Two sides of the same coin

Living systems are all different from each other, but at the same time they have common characteristics. Some of these are always present, so much so that, in some way, they help define our very concept of life. 
 

There is a common assumption underlying these characteristics, and especially in our way of contemplating and describing them. This is the relationship between the form and function of a biological structure. 
 

Biological systems have a structure, meaning their parts are organized in space according to a precise scheme that relates them. The functioning of a biological system emerges as a result of this organization, which in turn evolves according to the course of the biological processes involving the system. Even better, we can consider form and function as two sides of the same coin, two interdependent aspects that help us understand different things about a system, and influence each other at different moments of a process. 
 

Biological systems present multiple organizational levels, and the relationship between form and function occurs in each of them: from the molecule, to the cell, to the group of cells, and so on. 
 

The diversity of structure underlies a diversity of function. In an organism composed of many cells, there are different cell types, that is, different functional specializations for different cells that have different roles in contributing to the life of the organism. The set of these different cells makes up the structural and functional complexity of an organism, in which different interdependent functionalities result in an overall functionality. This functional diversity corresponds to an equally rich structural diversity. 
 

Each cell is specialized to perform a series of functions. Comparing two cells that perform different functions, their morphological difference is often also evident, that is, relating to their shape or structure. For example, a squamous epithelial cell has a flat shape, which allows the formation of layers of cells with a covering and protective function, such as that of the epithelium, that is, the layer of skin in contact with the outside.

Squamous epithelium cell - from Wikimedia commons)

A neuronal cell, on the other hand, is articulated into different types of branched structures to allow the exchange of signals with other nerve cells.

(Diagram of a complete neuronal cell - from Wikimedia commons) 
 

This great diversity of cell types within an organism has emerged during evolution, creating an ever wider and more effective variety of functions in responding to the challenges posed by the environment to the organism. 
 

In this process of mutation and selection, selective pressure is exerted on the function, not directly on the structure. 
 

Those who prevail in evolutionary competition are in fact the organisms that are best suited to the environment in which they find themselves. These organisms are structured, generation after generation, supporting the functional capacity that gives them an evolutionary advantage. 
 

Since different environments sometimes present similar evolutionary challenges, the same functionality is positively selected several times, over the course of evolutionary history, because it effectively responds to such challenges. This means that structural solutions that nature brings forth have very different starting points, but become similar during evolution. 
 

This is called convergent evolution, that is, the process (defined in evolutionary biology) by which organisms that are not closely related evolve, independently, similar traits. This results from the necessity, which they share, to adapt to similar environments or evolutionary niches. 
 

An example of this phenomenon is the similarity between the structures of pterodactyls, insects, birds, and bats for flying. All perform the same function of "wings," and to do so they are articulated into very similar structures, but they do not derive evolutionarily from one another: they evolved independently.

(© Sinauer Associates 2001 )

If observed from the perspective of the relationship between structure and function, biology can become a catalog of optimal solutions to inspire the resolution of engineering and design problems. Biomimetics refers to the transfer of biological processes from the natural world to the artificial one, making the imitation of solutions found by evolution a true design process. 
 

Nature is a great inspiration for finding new technological and design solutions to formalize and implement. However, it is essential, in order to admire biological systems in their complexity, to understand that they do not derive from the implementation of a rational design aimed at solving a problem. Rather, their functional, and therefore structural, complexity emerges through processes of random mutation and natural selection, without guidance. These processes are in fact local and distributed, that is, they do not belong to any intelligence that coordinates them, but happen according to the specific situations in which organisms find themselves at a given moment, and according to what has happened previously. This makes the beautiful—and functional—forms of nature even more interesting. 
 

Roberta Bardini is a researcher in the field of computational and systems biology. She currently works at the Sysbio Group, Politecnico di Torino, where she obtained her PhD. She deals with the development of multicellular organisms, and their valorization in the entrepreneurial field.

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