Encyclopedia of Evolution.pdf - Online Reading Center

iology
design) or adaptation to their environments and can only
be understood in terms of evolutionary ancestry. This applies
to some of their structural and functional characteristics (see
vestigial characteristics) and particularly to much of
the DNA (see noncoding DNA). This is what evolutionary
geneticist Theodosius Dobzhansky meant by his famous statement,
“Nothing in biology makes sense except in the light of
evolution” (see Dobzhansky, Theodosius). Furthermore,
evolution has given an organizing principle to biology, which
might otherwise have continued to be a cataloguing of types
of organisms and their structures and functions.
Although biologists study life, they cannot precisely
define it. Ernst Mayr (see Mayr, Ernst), perhaps the most
prominent biologist of modern times, indicated that a lifeform
must have the following capacities:
• For metabolism, in which energy is bound and released, for
example, to digest food molecules
• For self-regulation, whereby the chemical reactions of
metabolism are kept under control and in homeostasis, for
example, to maintain relatively constant internal conditions
of temperature, or moisture, or chemical composition
• To respond to environmental stimuli, for example, moving
toward or away from light
• To store genetic information that determines the chemical
reactions that occur in the organism, for example, DNA,
and to use this information to bring about changes in the
organism
• For growth
• For differentiation, for example to develop from an embryo
into a juvenile into an adult
• For reproduction
• To undergo genetic change which, in a population, allows
evolution to occur
While most of these characteristics may not be much in
dispute, it is impossible for scientists to imagine all the possible
forms that they could take. In addition, no life-form
carries out all of these activities all of the time or under all
conditions. These considerations become important in two
respects. First, would it be possible to recognize a life-form
on another planet? Scientists may not be able to witness
putative life-forms carrying out metabolic and other activities
(see Mars, life on). Second, at what point might scientists
be able to construct a mechanical life-form? Although
computerized robots cannot grow or reproduce themselves,
they can construct new components and whole new robots.
Many computer algorithms already utilize natural selection
to generate improvements in structure and function (see evolutionary
algorithms). Should robots, or even computer
programs, be considered life-forms?
Fundamental assumptions of biology include the following:
• The physical and chemical components and processes of
organisms are the same as those of the nonliving world.
That is, organisms are constructed of the same kinds of
atoms (though not necessarily in the same relative amounts)
as the nonliving world; and the laws of physics and chemistry
are the same inside an organism as outside. The alterna-
tive to this view, vitalism, claimed that organisms were made
out of material that is different from the nonliving world.
This view was widespread along with many biblical views
of the natural world until the 19th century, despite the fact
that the Bible says “Dust thou art and to dust thou shalt
return.” The German chemist Friedrich Wöhler put an end
to vitalism in 1815 when he synthesized urea, a biological
molecule, from ammonia, an inorganic molecule. Some people
continue to believe, or hope, that there is some further
essence within organisms, at least within humans, that is not
shared with the nonliving environment, but no evidence of
such an essence has been found. Many of the processes that
occur in organisms are more complex than those in the nonliving
world; in addition, when complex molecules interact,
they can produce emergent properties (see emergence) that
could not have been predicted from a study of the atoms
themselves. But this is no different from what happens in the
nonliving world. One cannot explain water in terms of the
properties of hydrogen and oxygen; the properties of water
are emergent; yet nobody claims that water molecules violate
the laws of physics and chemistry, or that a nonmaterial
essence is needed to make water what it is.
• Explanations of biological phenomena can be made at
two levels. Consider, for example, why a mockingbird
sings. First, scientists can explain the immediate physical
and chemical causes of the singing. The pineal gland in
the mockingbird’s head senses the increase in the length of
the day; this indicates that spring is coming. In response
to this information, the mockingbird’s brain produces
enhanced levels of the hormone melatonin, which activates
the nerve pathways that cause the production of
sound. The brain uses both instinctive and learned information
to determine which songs the mockingbird sings.
The brain also stimulates the bird to leap and dance. This
complex network of immediate causation, which involves
sunlight, a gland, the brain, a hormone, the voice box,
and muscles, is the proximate causation of the mockingbird’s
singing. Second, scientists can explain the advantage
that the ancestors of the mockingbird obtained from
undertaking such behavior. Singing and dancing was a
territorial display that allowed dominant male mockingbirds
to keep other male mockingbirds away and to
attract female mockingbirds. Natural selection in the past
favored these activities and selected the genes that now
determine this behavior. The evolutionary causation is
the ultimate causation of the mockingbird’s singing (see
behavior, evolution of).
• Structure and function are interconnected in organisms.
That is, the structures do what they look like, and they
look like what they do. Both the xylem cells of plants and
the blood vessels of animals conduct fluid, and they have a
long, cylindrical structure. They act, and look, like pipes.
A student may memorize anatomical structures, or physiological
processes, but will understand biology only when
bringing the two together.
• Large organisms have less external surface area relative to
their volume than do small organisms (see allometry).