Encyclopedia of Evolution.pdf - Online Reading Center

Further Reading
Eldredge, Niles, and Stephen Jay Gould. “Punctuated equilibria: An
alternative to phyletic gradualism.” In Models in Paleobiology,
edited by T. J. M. Schopf, 82–115. San Francisco, Calif.: Freeman,
Cooper and Co., 1972.
———. Darwin: Discovering the Tree of Life. New York: Norton,
2005.
———. Dominion. Berkeley, Calif.: University of California Press,
1997.
———. Life in the Balance: Humanity and the Biodiversity Crisis.
Princeton, N.J.: Princeton University Press, 1998.
———. The Pattern of Evolution. New York: Freeman, 1999.
———. Reinventing Darwin: The Great Debate at the High Table of
Evolutionary Theory. New York: John Wiley, 1995.
———. The Triumph of Evolution and the Failure of Creationism.
New York: Freeman, 2000.
———. Why We Do It: Rethinking Sex and the Selfish Gene. New
York: Norton, 2004.
emergence Emergent properties are complex properties
that result from the interaction of simpler components.
Emergent properties are essential for understanding evolution
because they reveal how complexity can result without
a complex control system to produce and maintain it. The
information that is contained within an emergent system
does not have to be stored in DNA or imposed by an intelligent
designer (see intelligent design). The simpler components
of an emergent system do not themselves contain
the information for the structure of the complex system of
which they are a part. One important reason for emergent
properties is that the components of the system interact with
one another and modify their behavior in response to one
another, a process which could be called learning. The following
are examples, rather than a thorough explanation of
the topic.
Emergent properties of atoms and molecules. Atoms are
made of particles, but atoms have properties that the particles
do not. Particles interact with one another when they
make up atoms; electrons in atoms do not act like individual
electrons. Molecules are made of atoms, but molecules have
properties that the atoms do not have. Neither hydrogen nor
oxygen has the properties of water. The atoms interact with
one another when they make up molecules; oxygen atoms in
water do not behave like individual oxygen atoms but share
their electrons with the hydrogen atoms.
Emergent properties of cells in an organism. As an organism
develops from a single egg cell, the cells respond to one
another. Development occurs as a cascade of reactions, beginning
with instructions from some homeotic genes, which cause
other gene expressions, which cause yet other gene expressions
(see developmental evolution). While each cell contains a
complete set of instructions in its DNA regarding what kinds
of proteins to make (see DNA [raw material of evolution]),
no cell consults all of that information. Each cell has its
own particular job that it performs in response to the instructions
that it originally received. The complexity of a body
results from the very limited range of tasks performed by each
emergence
kind of cell. The genes code not for the structure but for the
instructions to make the structure emerge.
In complex animals, nervous systems coordinate movements
and responses. Sponges are multicellular animals but
do not have nervous systems (see invertebrates, evolution
of). Choanocytes are cells that line the internal passages of a
sponge. The movements of their whip-like flagella create currents
of water that flow in through pores on the side, and out
through a chimney-like structure at the top, of the animal.
The choanocytes beat their flagella in rhythm, but there is no
nervous system that coordinates this rhythm. The chaonocytes
develop the rhythm in response to one another, which
makes it an emergent property.
Emergent systems of organisms. A colony of ants is a
very complex structure. The queen lays eggs, and workers
carry out numerous tasks. As a result, many jobs get done
by the ant colony. Workers explore their environments, find
food, and bring it back to the colony. Yet no individual,
and no set of instructions, is in control of the process. The
queen is an egg-laying machine, rather than a ruler. None
of the individual worker ants have information about the
entire colony or about all of the surrounding environment.
Each worker explores more or less at random, and if she
finds a food source, she deposits pheromones, which are
chemicals that signal to the other ants that this is the direction
to go in order to find food. The crucial step is that
each individual learns a little bit from other individuals.
This limited amount of learning on the part of the individuals
allows the colony as a whole to learn complex behavior.
The ant colony displays what looks like intelligence to a
human observer, but none of the individual ants have intelligence.
The complex behavior of the colony is an emergent
property.
A similar process of chemical communication causes
individual slime mold cells to coalesce into a single reproductive
structure. Slime mold cells live underneath leaf litter, each
one obtaining its own food individually under warm, moist
conditions. If conditions become cooler or drier, the behavior
of the individual cells changes, and they begin to come
together. They form a stalk, in a capsule at the top of which
some of the individuals produce spores that blow away in the
wind. None of the slime mold cells is in charge of organizing
this activity. But as each cell responds to and learns from
a few others, they assemble themselves into a complex structure.
The reproductive behavior of slime molds is an emergent
property.
Emergent properties of brains. A brain, especially the
human brain, can store an immense amount of information.
Yet each individual neuron stores very little information.
The brain as a whole is able to learn and store information,
because each neuron responds to and learns from the other
neurons with which it is in contact. A complex structure of
information storage emerges. While parts of the brain specialize
on different functions, most brain functions are shared by
several parts of the brain. PET studies of the brain (positronemission
tomography, which reveals the parts of the brain
that are active during different functions such as talking,