Biological Complexity and Integrative Levels of Organization

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).

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.

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.