Philosophy of Biology and Cognitive Sciences

Organisational accounts of biological functions typically identify a trait’s function as the contribution it makes towards the maintenance of an organisation, which in turn maintains the trait. On this account, it is the unique ‘quasiholistic’, ‘circular’ or ‘looping’ causal structure of biological systems which grounds function descriptions. Specifically, it is claimed that this causal structure naturalises both the normative and teleological aspects of functional explanations. Living systems differ from non-living systems because they can only maintain their organisation by interacting with their surroundings to reproduce their parts. This, it is argued, can account for the normative status of functional traits; traits ought to do that activity on which the system they are part of, and therefore their own existence, depends. In addition, it explains the teleological aspect of function ascriptions: because the activity of a trait contributes to the maintenance of a system on which its own existence depends, it contributes to the maintenance of its own existence. It therefore seems reasonable to explain the activity of a trait by appealing to the effects of its activity; the hallmark of a teleological explanation.

 

Colloquially referred to as programmed cell death; apoptosis is the process by which both healthy and unhealthy cells actively participate in their own destruction. There are many different types of apoptotic triggers and apoptotic pathways; in common, all these pathways are distinguished by the extremely regular and organised process by which a cell is dismantled as well as the active role which the cell takes in its own destruction. From the get-go, apoptosis seems to present a clear problem-case for organisational accounts which by and large, tie functional status to self- maintenance.

There are, however, many different formulations of organisational accounts. Some emphasize the contribution of the trait to its own self-reproduction (e.g. Schlosser 1998), others emphasize the trait’s contribution to the self-maintenance of the system it is part of (e.g. Christensen & Bickhard 2002, Sabirido et al. 2011, Mossio & Moreno 2015). In the quest to ascribe apoptosis an organisational function, the first approach, in which a function is identified as the contribution a trait makes to its own self-reproduction seems to me an obvious non-starter. Self-annihilation is quite plainly a very different end than self-reproduction. Given this, apoptosis gives us a good reason to favour those accounts which emphasize system self-maintenance rather than trait self-reproduction.

That said, system self-maintenance accounts still face a number of difficulties if they are to ascribe apoptosis a function. For a start, although system self-maintenance accounts emphasize the contribution to the trait makes to a system; in both formal and informal formulations, the loop is still made which connects the effect of a trait to the maintenance of the trait (via the system). As Sabirido et al. puts it: ‘within a self-maintaining organization, in particular, functions are interpreted as specific causal effects of a part or trait, which contribute to the maintenance of the organization and, consequently, of the part itself’ (2011, 584, emphasis added). It is this causal loop which explains the teleological nature of functional traits. Discussing the classic example of the heart which pumps blood, Mossio & Moreno assert, ‘the heart is there because it pumps blood (otherwise the organism, and thus the heart, would disappear), and pumping blood is a consequence of the heart’s being there’ (2015: 73). It is simply not possible to apply this kind of story to apoptotic traits such as caspases. To the contrary, it is the function of caspases to make the system it is part of disappear and, as a consequence, to make themselves disappear too.

 

I will argue that the troubles organizational accounts face with regard to apoptotic traits stems from two base causes. First, organisational accounts of functions assume that existence has some kind of intrinsic value for living systems: this has been variously expressed as both intrinsic relevance and intrinsic goodness. This is what grounds the normative aspect of function ascriptions. Second, and related, organisational accounts seem to posit that living systems have trait self-reproduction or system self-maintenance as their goal. This is the end towards which biological functions operate. Apoptotic pathways challenge both these assumptions.

 

In addition, the case of apoptosis points to an area where organisational accounts need further elaboration and clarification. The occurrence of apoptosis in multicellular systems highlight the nested nature of biological organisation: the fact that self-maintaining systems occur within self-maintaining systems within self-maintaining systems. Common sense tells us that apoptotic traits (e.g. caspases) contribute to the maintenance of the wider multicellular system of which the individual cell is a part. But if we tie the function of apoptotic pathways to the maintenance of the wider multicellular system, what is to stop us from tying the function of all intra-cellular pathways to the maintenance of the multicellular system? Such a move risks dissolving the boundaries which exist between an individual cell and the multicellular system of which it is a part. It also seems to undermine the entire concept of self-maintenance.

 

In this respect, the problem of apoptotic pathways is very similar to the problem of reproductive traits; the system which the trait contributes to maintaining seems to be different to the system of which it is a component. But the case is also markedly different. In the case of reproductive traits, the organisation which the trait contributes to maintaining is a continuation of the organisation of which it is a part. That is to say, the two organisations have the same fundamental structure and there is material continuity between the two instances of the same organisation. None of this is apparent in the case of apoptotic traits. In order to deal with the problem of apoptosis, organisational accounts will first have to clarify the relations between levels of autonomy.