Biology subjects

Bourrat, P.

Publications and source records attributed to Bourrat, P..

6 recordsLinked to original sources

bioRxiv2023

Stability of Ecologically Scaffolded Traits During Evolutionary Transitions in Individuality

Evolutionary transitions in individuality (ETIs), such as the emergence of multicellularity, are events in the history of life during which entities at one level of organisation (particles) form collective-level entities that subsequently become individuals in their own right. Recent empirical and theoretical studies advocate the importance of an externally imposed meta-population structure or "ecological scaffold" for the emergence of new levels of individuality. Such a scaffold enables survival and reproduction at the collective level and thus the possibility of selection for beneficial traits on that level. However, a long-standing difficulty for the ecological scaffolding approach has been its inability to adequately explain how collective-level trait values that evolved under scaffolding conditions can be retained once these conditions are lifted. We call this difficulty "the problem of endogenisation." Here, we derive general conditions for the possibility of endogenisation. Key to endogenisation is the existence of a fitness valley that can be circumvented when scaffolding occurs. Using a stochastic meta-population model, we implement two versions of ecological scaffolding (one temporal and one spatial) and study subsequent evolutionary trajectories using the modelling techniques of adaptive dynamics. Our analysis yields several important results. The temporal model reveals that only collective traits based on particle-particle interactions can be endogenised when a temporary scaffold is applied to the entire population. The spatial model shows that, given the presence of an environmental gradient of externally imposed meta-population structure, ecological scaffolding can only occur in a limited "Goldilocks" zone of the environment. Further, if endogenisation conditions are also fulfilled, scaffolded collectives can colonise non-scaffolding areas of the environment. We conjecture that Goldilocks zones could act as initiators of ETIs and help explain the near ubiquity of collective-level individuality even if the conditions that promote it prove to be rare.

evolutionary biology↗

bioRxiv2022

The Effect of Bottleneck Size on Evolution in Nested Darwinian Populations

Previous work has shown how a minimal ecological structure consisting of patchily distributed resources and recurrent dispersal between patches can scaffold Darwinian properties onto collections of cells. When the timescale of dispersal is long compared with the time to consume resources, patches evolve such that their size increases, but at the expense of cells whose growth rate decreases within patches. This creates the conditions that initiate evolutionary transitions in individuality. A key assumption of this scaffolding is that a bottleneck is created during dispersal, so patches are founded by single cells. The bottleneck decreases competition within patches and hence creates a strong hereditary link at the level of patches. Here we construct a fully stochastic model of nested Darwinian populations and investigate how larger bottlenecks affect the evolutionary dynamics at both cell and collective levels. It is shown that, up to a point, larger bottlenecks simply slow the dynamics, but at some point, which depends on the parameters of the within-patch model, the direction of evolution toward the equilibrium is reversed. Introducing random bottleneck sizes with some positive probability of smaller sizes can counteract this, even if the probability of smaller bottlenecks is small.

evolutionary biology↗

bioRxiv2021

Beyond Fitness Decoupling: Tradeoff-breaking during Evolutionary Transitions in Individuality

Evolutionary transitions in individuality (ETIs) involve the formation of Darwinian collectives from Darwinian particles. The transition from cells to multicellular life is a prime example. During an ETI, collectives become units of selection in their own right. However, the underlying processes are poorly understood. One observation used to identify the completion of an ETI is an increase in collective-level performance accompanied by a decrease in particle-level performance, for example measured by growth rate. This seemingly counterintuitive dynamic has been referred to as "fitness decoupling" and has been used to interpret both models and experimental data. Using a mathematical approach, we show this concept to be problematic in that the fitness of particles and collectives can never decouple--calculations of particle and collective fitness performed over appropriate and equivalent time intervals are necessarily the same. By way of solution, we draw attention to the value of mechanistic approaches that emphasise traits, and tradeoffs among traits, as opposed to fitness. This trait-based approach is sufficient to capture dynamics that underpin evolutionary transitions. In addition, drawing upon both experimental and theoretical studies, we show that while early stages of transitions might often involve tradeoffs among particle traits, later--and critical--stages are likely to involve the rupture of such tradeoffs. Thus, tradeoff-breaking stands as a useful marker for ETIs.

evolutionary biology↗

bioRxiv2020

Division of labor and fitness decoupling in evolutionary transitions in individuality

Reproductive division of labor has been proposed to play a key role for evolutionary transitions in individuality (ETIs). This chapter provides a guide to a theoretical model that addresses the role of a tradeoff between life-history traits in selecting for a reproductive division of labor during the transition from unicellular to multicellular organisms. In particular, it focuses on the five key assumptions of the model, namely (1) fitness is viability times fecundity; (2) collective traits are linear functions of their cellular counterparts; (3) there is a tradeoff between cell viability and fecundity; (4) cell contribution to the collective is optimal; and (5) there is an initial reproductive cost in large collectives. Thereafter the chapter contrasts two interpretations of the model in the context of ETIs. Originally, the model was interpreted as showing that during the transition to multicellularity the fitness of the lower-level (the cells) is "transferred" to the higher level (the collective). Despite its apparent intuitiveness, fitness transfer may obscure actual mechanisms in metaphorical language. Thus, an alternative and more conservative interpretation of the model that focuses on cell traits and the evolutionary constraints that link them is advocated. In addition, it allows for pursuing subsequent questions, such as the evolution of development.

evolutionary biology↗

bioRxiv2019

Ecological scaffolding and the evolution of individuality: the transition from cells to multicellular life

Evolutionary transitions in individuality are central to the emergence of biological complexity. Recent experiments provide glimpses of processes underpinning the transition from single cells to multicellular life and draw attention to the critical role of ecology. Here we emphasise this ecological dimension and argue that its current absence from theoretical frameworks hampers development of general explanatory solutions. Using mechanistic mathematical models, we show how a minimal ecological structure comprised of patchily distributed resources and between patch dispersal can scaffold Darwinian-like properties on collectives of cells. This scaffolding causes cells to participate directly in the process of evolution by natural selection as if they were members of multicellular collectives, with collectives participating in a death-birth process arising from the interplay between the timing of dispersal events and the rate of resource utilisation by cells. When this timescale is sufficiently long and new collectives are founded by single cells, collectives experience conditions that favour evolution of a reproductive division of labour. Together our simple model makes explicit key events in the major evolutionary transition to multicellularity. It also makes predictions concerning the life history of certain pathogens and serves as an ecological recipe for experimental realisation of evolutionary transitions.

evolutionary biology↗

bioRxiv2017

Evolutionary Transitions in Heritability and Individuality

With a few exceptions, the literature on evolutionary transitions in individuality (ETIs) has mostly focused on the relationships between lower-level (particle-level) and higher-level (collective-level) selection, leaving aside the question of the relationship between particle-level and collective-level inheritance. Yet, without an account of this relationship, our hope to fully understand the evolutionary mechanisms underlying ETIs is impeded. To that effect, I present a highly idealized model to study the relationship between particle-level and collective-level heritability both when a collective-level trait is a linear function and when it is a non-linear function of a particle-level trait. I first show that when a collective trait is a linear function of a particle-level trait, collective-level heritability is a by-product of particle-level heritability. It is equal to particle-level heritability, whether the particles interact randomly or not to form collectives. Second, I show that one effect of population structure is the reduction in variance in offspring collective-level character for a given parental collective. I propose that this reduction in variance is one dimension of individuality. Third, I show that even in the simple case of a non-linear collective-level character, collective-level heritability is not only weak but also highly dependent on the frequency of the different types of particles in the global population. Finally, I show that population structure, because one of its effects is to reduce the variance in offspring collective-level character, allows not only for an increase in collective-level character but renders it less context dependent. This in turns permits a stable collective-level response to selection. The upshot is that population structure is a driver for ETIs. These results are particularly significant in that the relationship between population structure and collective-level heritability has, to my knowledge, not been previously explored in the context of ETIs.

evolutionary biology↗