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Function as "Causal Role"

Previously, we discussed the selected-effects view, which considered selective history, or the reasons for selection in the past, as what constitutes the function in biological systems like organisms. The causal-role view steps away from this focus on history, looking for an understanding of function based instead on what is happening in the present.

There are many different causal-role views, but the details that distinguish them don’t matter for now, so we will talk about these views in a general way to get started. The causal-role view of function, also known as the functional-role view, is another perspective that aims to explain the concept of function, particularly in the context of biology. According to this view, the function of a biological trait or organ is determined by its causal role within a larger system, such as the organism it belongs to.

For example, the role of a kidney is the effects it has on the larger body, which is to filter blood. Let's consider the function of the kidney in the human body according to the causal-role view. The kidney's function is to filter blood by removing waste products. From a causal-role perspective, the kidney's function is its causal role in maintaining blood filtration, and ensuring the ongoing functioning of other organs and systems that rely on blood filtration.

The causal-role view focuses on the contribution of a particular trait or organ to the overall life processes and behavior of an organism. Rather than appealing to the historical selection process as in the selected-effects view, this perspective emphasizes the current and ongoing causal relationships and interactions between different parts of an organism or system. This provides a unique way to understand how functions can change. It recognizes that functions are not fixed or predetermined but arise from the dynamic interactions and relationships between different components of a system. If what’s happening within the organism changes, or its relationship to its environment changes, the functions of its parts can change too.

We may think this is a theoretical point with no consequences for how we see evolution in the natural world. But we would be wrong in thinking this way! For example, cases of “exaptation” or evolutionary repurposing might be better explained by the causal-role model than the selected-effects model. Exaptation occurs when an organ or trait evolved to serve one function, but is later re-purposed to serve a completely different one. An example is feathers, which likely evolved in birds to keep them warm. Many scientists think that the function of flight came much later.

The selected-effects model would need to appeal to the new selective pressure of flight to say that feathers have the function of helping birds fly. The causal-role view can explain the new function of flight without having to wait for selection to come along. In effect, the causal-role model has a way to make sense of new functions before selection occurs, whereas the selected-effects view seemingly does not.

Many philosophers think that the selected-effects view and the causal-role view are entirely compatible with each other. This would mean that explaining why a system has a function on the selected-effects account is distinct from explaining what the functions of the system are on the causal-role account. We can use one to explain historically why the organism evolved to have the features it now has, and the other to understand how it now works.

The causal-role view can also explain goal-oriented behavior. The causal role that the kidneys play in aiding in proper blood filtration contributes to the overall goals of the organism. This story also works for any other organ systems, and even for the motivational systems which set our psychological goals! It looks at how a particular trait or organ contributes to the overall functional organization of an organism, enabling it to perform certain tasks or fulfill specific purposes.

While the selected-effects view focuses on the historical selection process as the basis for determining function, the causal-role view emphasizes the present causal contributions and roles within an organism's functioning. By considering the functional organization and causal relationships within a system, this view provides an alternative perspective for understanding the notion of function in biology.

This gives us a sense of the unique benefits the causal-role view offers. Next we will discuss the problems associated with the view. In later posts we will discuss the persistence view of function (developed for thinking about functions in ecology), and examine the unique tools it offers for discussing function and the problems for the view.

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