Encyclopedia of Evolution.pdf - Online Reading Center

thermodynamics
return to their original orderly arrangement simply by random
movements.
Electricity. Electricity flows from regions of high voltage
to regions of lower voltage. Electric current occurs when
electrons flow from regions in which they have high potential
energy (high voltage) to regions in which electrons have
low voltage. This process continues until the voltage reaches
equilibrium, as when a battery runs down. Energy must be
expended, as in an electric generator, to raise the voltage of
some electrons and thus create an electric current. The First
Law indicates that the total amount of energy has remained
constant, although it changed form from potential to electrical,
and the Second Law indicates that the electrons have
reached maximum uniformity, when equilibrium is reached.
Chemical reactions. Chemical reactions (in which atoms
change molecular arrangements) also follow this pattern. A
great deal of potential energy is stored within the chemical
bonds of molecules, particularly in large biological or other
organic molecules. If the reactant or reactants at the beginning
of the reaction have more potential energy than the
product or products have at the end of the reaction, then
the energy has been released into another form. It could be
released as kinetic energy (the reaction can release heat).
Some reactions release light, while others release electricity.
The First Law indicates that during these energy-releasing
reactions, potential energy has changed into other forms
of energy, but the total amount of energy remains constant,
and the Second Law indicates that disorder has increased as
a large, orderly molecule has broken down into smaller, less
orderly molecules. Large molecules tend to fall apart into
smaller components because large molecules contain more
potential energy and are more orderly than small molecules.
The total amount of energy remains the same, though it was
transformed from potential to kinetic forms.
In other chemical reactions, the reactant or reactants
have less potential energy than the product or products.
These reactions will not occur unless energy is put into the
reaction. In some of these reactions, kinetic energy of molecules
becomes potential energy within bonds: The reactions
cause their environment to become colder. In other reactions,
light is absorbed by the electrons. In organisms, the energy
that is released from one reaction can be used as an energy
source by another reaction. The First Law indicates that during
these energy-absorbing reactions, various forms of energy
become potential energy, but the total amount of energy
remains constant, and the Second Law indicates that entropy
has actually decreased, as (usually) smaller, disorderly molecules
have come together to form a larger, orderly molecule.
Biological processes can assemble small molecules into larger
ones. This process requires an input of energy and represents
a decrease in entropy. For reasons described below, events
that build small molecules into large ones do not represent
violations of the Second Law.
For any of the events listed above to go in the reverse
direction, inputs are necessary. It has already been noted that
inputs of energy are necessary. However, a simple addition of
energy is usually not enough to reverse the tendency toward
disorder. If one puts kinetic energy into a sample of small mol-
ecules by heating them, one will not necessarily cause the small
molecules to assemble into large molecules. It would probably
just make them hotter and even more disorderly. A process
that uses some kind of information is necessary to direct the
use of the energy. Organisms have enzymes by which energy
is directed to assemble small molecules into large ones and to
make molecules to move from regions of lower concentration
to regions of higher concentration. This represents an input
of information. The information source need not be complex.
Many inorganic catalysts are quite simple but can allow simple
molecules to react into more complex forms.
What does all this have to do with evolution? Although
evolution does not require an increase in complexity (see
progress, concept of), the history of the Earth has been
characterized by the emergence of more and more complex
organisms. This means that, at least in some cases,
entropy has decreased during evolution. Some critics (see
creationism) have claimed that evolution therefore violates
the Second Law. This is not the case. A decrease in
entropy requires an input of information. In biological systems,
most of this information is in the form of enzymes,
which control chemical reactions, and which are produced
using information contained in nucleic acids (see DNA [raw
material of evolution]). If mutations accumulated without
any restraints, information would degrade and entropy
would increase. However, natural selection removes deleterious
mutations, causing an accumulation of neutral or
even beneficial mutations, which represent new information
and a potential decrease in entropy. Sometimes, as in gene
duplication (followed by mutation), horizontal gene transfer,
or symbiogenesis, the information in a cell can suddenly
increase.
Thermodynamics is especially relevant to an understanding
of the origin of the first biological molecules and living
systems (see origin of life). Life originated when small
molecules assembled into large organic molecules. There was
no preexisting life that could provide information (for example,
in the form of enzymes) to direct this process. Yet experimental
evidence indicates that entropy can decrease even
without an input of complex information. Miller’s classic
experiment (see Miller, Stanley) brought small molecules
and an energy source together to produce larger molecules.
Subsequent experimental simulations of the origin of life
have used clay surfaces that nonrandomly orient the smaller
molecules and promote their synthesis into large molecules.
The processes that would break large molecules down into
smaller ones would overwhelm the processes that build small
molecules into large ones, according to the Second Law, if it
were not for the clay surfaces, which represent a source of
information. The clay surfaces allow a spatial separation of
the large molecules, so that they are no longer in circulation
and exposed to the processes that would break them down.
The total amount of energy in the universe remains constant,
but the total amount of disorder is always increasing.
Eventually all the energy in the universe will be uniformly
distributed and very weak, producing an equilibrium of maximum
disorder from which nothing from within the universe
itself can lift it.