PGS defines a Darwinian individual as a member of a Darwinian population: a collection of entities that undergo evolution by natural selection. A transition in individuality therefore involves the appearance a new kind of Darwinian population. These populations do not suddenly appear as unequivocal collections of individuals, but instead exist within a wide spectrum of possible Darwinian populations. Populations residing in the large ‘grey area’ of the spectrum are perhaps those populations currently undergoing an evolutionary transition – they are ‘mid-transition’ and have yet to complete Stage Two and solve the levels of selection problem.
The gradient of populations ranges from marginal, to minimal, to paradigm Darwinian populations. The very permissive ‘minimal concept’ of a Darwinian population features three elements that closely resemble Lewontin’s (1970) classic requirements for individuality (Section 1.1): (1) variation in individual character, (2) which affects reproductive output, and (3) which is heritable. The category of ‘paradigm Darwinian populations’ is the subset of the minimal concept area of the spectrum that can produce novel and complex organisms. If only paradigm Darwinian populations can evolve complexity (Stage Three), then according to the framework outlined in Section 1.4.2 it is only this subset of populations that can be said to have completed the transition to individuality (Stage Two). ‘Marginal Darwinian populations’ reside at the opposite end of the spectrum and outside the minimal concept boundary because they do not clearly satisfy all three minimal requirements. These marginal populations have a partially Darwinian character but cannot complete the evolutionary transition to individuality.
PGS introduces a range of parameters that can be applied to real biological populations in order to determine their position in the spectrum of Darwinian
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populations. The extent to which reproductive differences of members of a population depend on their intrinsic character is captured by the parameter ‘S’ (for ‘supervenience’). Intrinsic features of an individual are those that are not altered by other individuals in the population. By this definition, the transition rate in the P. fluorescens collectives is an intrinsic character (upon which collective fitness highly depends) because it is not affected by the existence or arrangement of the other collectives in the higher-level population. An evolving population exhibits high ‘C’
(‘continuity’) when small changes in an individual’s character lead to small changes in fitness. The C parameter is related to Sewall Wright’s famous ‘fitness landscape’ (S Wright, 1932). High values of C correspond to a smooth landscape in which similar properties are associated with similar fitnesses, as opposed to a rugged landscape that results from similar features associated with very different fitness values (low C). The heritability parameter ‘H’ distinguishes between reliable and unreliable inheritance of characters, while ‘V’ represents the abundance of variation present in a population. In the experiment outlined in Chapter 4, the mixed regimes exhibit much less variation between collectives compared to the non-mixed regimes. Inclusion of the V parameter in the spectrum of Darwinian populations makes it apparent that the higher-level populations in the mixed regimes are less ‘Darwinian’ than those in the non-mixed. Finally, a measure of the strength of competition, α, encapsulates the extent to which the reproductive success of one individual affects that of another. PGS suggests that paradigm Darwinian populations have high values of α. The P. fluorescens populations exhibit strong competitive interaction with respect to reproduction (α): the death of one collective has a direct causal effect on another’s reproduction. This is in contrast to the within-collective control regime imposed in Chapter 4 in which there was no
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replacement of failed collectives: α had a value of 0 and the population of collectives was ultimately extinguished.
PGS introduces a spatial tool that enables us to easily visualise the spectrum of possible Darwinian populations in a 3D ‘Darwinian Space’ displaying the relationships between these evolutionarily important parameters (Figure 5.3). High values of H, C
and S correspond to paradigm Darwinian populations. The list of parameters described above is incomplete. Population size, population structure, strength of selection, sex and niche construction are also important variables in an evolving population. However, in this discussion we are interested in discovering which of these factors make a tangible difference to the Darwinian nature of a population – what are the ‘difference-makers’? (Calcott, 2011). PGS suggests that by assuming that all the other parameters have high values on various unseen dimensions, we can focus on the difference-making role of a few key elements. For example, low S indicates that reproductive differences are uncoupled from differences in intrinsic character and selection is instead dependent on extrinsic factors. This is the case for populations of human cells: a character-fitness covariance barely exists at the lower (cell) level, whereas the higher-level populations (of humans themselves) - are paradigm Darwinian populations that exhibit strong character-fitness covariance (high S). If populations with low S also exhibit low C then selection is essentially random – the ultra-rugged landscape can lead to the fixation of characters that are not necessarily associated with high fitness – the population evolves by drift rather than by natural selection. In populations with low H, paradigmatic Darwinian evolution cannot occur because adaptations cannot accumulate before they disappear again due to mistaken transmission. The low H corner of Darwinian space is perhaps the most difficult to overcome during an evolutionary transition in individuality because of the collective reproduction problem introduced in Section 1.6.1. A certain
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amount of biological complexity is required before an inheritance system can be reasonably reliable, however paradoxically this complexity is composed of adaptations that must evolve by paradigmatic natural selection. The life cycle experiment described in Chapter 4 attempted to overcome this ‘error catastrophe’ by providing a mechanism for reproduction of nascent multicellular collectives.
Figure 5.3 A Darwinian space. From (Godfrey-Smith, 2009).