General Properties of Living Systems
The most outstanding general features in life’s history include
chemical uniqueness; complexity and hierarchical organization; reproduction
(heredity and variation); possession of a genetic program; metabolism; development;
environmental interaction; and movement.
1. Chemical uniqueness. Living systems demonstrate a unique and
complex molecular organization. Living
systems assemble
large molecules, known as macromolecules, that are far more
complex than the small molecules of nonliving matter.
These macromolecules are composed of the same kinds of
atoms and chemical bonds that occur in nonliving matter
and they obey all fundamental laws of chemistry; it
is only the complex organizational structure of these macromolecules
that makes them unique. We recognize
four major categories of biological
macromolecules: nucleic
acids, proteins, carbohydrates, and
lipids. These
categories differ in the structures of their component parts,
the kinds of chemical bonds that link their subunits together,
and their functions in living systems.
The general structures of these macromolecules
evolved and
stabilized early in the history of life. With some modify cautions,
these same general structures are found in every form
of life today. Proteins, for example, contain about 20 specific
kinds of amino acid subunits linked together by peptide
bonds in a linear sequence. Additional
bonds occurring between amino acids that
are not adjacent to
each other in the protein chain give the protein a complex, three-dimensional
structure A typical protein contains several hundred amino acid subunits. Despite
the stability of this basic protein structure, the ordering
of the different amino acids in the protein molecule is
subject to enormous variation. This variation underlies much
of the diversity that we observe among different kinds
of living forms. The nucleic acids, carbohydrates, and lipids
likewise contain characteristic bonds that link variable subunits.
This organization gives living systems
both a biochemical unity and great
potential diversity.
2. Complexity and hierarchical organization. Living systems demonstrate a unique
and complex hierarchical organization. Nonliving
matter is organized at least into atoms
and molecules and often has a higher
degree of organization as
well. However, atoms and molecules are combined into
patterns in the living world that do not exist in the
nonliving world. In living systems, we fi nd a hierarchy of
levels that includes, in ascending order of complexity, and has its own
internal structure, which is also often hierarchical. Within the cell, for
example, macromolecules are compounded into structures such as ribosomes,
chromosomes, and membranes, and these are likewise combined in various ways to
form even more complex subcellular structures called organelles, such as
mitochondria. The organismal level also has a hierarchical substructure; cells
combine to form tissues, which combine to form organs, which likewise combine to
form organ systems.
ZOOLOGY AS A PART OF BIOLOGY
Animals form a distinct branch
on the evolutionary tree of life. It is a large and old branch that originated
in the Precambrian seas over 600 million years ago. Animals form part of an
even larger limb known as eukaryotes,
organisms whose cells contain
membrane- enclosed nuclei. This larger limb includes plants, fungi and numerous
unicellular forms. Perhaps the most distinctive characteristic of the animals
as a group is their means of nutrition, which consists of eating other organisms.
Evolution has elaborated this basic way of life through diverse systems for
capturing and processing a wide array of food items and for locomotion. Animals
are distinguished also by the absence of characteristics that have evolved in
other eukaryotes. Plants, for example, use light energy to produce organic
compounds (photosynthesis), and they have evolved rigid cell walls that
surround their cell membranes; photosynthesis and cell walls are absent from animals.
Fungi acquire nutrition by absorption of small organic molecules from their
environments, and their body plan contains tubular fi laments called hyphae; these structures are absent from the animal
kingdom.
Some organisms combine properties of animals and plants. For
example, Euglena is a motile, single-celled organism that resembles plants in
being photosynthetic, but it resembles animals in its ability to eat food
particles. Euglena is part of a separate eukaryotic lineage that diverged from
those of plants and animals early in the evolutionary history of eukaryotes. Euglena and other unicellular eukaryotes are sometimes grouped as the
kingdom Protista, although this kingdom is an arbitrary grouping of unrelated lineages
that violates taxonomic principles.
PRINCIPLES OF SCIENCE
Nature of Science
We stated in the first
sentence of this chapter that zoology is the scientific study of animals. A
basic understanding of zoology therefore requires an understanding of what
science is, what it is not, and how knowledge is gained using the scientific
method. Science is a way of asking questions about the natural world and
sometimes obtaining precise answers to them. Although science, in the modern
sense, has arisen recently in human history (within the last 200 years or so),
the tradition of asking questions about the natural world is an ancient one. In
this section, we examine the methodology that zoology shares with science as a whole.
These features distinguish sciences from activities that we exclude from the
realm of science, such as art and religion.
Despite an enormous impact of science on our
lives, many people have only a minimal understanding of the nature of science. For
example, on March 19, 1981, the governor of Arkansas signed into law the
Balanced Treatment for Creation-Science and Evolution-Science Act (Act 590 of
1981). This act falsely presented “creation-science” as a valid scientifi c
endeavor. “Creationscience” is actually a religious position advocated by a
minority of the American religious community, and it does not qualify as science.
The enactment of this law led to a historic lawsuit tried in December 1981 in
the court of Judge William R. Overton, U.S. District Court, Eastern District of
Arkansas. The suit was brought by the American Civil Liberties Union on behalf
of 23 plaintiffs, including religious leaders and groups representing several denominations,
individual parents, and educational associations. The plaintiffs contended that
the law was a violation of the First Amendment to the U.S. Constitution, which
prohibits “establishment of religion” by government. This prohibition includes passing
a law that would aid one religion or prefer one religion over another. On
January 5, 1982, Judge Overton permanently
enjoined the State of Arkansas from enforcing Act
590. Considerable testimony during the trial dealt with the nature of science.
Some witnesses defined science simply, if not very informatively, as “what is
accepted by the scientific community” and “what scientists do.” However, on
the basis of other testimony
by scientists, Judge Overton was able to state
explicitly these essential characteristics of science:
1. It is guided by natural law.
2. It has to be explanatory by reference to natural
law.
3. It is testable against the observable world.
4. Its conclusions are tentative and therefore not
necessarily the final word.
5. It is falsifiable.
Pursuit of scientific
knowledge must be guided by the physical and chemical laws that govern the
state of existence. Scientific knowledge must explain what is observed by
reference to natural law without requiring intervention of a supernatural being
or force. We must be able to observe events in the real world, directly or
indirectly, to test hypotheses about nature. If we draw a conclusion relative
to some event, we must be ready always to discard or to modify our conclusion
if further observations contradict it. As Judge Overtonstated, “While anybody
is free to approach a scientific inquiry in any fashion they choose, they cannot
properly describe the methodology used as scientific if they start with a
conclusion and refuse to change it regardless of the evidence developed during
the course of the investigation.”
Science is separate from
religion, and the results of science do not favor one religious position over
another. Unfortunately, the religious position formerly called “creation science has reappeared in American politics with the name “intelligent- design theory.”
We are forced once again to defend the teaching of science against this
scientifically meaningless dogma.
Scientifi c Method
These essential criteria of science form the hypotheticodeductive method. The first step of this method is the generation of
hypotheses or potential answers to the question being asked. These hypotheses
are usually based on prior observations of nature or derived from theories
based on such observations. Scientif c hypotheses often constitute general statements
about nature that may explain a large number of diverse observations. Darwin’s
hypothesis of natural selection, for example, explains the observations that
many different species have properties that adapt them to their environments.
On the basis of the
hypothesis, a scientist must make a prediction about future observations. The
scientist must say, “If my hypothesis is a valid explanation of past observations,
then future observations ought to have certain characteristics.” The east
hypotheses are those that make many predictions which, if found erroneous, will
lead to rejection, or falsification, of the hypothesis. The scientific method may
be summarized as a series of steps:
1. Observation
2. Question
3. Hypothesis
4. Empirical test
5. Conclusions
6. Publication
form a critical first step in evaluating the life histories of
natural populations. might cause the
observer to question whether rate of larval growth is higher in undisturbed
populations than in ones exposed to a chemical pollutant. A null hypothesis is then
generated to permit an empirical test. A null hypothesis is one worded in a way
that would permit data to reject it if it is false. In this case, the null hypothesis
is that larval growth rates for crabs in undisturbed habitats are the same as
those in polluted habitats. The investigator then performs an empirical test by
gathering data on larval growth rates in a set of undisturbed crab populations
and a set of populations subjected to the chemical pollutant. Ideally, the undisturbed
populations and the chemically treated populations are equivalent for all
conditions except presence of the chemical in question. If measurements show
consistent differences in growth rate between the two sets of populations, the
null hypothesis is rejected. One then concludes that the chemical pollutant does
alter larval growth rates. A statistical test is usually needed to ensure that
the differences between the two groups are greater than would be expected from
chance fluctuations alone. If the null hypothesis cannot be rejected, one
concludes that the data do not show any effect of the chemical treatment. The
results of the study are then published to communicate findings to other researchers,
who may repeat the results, perhaps using additional populations of the same or
a different species. Conclusions of the initial study then serve as the observations
for further questions and hypotheses to reiterate the scientifc process.
References : Hickman.2008.INTEGRATED PRINCIPLES OF ZOOLOGY, FOURTEENTH EDITION. New york. McGraw-Hill Companies