Download Biological Complexity and Integrative Levels of Organization

Biological Complexity and Integrative Levels of
Organization
By: Ingrid Lobo, Ph.D. (Write Science Right) © 2008 Nature Education Citation: Lobo, I. (2008) Biological complexity and integrative levels of
organization. Nature Education 1(1):141
If someone gave you a stranger’s complete genetic code, could you predict
everything about that person? Of course not, but why isn't there one code to
explain how everything works?
It may be difficult to imagine that your body is made of spinning protons, neutrons, and electrons,
but this is indeed the case. It's slightly easier, however, to picture forms of matter in levels that
increase in complexity. For example, subatomic particles can be organized into atoms, which are
the components of molecules, and molecules can be organized into macromolecules, such as DNA
and proteins, which can be built into cells. Cells can then be organized into tissues, which form
organs, and organs can be grouped into organ systems, which are built into entire organisms—
including humans like you. Organisms are units that can form populations, and then biospheres,
which go on to make up even greater levels of complexity.
As this example illustrates, units of matter are organized and integrated into levels of increasing
complexity; this is a concept referred to as integrative levels of organization. Integrative levels of
organization allow researchers to describe the evolution from the inanimate to the animate and
social worlds (Novikoff, 1945). Higher integrative levels are more complex and demonstrate more
variation and characteristics than lower integrative levels. These levels are based upon a physical
foundation, with the lowest level appearing to consist of subatomic particles. In order to study
genetics, however, we don't need to consider objects as tiny as subatomic particles. Rather, the
spectrum of integrative levels that ranges from macromolecules to populations is most relevant
(Figure 1).
Emergent Properties
When units of biological material are put together, the properties of the new material are not always
additive, or equal to the sum of the properties of the components. Instead, at each level, new
properties and rules emerge that cannot be predicted by observations and full knowledge of the
lower levels. Such properties are called emergent properties (Novikoff, 1945).
Life itself is an example of an emergent property. For instance, a single‑celled bacterium is alive,
but if you separate the macromolecules that combined to create the bacterium, these units are not
alive. Based on our knowledge of macromolecules, we would not have been able to predict that they
could combine to form a living organism, nor could we have predicted all of the characteristics of
the resulting bacterium.
Thus, our understanding of physical and chemical properties in lower levels of organization helps
us understand only some of the properties of living organisms, which prevents use of a reductionist
approach. No matter how well we understand the physics and chemistry of living systems, we must
recognize that living systems, and other high integrative levels, have new and unique properties
that emerge through the combination of the lower‑level units of matter (Novikoff, 1945). Likewise,
our understanding of the new emergent properties at a higher level does not help us understand
the properties of the lower levels, because each integrative level of organization has its own
particular structure and emergent properties.
As an example of why the reductionist approach fails, consider the function of one cell within a
multicellular organism. Even if we understand the cell's function, that does not mean we fully
understand the organism's physiology. After all, the activity of each cell is affected by the activity of
other cells in the tissues, organs, and organ
systems within the organism. The cell is thus no
longer in isolation, and its integration into a
system provides that system with emergent
properties (Novikoff, 1945).
Let's consider how the effect of an allele varies
according to levels of organization. At the
macromolecular level, an allele is encoded as
DNA, which is transcribed into another
macromolecule, RNA, and then translated into a
third macromolecule, protein. Thus, you could
study DNA sequence, RNA expression, or protein
expression at the macromolecular level. Now,
imagine that this particular allele encodes the
protein hexokinase. At the cellular level,
hexokinase helps break down glucose in the
glycolysis pathway to provide cells with energy.
The glycolysis pathway is an emergent property
that functions because many enzymes have
formed a biochemical pathway in single cells.
Next, the cells that express the hexokinase allele
can be organized into tissues, such as skeletal
muscle. At the organ system level, skeletal
muscles sustain movements throughout an
organism. Now, imagine that this allele is acting
in concert with many other alleles in the muscles
of a songbird to create energy. The allele's
actions throughout all of these levels can be
integrated with other processes in the bird to
allow complex movement, such as flight. Flight is
an example of an emergent property that exists
at the level of the organism but not at lower
levels. Higher up, at the population level, the
allele might also help birds synchronize their
feeding, metabolism, muscle movements, and
flight patterns to allow migration in a flock.
Figure 1: Biological matter can be
organized into levels of increasing
complexity.
Many different kinds of macromolecules are
used to build cells, which in turn can be
organized into tissues. Tissues form organs,
and several organs may have interrelated
functions in a cohesive organ system, such
as the digestive system. A complex
organism contains multiple organ systems
with different functions. Multiple organisms
of a single species may form a group, called
a population. Many populations of different
species form diverse communities, and
communities that share the same
geographical space are part of a larger
ecosystem. The Earth’s biosphere is made
up of many diverse ecosystems.
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Figure Detail
Dimensions of Analysis
A very small change in a single macromolecule can have a profound effect on an organism, or even
a population, when magnified through levels of complexity. For instance, when a disturbance such
as a genetic mutation is introduced in any level, it can affect all of the higher levels of organization.
The effect of such a disturbance can be either severe or trivial. For example, a mutation, or change,
in a single DNA base in a single gene can result in diseases such as cystic fibrosis and Duchenne
muscular dystrophy in humans at the organismal level.
This means that the mechanism behind an organism's phenotype can be observed at the integrative
level immediately below it. Likewise, a change in matter at a lower level can produce a phenotype
that is observable at a higher integrative level. Therefore, a phenotype must only be defined
according to the integrative level under consideration. For example, a mutation in a gene can be
observed as a change in DNA and protein at the macromolecular level. At the tissue level, the same
mutation could cause changes in histology. Meanwhile, at the level of the organism, the mutation
could result in behavioral changes. For these reasons, each integrative level must be studied with
the tools available for that level, which are called the dimensions of analysis. Moreover, the change
or changes at any one level must be related to the changes at all higher levels (Novikoff, 1945).
Thus, understanding a disease phenotype or behavior at a higher level requires that we study
changes at many different integrative levels using the appropriate methodologies. For a geneticist,
chemistry, biochemistry, molecular biology, histology, and physiology are all important. Broad
training in all of these techniques allows a geneticist to study emergent interactions at multiple
levels.
References and Recommended Reading
Novikoff, A. B. The concept of integrative levels and biology. Science 101, 209‑215 (1945)
doi:10.1126/science.101.2618.209.