molecule might be called "the germ of a solid." On the other hand,
an amorphous solid, without crystalline structure, should be regarded as a liquid with a very high viscosity.
Schrödinger believes the heredity material to be a molecule, which
unlike a crystal does not repeat itself. He calls this an aperiodic
crystal. Its aperiodic nature allows it to encode an almost infinite
number of possibilities with a small number of atoms. He finally
compares this picture with the known facts and finds it in accordance
with them.
In chapter VI Schrödinger states:
...living matter, while not eluding the "laws of
physics" as established up to date, is likely to involve "other laws of
physics" hitherto unknown, which however, once they have been revealed,
will form just as integral a part of science as the former.
He knows that this statement is open to misconception and tries to
clarify it. The main principle involved with "order-from-disorder" is
the second law of thermodynamics, according to which entropy only increases in a closed system (such as the universe). Schrödinger explains that living matter evades the decay to thermodynamical equilibrium by homeostatically maintaining negative entropy in an open system.
In chapter VII, he maintains that "order-from-order" is not
absolutely new to physics; in fact, it is even simpler and more
plausible. But nature follows "order-from-disorder", with some
exceptions as the movement of the celestial bodies
and the behaviour of mechanical devices such as clocks. But even those
are influenced by thermal and frictional forces. The degree to which a
system functions mechanically or statistically depends on the
temperature. If heated, a clock ceases to function, because it melts.
Conversely, if the temperature approaches absolute zero,
any system behaves more and more mechanically. Some systems approach
this mechanical behaviour rather fast with room temperature already
being practically equivalent to absolute zero.
Schrödinger concludes this chapter and the book with philosophical speculations on determinism, free will, and the mystery of human consciousness.
He attempts to "see whether we cannot draw the correct
non-contradictory conclusion from the following two premises: (1) My
body functions as a pure mechanism according to Laws of Nature; and (2)
Yet I know, by incontrovertible direct experience, that I am directing
its motions, of which I foresee the effects, that may be fateful and
all-important, in which case I feel and take full responsibility for
them. The only possible inference from these two facts is, I think, that
I – I in the widest meaning of the word, that is to say, every
conscious mind that has ever said or felt 'I' – am the person, if any,
who controls the 'motion of the atoms' according to the Laws of Nature".
Schrödinger then states that this insight is not new and that Upanishads
considered this insight of "ATHMAN = BRAHMAN" to "represent
quintessence of deepest insights into the happenings of the world."
Schrödinger rejects the idea that the source of consciousness should
perish with the body because he finds the idea "distasteful". He also
rejects the idea that there are multiple immortal souls that can exist
without the body because he believes that consciousness is nevertheless
highly dependent on the body. Schrödinger writes that, to reconcile the
two premises,
The only possible alternative is simply
to keep to the immediate experience that consciousness is a singular of
which the plural is unknown; that there is only one thing and that what
seems to be a plurality is merely a series of different aspects of this
one thing…
Any intuitions that consciousness is plural, he says, are illusions. Schrödinger is sympathetic to the Hindu concept of Brahman, by which each individual's consciousness is only a manifestation of a unitary consciousness pervading the universe
— which corresponds to the Hindu concept of God. Schrödinger concludes
that "...'I' am the person, if any, who controls the 'motion of the
atoms' according to the Laws of Nature." However, he also qualifies the
conclusion as "necessarily subjective" in its "philosophical
implications". In the final paragraph, he points out that what is meant
by "I" is not the collection of experienced events but "namely the
canvas upon which they are collected." If a hypnotist succeeds in
blotting out all earlier reminiscences, he writes, there would be no
loss of personal existence — "Nor will there ever be."
Schrödinger's "paradox"
In a world governed by the second law of thermodynamics, all isolated systems
are expected to approach a state of maximum disorder. Since life
approaches and maintains a highly ordered state, some argue that this
seems to violate the aforementioned second law, implying that there is a
paradox. However, since the biosphere
is not an isolated system, there is no paradox. The increase of order
inside an organism is more than paid for by an increase in disorder
outside this organism by the loss of heat into the environment. By this
mechanism, the second law is obeyed, and life maintains a highly ordered
state, which it sustains by causing a net increase in disorder in the
Universe. In order to increase the complexity on Earth—as life does—free energy is needed and in this case is provided by the Sun.
The Africanized bee, also known as the Africanized honey bee and known colloquially as the "killer bee", is a hybrid of the western honey bee (Apis mellifera), produced originally by crossbreeding of the East African lowland honey bee (A. m. scutellata) with various European honey bee subspecies such as the Italian honey bee (A. m. ligustica) and the Iberian honey bee (A. m. iberiensis).
The East African lowland honey bee was first introduced to Brazil
in 1956 in an effort to increase honey production, but 26 swarms
escaped quarantine in 1957. Since then, the hybrid has spread throughout
South America and arrived in North America in 1985. Hives were found in
south Texas in the United States in 1990.
Africanized honey bees are typically much more defensive than
other varieties of honey bees, and react to disturbances faster than
European honey bees. They can chase a person a quarter of a mile
(400 m); they have killed some 1,000 humans, with victims receiving 10
times more stings than from European honey bees. They have also killed horses and other animals.
History
There are 29 recognized subspecies of Apis mellifera
based largely on geographic variations. All subspecies are
cross-fertile. Geographic isolation led to numerous local adaptations.
These adaptations include brood cycles synchronized with the bloom
period of local flora, forming a winter cluster in colder climates,
migratory swarming in Africa, enhanced (long-distance) foraging behavior
in desert areas, and numerous other inherited traits.
The Africanized honey bees in the Western Hemisphere are descended from hives operated by biologistWarwick E. Kerr, who had interbred honey bees from Europe and southern Africa. Kerr was attempting to breed a strain of bees that would produce more honey in tropical conditions than the European strain of honey bee currently in use throughout North, Central and South America. The hives containing this particular African subspecies were housed at an apiary near Rio Claro, São Paulo, in the southeast of Brazil, and were noted to be especially defensive. These hives had been fitted with special excluder screens (called queen excluders)
to prevent the larger queen bees and drones from getting out and mating
with the local population of European bees. According to Kerr, in
October 1957 a visiting beekeeper, noticing that the queen excluders
were interfering with the worker bees' movement, removed them, resulting
in the accidental release of 26 Tanganyikan swarms of A. m. scutellata.
Following this accidental release, the Africanized honey bee swarms
spread out and crossbred with local European honey bee colonies.
The descendants of these colonies have since spread throughout the Americas, moving through the Amazon Basin in the 1970s, crossing into Central America in 1982, and reaching Mexico in 1985.
Because their movement through these regions was rapid and largely
unassisted by humans, Africanized honey bees have earned the reputation
of being a notorious invasive species.
The prospect of killer bees arriving in the United States caused a
media sensation in the late 1970s, inspired several horror movies, and sparked debate about the wisdom of humans altering entire ecosystems.
The first Africanized honey bees in the U.S. were discovered in 1985 at an oil field in the San Joaquin Valley
of California. Bee experts theorized the colony had not traveled
overland but instead "arrived hidden in a load of oil-drilling pipe
shipped from South America." The first permanent colonies arrived in Texas from Mexico in 1990.
In the Tucson region of Arizona, a study of trapped swarms in 1994
found that only 15 percent had been Africanized; this number had grown
to 90 percent by 1997.
Characteristics
Though
Africanized honey bees display certain behavioral traits that make them
less than desirable for commercial beekeeping, excessive defensiveness
and swarming foremost, they have now become the dominant type of honey
bee for beekeeping
in Central and South America due to their genetic dominance as well as
ability to out-compete their European counterpart, with some beekeepers
asserting that they are superior honey producers and pollinators.
Africanized honey bees, as opposed to other Western bee types:
Tend to swarm more frequently and go farther than other types of honey bees.
Are more likely to migrate as part of a seasonal response to lowered food supply.
Are more likely to "abscond"—the entire colony leaves the hive and relocates—in response to stress.
Have greater defensiveness when in a resting swarm, compared to other honey bee types.
Live more often in ground cavities than the European types.
Guard the hive aggressively, with a larger alarm zone around the hive.
Have a higher proportion of "guard" bees within the hive.
Deploy in greater numbers for defense and pursues perceived threats over much longer distances from the hive.
Cannot survive extended periods of forage deprivation, preventing introduction into areas with harsh winters or extremely dry late summers.
Live in dramatically higher population densities.
North American distribution
Map showing the spread of Africanized honey bees in the United States from 1990 to 2003
Africanized honey bees are considered an invasive species in the Americas. As of 2002, the Africanized honey bees had spread from Brazil south to northern Argentina and north to Central America, Trinidad (the West Indies), Mexico, Texas, Arizona, Nevada, New Mexico, Florida,
and southern California. Their expansion stopped for a time at eastern
Texas, possibly due to the large population of European honey bee hives
in the area. However, discoveries of the Africanized honey bees in
southern Louisiana show that they have gotten past this barrier, or have come as a swarm aboard a ship.
In June 2005, it was discovered that the bees had entered Texas and had spread into southwest Arkansas. On 11 September 2007, Commissioner Bob Odom of the Louisiana Department of Agriculture and Forestry said that Africanized honey bees had established themselves in the New Orleans area. In February 2009, Africanized honey bees were found in southern Utah. The bees had spread into eight counties in Utah, as far north as Grand and Emery Counties by May 2017.
In October 2010, a 73-year-old man was killed by a swarm of Africanized honey bees while clearing brush on his south Georgia property, as determined by Georgia's Department of Agriculture. In 2012, Tennessee state officials reported that a colony was found for the first time in a beekeeper's colony in Monroe County in the eastern part of the state. In June 2013, 62-year-old Larry Goodwin of Moody, Texas was killed by a swarm of Africanized honey bees.
In May 2014, Colorado State University confirmed that bees from a swarm which had aggressively attacked an orchardist near Palisade, in west-central Colorado, were from an Africanized honey bee hive. The hive was subsequently destroyed.
In tropical climates they effectively out-compete European honey
bees and, at their peak rate of expansion, they spread north at almost
two kilometers (about one mile) a day. There were discussions about
slowing the spread by placing large numbers of docile European-strain
hives in strategic locations, particularly at the Isthmus of Panama,
but various national and international agricultural departments could
not prevent the bees' expansion. Current knowledge of the genetics of
these bees suggests that such a strategy, had it been tried, would not
have been successful.
As the Africanized honey bee migrates further north, colonies
continue to interbreed with European honey bees. In a study conducted in
Arizona in 2004 it was observed that swarms of Africanized honey bees
could take over weakened European honey bee hives by invading the hive,
then killing the European queen and establishing their own queen.
There are now relatively stable geographic zones in which either
Africanized honey bees dominate, a mix of Africanized and European honey
bees is present, or only non-Africanized honey bees are found, as in
the southern portions of South America or northern North America.
African honey bees abscond (abandon the hive and any food store to
start over in a new location) more readily than European honeybees. This
is not necessarily a severe loss in tropical climates where plants
bloom all year, but in more temperate climates it can leave the colony
with not enough stores to survive the winter. Thus Africanized honey
bees are expected to be a hazard mostly in the southern states of the United States, reaching as far north as the Chesapeake Bay
in the east. The cold-weather limits of the Africanized honey bee have
driven some professional bee breeders from Southern California into the
harsher wintering locales of the northern Sierra Nevada and southern Cascade Range. This is a more difficult area to prepare bees for early pollination placement in, such as is required for the production of almonds. The reduced available winter forage in northern California means that bees must be fed for early spring buildup.
The arrival of the Africanized honey bee in Central America is threatening the ancient art of keeping Meliponastingless bees in log gums,
although they do not interbreed or directly compete with each other.
The honey production from a single hive of Africanized honey bees can be
100 kg annually and far exceeds the much smaller 3–5 kg of the various Melipona
stingless bee species. Thus economic pressures are forcing beekeepers
to switch from the traditional stingless bees of their ancestors to the
new reality of the Africanized honey bee. Whether this will lead to
their extinction is unknown, but they are well adapted to exist in the
wild, and there are a number of indigenous plants that the Africanized
honey bees do not visit, so their fate remains to be seen.
Africanized
honey bees have a set of characteristics with respect to foraging
behavior. Africanized honey bees begin foraging at young ages and
harvest a greater quantity of pollen with respect to their European
counterparts (Apis mellifera ligustica). This may be linked to
the high reproductive rate of the Africanized honey bee which requires
pollen to feed the greater number of larvae.
Africanized honey bees are also sensitive to sucrose at lower
concentrations. This adaptation causes foragers to harvest resources
with low concentrations of sucrose that include water, pollen, and
unconcentrated nectar. A study comparing A. m. scutellata and A. m. ligustica
published by Fewell and Bertram in 2002 suggests that the differential
evolution of this suite of behaviors is due to the different
environmental pressures experienced by African and European subspecies.
Proboscis extension responses
Honey
bee sensitivity to different concentrations of sucrose is determined by
a reflex known as the proboscis extension response or PER. Different
species of honey bees that employ different foraging behaviors will vary
in the concentration of sucrose that elicits their proboscis extension
response.
For example, European honey bees (Apis mellifera ligustica)
forage at older ages and harvest less pollen and more concentrated
nectar. The differences in resources emphasized during harvesting are a
result of the European honey bee's sensitivity to sucrose at higher
concentrations.
Evolution
The differences in a variety of behaviors between different species of honey bees are the result of a directional selection that acts upon several foraging behavior traits as a common entity.
Selection in natural populations of honey bees show that positive
selection of sensitivity to low concentrations of sucrose are linked to
foraging at younger ages and collecting resources low in sucrose.
Positive selection of sensitivity to high concentrations of sucrose were
linked to foraging at older ages and collecting resources higher in
sucrose. Additionally of interest, “change in one component of a suite of behaviors appear[s] to direct change in the entire suite.”
When resource density is low in Africanized honey bee habitats,
it is necessary for the bees to harvest a greater variety of resources
because they cannot afford to be selective. Honey bees that are
genetically inclined towards resources high in sucrose like concentrated
nectar will not be able to sustain themselves in harsher environments.
The noted PER to low sucrose concentration in Africanized honey bees may
be a result of selective pressure in times of scarcity when their
survival depends on their attraction to low quality resources.
Morphology and genetics
The
popular term "killer bee" has only limited scientific meaning today
because there is no generally accepted fraction of genetic contribution
used to establish a cut-off.
Morphological tests
Although the native East African lowland honey bees (Apis mellifera scutellata)
are smaller and build smaller comb cells than the European honey bees,
their hybrids are not smaller. Africanized honey bees have slightly
shorter wings, which can only be recognized reliably by performing a
statistical analysis on micro-measurements of a substantial sample.
An African honey bee extracts nectar from a flower as pollen grains stick to its body in Tanzania (this is a purebred African honey bee, not an 'Africanized' hybrid honey bee).
One of the problems with this test is that there are other subspecies, such as Apis mellifera iberiensis, which also have shortened wings. This trait is hypothesized to derive from ancient hybrid haplotypes thought to have links to evolutionary lineages from Africa. Some belong to Apis mellifera intermissa, but others have an indeterminate origin; the Egyptian honeybee (Apis mellifera lamarckii), present in small numbers in the southeastern U.S., has the same morphology.
DNA tests
Currently testing techniques have moved away from external measurements to DNA analysis, but this means the test can only be done by a sophisticated laboratory. Molecular diagnostics using the mitochondrial DNA (mtDNA) cytochrome b gene can differentiate A. m. scutellata from other A. mellifera
lineages, though mtDNA only allows one to detect Africanized colonies
that have Africanized queens and not colonies where a European queen has
mated with Africanized drones.
A test based on single nucleotide polymorphisms was created in 2015 to
detect Africanized bees based on the proportion of African and European
ancestry.
Western variants
The
western honey bee is native to the continents of Europe, Asia, and
Africa. As of the early 1600s, it was introduced to North America, with
subsequent introductions of other European subspecies 200 years later.
Since then, they have spread throughout the Americas. The 29 subspecies
can be assigned to one of four major branches based on work by Ruttner
and subsequently confirmed by analysis of mitochondrial DNA.
African subspecies are assigned to branch A, northwestern European
subspecies to branch M, southwestern European subspecies to branch C,
and Mideast subspecies to branch O. The subspecies are grouped and
listed. There are still regions with localized variations that may
become identified subspecies in the near future, such as A. m. pomonella from the Tian Shan Mountains, which would be included in the Mideast subspecies branch.
The western honey bee is the third insect whose genome has been mapped, and is unusual in having very few transposons.
According to the scientists who analyzed its genetic code, the western
honey bee originated in Africa and spread to Eurasia in two ancient
migrations. They have also discovered that the number of genes in the honey bee related to smell outnumber those for taste. The genome sequence revealed several groups of genes, particularly the genes related to circadian rhythms, were closer to vertebrates than other insects. Genes related to enzymes that control other genes were also vertebrate-like.
African variants
There are two lineages of the East African lowland subspecies (Apis mellifera scutellata)
in the Americas: actual matrilineal descendants of the original escaped
queens and a much smaller number that are Africanized through
hybridization. The matrilineal descendants carry African mtDNA, but
partially European nuclear DNA, while the honey bees that are
Africanized through hybridization carry European mtDNA, and partially
African nuclear DNA. The matrilineal descendants are in the vast
majority. This is supported by DNA analyses performed on the bees as
they spread northwards; those that were at the "vanguard" were over 90%
African mtDNA, indicating an unbroken matriline,
but after several years in residence in an area interbreeding with the
local European strains, as in Brazil, the overall representation of
African mtDNA drops to some degree. However, these latter hybrid lines
(with European mtDNA) do not appear to propagate themselves well or
persist.
Population genetics analysis of Africanized honey bees in the United
States, using a maternally inherited genetic marker, found 12 distinct
mitotypes, and the amount of genetic variation observed supports the
idea that there have been multiple introductions of AHB into the United
States.
A newer publication shows the genetic admixture of the
Africanized honey bees in Brazil. The small number of honey bees with
African ancestry that were introduced to Brazil in 1956, which dispersed
and hybridized with existing managed populations of European origin and
quickly spread across much of the Americas, is an example of a massive
biological invasion as earlier told in this article. Here, they analysed
whole‐genome sequences of 32 Africanized honey bees sampled from
throughout Brazil to study the effect of this process on genome
diversity. By comparison with ancestral populations from Europe and
Africa, they infer that these samples had 84% African ancestry, with the
remainder from western European populations. However, this proportion
varied across the genome and they identified signals of positive
selection in regions with high European ancestry proportions. These
observations are largely driven by one large gene‐rich 1.4 Mbp segment
on chromosome 11 where European haplotypes are present at a
significantly elevated frequency and likely confer an adaptive advantage
in the Africanized honey bee population.
Consequences of selection
The chief difference between the European subspecies of honey bees
kept by beekeepers and the African ones is attributable to both
selective breeding and natural selection. By selecting only the most
gentle, non-defensive subspecies, beekeepers have, over centuries,
eliminated the more defensive ones and created a number of subspecies
suitable for apiculture. The most common subspecies used in Europe and the United States today is the Italian honey bee (Apis mellifera ligustica),
which has been used for over 1,000 years in some parts of the world and
in the Americas since the arrival of the European colonists.
In Central and southern Africa there was formerly no tradition of
beekeeping, and the hive was destroyed in order to harvest the honey,
pollen and larvae. The bees adapted to the climate of Sub-Saharan
Africa, including prolonged droughts. Having to defend themselves
against aggressive insects such as ants and wasps, as well as voracious
animals like the honey badger, African honey bees evolved as a subspecies group of highly defensive bees unsuitable by a number of metrics for domestic use.
As Africanized honey bees migrate into regions, hives with an old
or absent queen can become hybridized by crossbreeding. The aggressive
Africanized drones out-compete European drones for a newly developed
queen of such a hive, ultimately resulting in hybridization of the
existing colony. Requeening, a term for swapping out the old queen with a
new, already fertilized one, can reduce hybridization in apiaries. As a
prophylactic measure, the majority of beekeepers in North America tend
to requeen their hives annually, maintaining strong colonies and
avoiding hybridization.
Defensiveness
Africanized honey bees exhibit far greater defensiveness than European honey bees and are more likely to deal with a perceived threat by attacking in large swarms. These hybrids have been known to pursue a perceived threat for a distance of well over 500 meters (1,640 ft).
The venom of an Africanized honey bee is the same as that of a
European honey bee, but since the former tends to sting in far greater
numbers, deaths from them are naturally more numerous than from European
honey bees.
While allergies to the European honey bee may cause death,
complications from Africanized honey bee stings are usually not caused
from allergies to their venom. Humans stung many times by the
Africanized honey bees can exhibit serious side effects such as
inflammation of the skin, dizziness, headaches, weakness, edema, nausea,
diarrhea, and vomiting. Some cases even progress to affecting different
body systems by causing increased heart rates, respiratory distress,
and even renal failure.
Africanized honey bee sting cases can become very serious, but they
remain relatively rare and are often limited to accidental discovery in
highly populated areas.
Impact on humans
Fear factor
The Africanized honey bee is widely feared by the public, a reaction that has been amplified by sensationalist movies (such as The Swarm) and some of the media reports. Stings from Africanized honey bees kill on average one or two people per year.
As the Africanized honey bee spreads through Florida, a densely
populated state, officials worry that public fear may force misguided
efforts to combat them.
News reports of mass stinging
attacks will promote concern and in some cases panic and anxiety, and
cause citizens to demand responsible agencies and organizations to take
action to help ensure their safety. We anticipate increased pressure
from the public to ban beekeeping in urban and suburban areas. This
action would be counter-productive. Beekeepers maintaining managed
colonies of domestic European bees are our best defense against an area
becoming saturated with AHB. These managed bees are filling an
ecological niche that would soon be occupied by less desirable colonies
if it were vacant.
— Florida African Bee Action Plan
Misconceptions
"Killer
bee" is a term frequently used in media such as movies that portray
aggressive behavior or actively seeking to attack humans. "Africanized
honey bee" is considered a more descriptive term in part because their
behavior is increased defensiveness compared to European honey bees that
can exhibit similar defensive behaviors when disturbed.
The sting of the Africanized honey bee is no more potent than any
other variety of honey bee, and although they are similar in appearance
to European honey bees, they tend to be slightly smaller and darker in
color. Although Africanized honey bees do not actively search for humans
to attack, they are more dangerous because they are more easily
provoked, quicker to attack in greater numbers, and then pursue the
perceived threat farther, sometimes for up to a kilometer (approx. 5⁄8 mile) or more.
While studies have shown that Africanized honey bees can
infiltrate European honey bee colonies and then kill and replace their
queen (thus usurping the hive), this is less common than other methods.
Wild and managed colonies will sometimes be seen to fight over honey
stores during the dearth (periods when plants are not flowering), but
this behavior should not be confused with the aforementioned activity.
The most common way that a European honey bee hive will become
Africanized is through crossbreeding during a new queen's mating flight.
Studies have consistently shown that Africanized drones are more
numerous, stronger and faster than their European cousins and are
therefore able to out-compete them during these mating flights. The
results of mating between Africanized drones and European queens is
almost always Africanized offspring.
Impact on apiculture
In
areas of suitable temperate climate, the survival traits of Africanized
honey bee colonies help them outperform European honey bee colonies.
They also return later and basically work under conditions that often
keep European honey bees hive-bound. This is the reason why they have
gained a well-deserved reputation as superior honey producers, and those
beekeepers who have learned to adapt their management techniques now
seem to prefer them to their European counterparts. Studies show that in
areas of Florida that contain Africanized honey bees, the honey
production is higher than in areas in which they do not live.
It is also becoming apparent that Africanized honey bees have another
advantage over European honey bees in that they seem to show a higher
resistance to several health issues, including parasites such as Varroa destructor, some fungal diseases like chalkbrood and even the mysterious colony collapse disorder
which is currently plaguing beekeepers. So despite all its negative
factors, it is possible that the Africanized honey bee might actually
end up being a boon to apiculture.
Queen management
In
areas where Africanized honey bees are well established, bought and
pre-fertilized (i.e. mated) European queens can be used to maintain a
hive's European genetics and behavior. However, this practice can be
expensive, since these queens must be bought and shipped from breeder
apiaries in areas completely free of Africanized honey bees, such as the
northern U.S. states or Hawaii.
As such, this is generally not practical for most commercial beekeepers
outside the U.S., and it is one of the main reasons why Central and
South American beekeepers have had to learn to manage and work with the
existing Africanized honey bee.
Any effort to crossbreed virgin European queens with Africanized drones
will result in the offspring exhibiting Africanized traits; only
26 swarms escaped in 1957, and nearly 60 years there does not appear to
be a noticeable lessening of the typical Africanized characteristics.
Gentleness
Not
all Africanized honey bee hives display the typical hyper-defensive
behavior, which may provide bee breeders a point to begin breeding a
gentler stock (gAHBs).
Work has been done in Brazil towards this end, but in order to maintain
these traits, it is necessary to develop a queen breeding and mating
facility in order to requeen colonies and to prevent reintroduction of
unwanted genes or characteristics through unintended crossbreeding with
feral colonies. In Puerto Rico, some bee colonies are already beginning
to show more gentle behavior. This is believed to be because the more
gentle bees contain genetic material that is more similar to the
European honey bee, although they also contain Africanized honey bee
material.
This degree of aggressiveness is surprisingly almost unrelated to
individual genetics - instead being almost entirely determined by the
entire hive's proportion of aggression genetics.
Also while bee incidents are much less common than they were during the
first wave of Africanized honey bee colonization, this can be largely
attributed to modified and improved bee management techniques. Prominent
among these are locating bee-yards much further from human habitation,
creating barriers to keep livestock at enough of a distance to prevent
interaction, and education of the general public to teach them how to
properly react when feral colonies are encountered and what resources to
contact. The Africanized honey bee is considered the honey bee of
choice for beekeeping in Brazil.
Health effects of pesticides may be acute or delayed in those who are exposed. A 2007 systematic review found that "most studies on non-Hodgkin lymphoma and leukemia showed positive associations with pesticide exposure" and thus concluded that cosmetic use of pesticides should be decreased. Strong evidence also exists for other negative outcomes from pesticide exposure including neurological problems, birth defects, fetal death, and neurodevelopmental disorder.
Acute health problems may occur in workers that handle pesticides,
such as abdominal pain, dizziness, headaches, nausea, vomiting, as well
as skin and eye problems. In China, an estimated half-million people are poisoned by pesticides each year, 500 of whom die. Pyrethrins, insecticides commonly used in common bug killers, can cause a potentially deadly condition if breathed in.
Long-term effects
Cancer
Many studies have examined the effects of pesticide exposure on the risk of cancer. Associations have been found with: leukemia, lymphoma, brain, kidney, breast, prostate, pancreas, liver, lung, and skin cancers. This increased risk occurs with both residential and occupational exposures. Increased rates of cancer have been found among farm workers who apply these chemicals. A mother's occupational exposure to pesticides during pregnancy is associated with an increases in her child's risk of leukemia, Wilms' tumor, and brain cancer. Exposure to insecticides within the home and herbicides outside is associated with blood cancers in children.
Neurological
Evidence links pesticide exposure to worsened neurological outcomes.
The United States Environmental Protection Agency finished a 10-year review of the organophosphate pesticides following the 1996 Food Quality Protection Act,
but did little to account for developmental neurotoxic effects, drawing
strong criticism from within the agency and from outside researchers. Comparable studies have not been done with newer pesticides that are replacing organophosphates.
Reproductive effects
Strong evidence links pesticide exposure to birth defects, fetal death and altered fetal growth. Agent Orange, a 50:50 mixture of 2,4,5-T and 2,4-D, has been associated with bad health and genetic effects in Malaya and Vietnam.
It was also found that offspring that were at some point exposed to
pesticides had a low birth weight and had developmental defects.
Fertility
A number of pesticides including dibromochlorophane and 2,4-D has been associated with impaired fertility in males.
Pesticide exposure resulted in reduced fertility in males, genetic
alterations in sperm, a reduced number of sperm, damage to germinal
epithelium and altered hormone function.
Other
Some studies have found increased risks of dermatitis in those exposed.
Additionally, studies have indicated that pesticide exposure is associated with long-term respiratory problems. Summaries of peer-reviewed research have examined the link between pesticide exposure and neurologic outcomes and cancer, perhaps the two most significant things resulting in organophosphate-exposed workers.
According to researchers from the National Institutes of Health
(NIH), licensed pesticide applicators who used chlorinated pesticides
on more than 100 days in their lifetime were at greater risk of diabetes.
One study found that associations between specific pesticides and
incident diabetes ranged from a 20 percent to a 200 percent increase in
risk. New cases of diabetes were reported by 3.4 percent of those in the
lowest pesticide use category compared with 4.6 percent of those in the
highest category. Risks were greater when users of specific pesticides
were compared with applicators who never applied that chemical.
Route of exposure
People
can be exposed to pesticides by a number of different routes including:
occupation, in the home, at school and in their food.
There are concerns that pesticides used to control pests on food
crops are dangerous to people who consume those foods. These concerns
are one reason for the organic food movement. Many food crops, including fruits and vegetables, contain pesticide residues
after being washed or peeled. Chemicals that are no longer used but
that are resistant to breakdown for long periods may remain in soil and
water and thus in food.
In the United States, levels of residues that remain on foods are limited to tolerance levels that are established by the U.S. Environmental Protection Agency and are considered safe. The EPA sets the tolerances based on the toxicity of the pesticide and its breakdown products, the amount and frequency of pesticide application, and how much of the pesticide (i.e., the residue) remains in or on food by the time it is marketed and prepared.
Tolerance levels are obtained using scientific risk assessments that
pesticide manufacturers are required to produce by conducting
toxicological studies, exposure modeling and residue studies before a
particular pesticide can be registered, however, the effects are tested
for single pesticides, and there is little information on possible synergistic effects of exposure to multiple pesticide traces in the air, food and water.
Strawberries and tomatoes are the two crops with the most
intensive use of soil fumigants. They are particularly vulnerable to
several types of diseases, insects, mites, and parasitic worms. In 2003,
in California alone, 3.7 million pounds (1,700 metric tons) of metham sodium
were used on tomatoes. In recent years other farmers have demonstrated
that it is possible to produce strawberries and tomatoes without the use
of harmful chemicals and in a cost-effective way.
Exposure routes other than consuming food that contains residues, in particular pesticide drift, are potentially significant to the general public.
Some pesticides can remain in the environment for prolonged
periods of time. For example, most people in the United States still
have detectable levels of DDT in their bodies even though it was banned
in the US in 1972.
Prevention
Pesticides exposure cannot be studied in placebo controlled trials as this would be unethical. A definitive cause effect relationship therefore cannot be established. Consistent evidence can and has been gathered through other study designs. The precautionary principle is thus frequently used in environmental law such that absolute proof is not required before efforts to decrease exposure to potential toxins are enacted.
The American Medical Association recommend limiting exposure to pesticides.
They came to this conclusion due to the fact that surveillance systems
currently in place are inadequate to determine problems related to
exposure.
The utility of applicator certification and public notification
programs are also of unknown value in their ability to prevent adverse
outcomes.
Epidemiology
The World Health Organization and the UN Environment Programme estimate that each year, 3 million workers in agriculture in the developing world experience severe poisoning from pesticides, about 18,000 of whom die. According to one study, as many as 25 million workers in developing countries may suffer mild pesticide poisoning yearly. Detectable levels of 50 different pesticides were found in the blood of a representative sample of the U.S. population.
A number of pesticides including the neonicotinoidsclothianidin, dinotefuran, imidacloprid are toxic to bees. Exposure to pesticides may be one of the contributory factors to colony collapse disorder.
A study in North Carolina indicated that more than 30 percent of the
quail tested were made sick by one aerial insecticide application. Once
sick, wild birds may neglect their young, abandon their nests, and
become more susceptible to predators or disease.
Quantum biology may use computations to model biological interactions in light of quantum mechanical effects. Quantum biology is concerned with the influence of non-trivial quantum phenomena, which can be explained by reducing the
biological process to fundamental physics, although these effects are difficult to study and can be speculative.
History
Quantum
biology is an emerging field; most of the current research is
theoretical and subject to questions that require further
experimentation. Though the field has only recently received an influx
of attention, it has been conceptualized by physicists throughout the
20th century. It has been suggested that quantum biology might play a
critical role in the future of the medical world. Early pioneers of quantum physics saw applications of quantum mechanics in biological problems. Erwin Schrödinger's 1944 book What is Life? discussed applications of quantum mechanics in biology. Schrödinger introduced the idea of an "aperiodic crystal" that contained genetic information in its configuration of covalent chemical bonds. He further suggested that mutations are introduced by "quantum leaps". Other pioneers Niels Bohr, Pascual Jordan, and Max Delbruck argued that the quantum idea of complementarity was fundamental to the life sciences. In 1963, Per-Olov Löwdin published proton tunneling as another mechanism for DNA mutation. In his paper, he stated that there is a new field of study called "quantum biology".
Applications
Photosynthesis
Diagram
of FMO complex. Light excites electrons in an antenna. The excitation
then transfers through various proteins in the FMO complex to the
reaction center to further photosynthesis.
Organisms that undergo photosynthesis absorb light energy through the process of electron excitation in antennae. These antennae vary among organisms. For example, bacteria use ring-like antennae, while plants and use chlorophyllpigments to absorb photons. Photosynthesis creates Frenkel excitons,
which provide a separation of charge that cells convert into usable
chemical energy. The energy collected in reaction sites must be
transferred quickly before it is lost to fluorescence or thermal vibrational motion.
Various structures, such as the FMO complex in green sulfur bacteria, are responsible for transferring energy from antennae to a reaction site. FT electron spectroscopy studies of electron absorption and transfer show an efficiency of above 99%, which cannot be explained by classical mechanical models like the diffusion model. Instead, as early as 1938, scientists theorized that quantum coherence was the mechanism for excitation energy transfer.
Scientists have recently looked for experimental evidence of this
proposed energy transfer mechanism. A study published in 2007 claimed
the identification of electronic quantum coherence
at −196 °C (77 K). Another theoretical study from 2010 provided
evidence that quantum coherence lives as long as 300 femtoseconds at
biologically relevant temperatures (4 °C or 277 K) . In that same year,
experiments conducted on photosynthetic cryptophyte algae using
two-dimensional photon echo spectroscopy yielded further confirmation
for long-term quantum coherence.
These studies suggest that, through evolution, nature has developed a
way of protecting quantum coherence to enhance the efficiency of
photosynthesis. However, critical follow-up studies question the
interpretation of these results. Single molecule spectroscopy now shows
the quantum characteristics of photosynthesis without the interference
of static disorder, and some studies use this method to assign reported
signatures of electronic quantum coherence to nuclear dynamics occurring
in chromophores.
A number of proposals emerged trying to explain unexpectedly long
coherence. According to one proposal, if each site within the complex
feels its own environmental noise, the electron will not remain in any
local minimum due to both quantum coherence and thermal environment, but proceed to the reaction site via quantum walks. Another proposal is that the rate of quantum coherence and electron tunneling create an energy sink that moves the electron to the reaction site quickly.
Other work suggested that geometric symmetries in the complex may favor
efficient energy transfer to the reaction center, mirroring perfect
state transfer in quantum networks.
Furthermore, experiments with artificial dye molecules cast doubts on
the interpretation that quantum effects last any longer than one hundred
femtoseconds.
In 2017, the first control experiment with the original FMO
protein under ambient conditions confirmed that electronic quantum
effects are washed out within 60 femtoseconds, while the overall exciton
transfer takes a time on the order of a few picoseconds.
In 2020 a review based on a wide collection of control experiments and
theory concluded that the proposed quantum effects as long lived
electronic coherences in the FMO system does not hold.
Instead, research investigating transport dynamics suggests that
interactions between electronic and vibrational modes of excitation in
FMO complexes require a semi-classical, semi-quantum explanation for the
transfer of exciton energy. In other words, while quantum coherence
dominates in the short-term, a classical description is most accurate to
describe long-term behavior of the excitons.
Another process in photosynthesis that has almost 100% efficiency is charge transfer, again suggesting that quantum mechanical phenomena are at play. In 1966, a study on the photosynthetic bacteria Chromatium found that at temperatures below 100 K, cytochrome
oxidation is temperature-independent, slow (on the order of
milliseconds), and very low in activation energy. The authors, Don
DeVault and Britton Chase, postulated that these characteristics of
electron transfer are indicative of quantum tunneling, whereby electrons penetrate a potential barrier despite possessing less energy than is classically necessary.
DNA mutation
Deoxyribonucleic acid, DNA,
acts as the instructions for making proteins throughout the body. It
consists of 4 nucleotides guanine, thymine, cytosine, and adenine. The order of these nucleotides gives the “recipe” for the different proteins.
Whenever a cell reproduces, it must copy these strands of DNA.
However, sometimes throughout the process of copying the strand of DNA a
mutation, or an error in the DNA code, can occur. A theory for the
reasoning behind DNA mutation is explained in the Lowdin DNA mutation model. In this model, a nucleotide may change its form through a process of quantum tunneling.
Because of this, the changed nucleotide will lose its ability to pair
with its original base pair and consequently changing the structure and
order of the DNA strand.
Exposure to ultraviolet lights and other types of radiation can
cause DNA mutation and damage. The radiations also can modify the bonds
along the DNA strand in the pyrimidines and cause them to bond with themselves creating a dimer.
In many prokaryotes and plants, these bonds are repaired to their
original form by a DNA repair enzyme photolyase. As its prefix implies,
photolyase is reliant on light in order to repair the strand.
Photolyase works with its cofactor FADH,
flavin adenine dinucleotide, while repairing the DNA. Photolyase is
excited by visible light and transfers an electron to the cofactor
FADH-. FADH- now in the possession of an extra electron gives the
electron to the dimer to break the bond and repair the DNA. This
transfer of the electron is done through the tunneling of the electron
from the FADH to the dimer.
Although the range of the tunneling is much larger than feasible in a
vacuum, the tunneling in this scenario is said to be
“superexchange-mediated tunneling,” and is possible due to the protein's
ability to boost the tunneling rates of the electron.
Vibration theory of olfaction
Olfaction,
the sense of smell, can be broken down into two parts; the reception
and detection of a chemical, and how that detection is sent to and
processed by the brain. This process of detecting an odorant is still under question. One theory named the “shape theory of olfaction”
suggests that certain olfactory receptors are triggered by certain
shapes of chemicals and those receptors send a specific message to the
brain.
Another theory (based on quantum phenomena) suggests that the olfactory
receptors detect the vibration of the molecules that reach them and the
“smell” is due to different vibrational frequencies, this theory is
aptly called the “vibration theory of olfaction.”
The vibration theory of olfaction, created in 1938 by Malcolm Dyson but reinvigorated by Luca Turin in 1996,
proposes that the mechanism for the sense of smell is due to G-protein
receptors that detect molecular vibrations due to inelastic electron
tunneling, tunneling where the electron loses energy, across molecules. In this process a molecule would fill a binding site with a G-protein
receptor. After the binding of the chemical to the receptor, the
chemical would then act as a bridge allowing for the electron to be
transferred through the protein. As the electron transfers through and
that usually would be a barrier for the electrons and would lose its
energy due to the vibration of the molecule recently bound to the
receptor, resulting in the ability to smell the molecule.
While the vibration theory has some experimental proof of concept,
there have been multiple controversial results in experiments. In some
experiments, animals are able to distinguish smells between molecules of
different frequencies and same structure, while other experiments show that people are unaware of distinguishing smells due to distinct molecular frequencies.
However, it has not been disproven, and has even been shown to be an
effect in olfaction of animals other than humans such as flies, bees,
and fish.
Vision
Vision relies on quantized energy in order to convert light signals to an action potential in a process called phototransduction. In phototransduction, a photon interacts with a chromophore in a light receptor. The chromophore absorbs the photon and undergoes photoisomerization. This change in structure induces a change in the structure of the photo receptor and resulting signal transduction pathways lead to a visual signal. However, the photoisomerization reaction occurs at a rapid rate, in under 200 femtoseconds, with high yield. Models suggest the use of quantum effects in shaping the ground state and excited state potentials in order to achieve this efficiency.
Quantum vision implications
Experiments have shown that the sensors in the retina of human eye is sensitive enough to detect a single photon. Single photon detection could lead to multiple different technologies. One area of development is in quantum communication and cryptography. The idea is to use a biometric system to measure the eye using only a small number of points across the retina with random flashes of photons that “read” the retina and identify the individual.
This biometric system would only allow a certain individual with a
specific retinal map to decode the message. This message can not be
decoded by anyone else unless the eavesdropper were to guess the proper
map or could read the retina of the intended recipient of the message.
Enzymatic activity (quantum biochemistry)
Enzymes may use quantum tunneling to transfer electrons long distances. It is possible that protein quaternary architecture may have evolved to enable sustained quantum entanglement and coherence. More specifically, they can increase the percentage of the reaction that occurs through hydrogen tunneling.
Tunneling refers to the ability of a small mass particle to travel
through energy barriers. This ability is due to the principle of complementarity,
which hold that certain objects have pairs of properties that cannot be
measured separately without changing the outcome of measurement.
Electrons have both wave and particle
properties, so they can pass through physical barriers as a wave
without violating the laws of physics. Studies show that long distance
electron transfers between redox centers through quantum tunneling plays important roles in enzymatic activity of photosynthesis and cellular respiration.
For example, studies show that long range electron tunneling on the
order of 15–30 Å plays a role in redox reactions in enzymes of cellular
respiration.
Without quantum tunneling, organisms would not be able to convert
energy quickly enough to sustain growth. Even though there are such
large separations between redox sites within enzymes, electrons
successfully transfer in a generally temperature independent (aside from
extreme conditions) and distance dependent manner.
This suggests the ability of electrons to tunnel in physiological
conditions. Further research is needed to determine whether this
specific tunneling is also coherent.
Magnetoreception
Magnetoreception refers to the ability of animals to navigate using the inclination of the magnetic field of the earth. A possible explanation for magnetoreception is the entangledradical pair mechanism. The radical-pair mechanism is well-established in spin chemistry,
and was speculated to apply to magnetoreception in 1978 by Schulten et
al.. The ratio between singlet and triplet pairs is changed by the
interaction of entangled electron pairs with the magnetic field of the
earth. In 2000, cryptochrome was proposed as the "magnetic molecule" that could harbor magnetically sensitive radical-pairs. Cryptochrome, a flavoprotein found in the eyes of European robins and other animal species, is the only protein known to form photoinduced radical-pairs in animals. When it interacts with light particles, cryptochrome goes through a redox
reaction, which yields radical pairs both during the photo-reduction
and the oxidation. The function of cryptochrome is diverse across
species, however, the photoinduction of radical-pairs occurs by exposure
to blue light, which excites an electron in a chromophore.
Magnetoreception is also possible in the dark, so the mechanism must
rely more on the radical pairs generated during light-independent
oxidation.
Experiments in the lab support the basic theory that radical-pair
electrons can be significantly influenced by very weak magnetic fields,
i.e. merely the direction of weak magnetic fields can affect
radical-pair's reactivity and therefore can "catalyze" the formation of
chemical products. Whether this mechanism applies to magnetoreception
and/or quantum biology, that is, whether earth's magnetic field
"catalyzes" the formation of biochemical products by the aid of
radical-pairs, is undetermined for two reasons. The first is that
radical-pairs may need not be entangled, the key quantum feature
of the radical-pair mechanism, to play a part in these processes. There
are entangled and non-entangled radical-pairs. However, researchers
found evidence for the radical-pair mechanism of magnetoreception when
European robins, cockroaches, and garden warblers, could no longer
navigate when exposed to a radio frequency that obstructs magnetic fields
and radical-pair chemistry. To empirically suggest the involvement of
entanglement, an experiment would need to be devised that could disturb
entangled radical-pairs without disturbing other radical-pairs, or vice
versa, which would first need to be demonstrated in a laboratory setting
before being applied to in vivo radical-pairs.
Other biological applications
Other examples of quantum phenomena in biological systems include the conversion of chemical energy into motion and brownian motors in many cellular processes.
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