Publications

Pre-Prints

2024

1. Eco-evolutionary dynamics for finite populations and the noise-induced reversal of selection

   Ananda Shikhara Bhat, and Vishwesha Guttal

   2024

   Being revised for The American Naturalist

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   Theoretical studies from diverse areas of population biology have shown that demographic stochasticity can substantially impact evolutionary dynamics in finite populations, including scenarios where traits that are disfavored by natural selection can nevertheless increase in frequency through the course of evolution. Historically, most general analytic frameworks have either restricted themselves to models with constant or deterministically varying total population size or have resorted to dynamically insufficient formulations. Here, we analytically describe the eco-evolutionary dynamics of finite populations from demographic first principles to investigate how noise-induced effects can alter the evolutionary fate of populations in which total population size may vary stochastically over time. Starting from a generic birth-death process describing a finite population of individuals with discrete traits, we derive a set of stochastic differential equations (SDEs) that recover well-known descriptions of evolutionary dynamics such as the replicator-mutator equation, the Price equation, and Fisher’s fundamental theorem in the infinite population limit. For finite populations, our SDEs reveal how stochasticity can induce a directional evolutionary force termed ‘noise-induced selection’ via two distinct mechanisms, one that operates over relatively faster (ecological) timescales and another that is only apparent over longer (evolutionary) timescales. Despite arising from the stochasticity of finite systems, the effects of noise-induced selection are predictable and may oppose natural selection. In some cases, noise-induced selection can even reverse the direction of evolution predicted by natural selection. By extending and generalizing some standard equations of population genetics, we thus describe how noise-induced selection appears alongside and interacts with the more well-understood forces of natural selection, neutral drift, and transmission effects (mutation/migration) to determine the eco-evolutionary dynamics of finite populations of non-constant size.

2. A stochastic field theory for the evolution of quantitative traits in finite populations

   Ananda Shikhara Bhat

   2024

   In review, Theoretical Population Biology

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   Infinitely many distinct trait values may arise in populations bearing quantitative traits, and modelling their population dynamics is thus a formidable task. While classical models assume fixed or infinite population size, stochasticity in finite, stochastically fluctuating populations can qualitatively affect evolutionary dynamics via noise-induced selection. In this paper, I present a stochastic field theory for the eco-evolutionary dynamics of finite populations bearing one-dimensional quantitative traits. I derive stochastic field equations that describe the evolution of population densities, trait frequencies, and the mean value of any trait in the population. These equations recover well-known results such as the replicator-mutator equation, Price equation, and gradient dynamics in the infinite population limit. For finite populations, the equations describe the intricate interplay between natural selection, noise-induced selection, eco-evolutionary feedback, and neutral genetic drift in determining evolutionary trajectories. My methods use ideas from statistical physics and present an alternative to some recently proposed measure-theoretic frameworks.

3. Spatial structure could explain the maintenance of alternative reproductive tactics in tree cricket males

   Mohammed Aamir Sadiq, Ananda Shikhara Bhat, Vishwesha Guttal, and Rohini Balakrishnan

   2024

   In review, Biology Open. I am a co-first author on this paper.

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   Trait polymorphisms are widespread in nature, and explaining their stable co-existence is a central problem in ecology and evolution. Alternative reproductive tactics, in which individuals of one or more sex exhibit discrete, discontinuous traits in response to reproductive competition, represent a special case of trait polymorphism in which the traits are often complex, behavioural, and dynamic. Thus, studying how alternative reproductive tactics are maintained may provide general insights into how complex trait polymorphisms are maintained in populations. We construct a spatially explicit individual-based model inspired from extensively collected empirical data to address the mechanisms behind the co-existence of three behavioural alternative reproductive tactics in males of a tree cricket (Oecanthus henryi). Our results show that the co-existence of these tactics over ecological time scales is facilitated by the spatial structure of the landscape they inhabit, which serves to equalize the otherwise unequal mating benefits of the three tactics. We also show that this co-existence is unlikely if spatial aspects of the system are not considered. Our findings highlight the importance of spatial dynamics in understanding ecological and evolutionary processes and underscore the power of integrative approaches that combine models inspired from empirical data.

2023

1. Mutualism Destabilizes Communities, but Competition Pays the Price

   Ananda Shikhara Bhat, Suryadeepto Nag, and Sutirth Dey

   2023

   In review, Oikos

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   A classic result in theoretical ecology states that an increase in the proportion of mutualistic interactions in unstructured ecological communities leads to a loss of stability to external perturbations. However, the fate and composition of the species that constitute an unstable ecology community following such perturbations remains relatively unexplored. In this paper, we use an individual-based model to study the population dynamics of unstructured communities following external perturbations to population numbers. We find that while mutualistic interactions do indeed destabilize communities, the entire community is rarely wiped out following a perturbation. Instead, only a subset of the ecological community is driven to extinction, and the species that go extinct are more likely to be those engaged in a greater number of competitive interactions. Thus, the resultant community formed after a perturbation has a higher proportion of mutualistic interactions than the original community. We show that this result can be explained by studying the dynamics of the species engaged in the highest number of competitive interactions: After an external perturbation, those species that compete with such a 'top competitor' are more likely to go extinct than expected by chance alone, whereas those that are engaged in mutualistic interactions with such a species are less likely to go extinct than expected by chance alone. Our results provide a potential explanation for the ubiquity of mutualistic interactions in nature despite the known negative effects of mutualism on community stability.

Journal Articles

2024

1. Studying the age of onset and detection of Chronic Myeloid Leukemia using a three-stage stochastic model

   Suryadeepto Nag, Ananda Shikhara Bhat, and Siddhartha P. Chakrabarty

   Journal of Biological Systems, 2024

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   Chronic Myeloid Leukemia (CML) is a biphasic malignant clonal disorder that progresses, first with a chronic phase, where the cells have enhanced proliferation only, and then to a blast phase, where the cells have the ability of self-renewal. It is well-recognized that the Philadelphia chromosome (which contains the BCR-ABL fusion gene) is the “hallmark of CML”. However, empirical studies have shown that the mere presence of BCR-ABL may not be a sufficient condition for the development of CML, and further modifications related to tumor suppressors may be necessary. Accordingly, we develop a three-mutation stochastic model of CML progression, with the three stages corresponding to the non-malignant cells with BCR-ABL presence, the malignant cells in the chronic phase and the malignant cells in the blast phase. We demonstrate that the model predictions agree with age incidence data from the United States. Finally, we develop a framework for the retrospective estimation of the time of onset of malignancy, from the time of detection of the cancer.

2023

1. Allopatric montane wren-babblers exhibit similar song notes but divergent vocal sequences

   Abhinava Jagan Madabhushi, Ananda Shikhara Bhat, and Anand Krishnan

   Behavioral Ecology and Sociobiology, Sep 2023

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   The songs of many passerine birds consist of notes temporally arranged into vocal sequences following syntactic structures and function both in courtship and territorial defense. Geographic barriers are important drivers of avian speciation and also influence the divergence of song. However, there is relatively little quantitative study of the relationship between geographic barriers and the syntactic structure of vocal sequences. Here, we investigate interspecific divergence in song notes and syntax within the allopatric montane Asian wren-babblers (Spelaeornis). Employing a quantitative analysis of note transitions and co-occurrence using song recordings from publicly accessible databases, we find that Spelaeornis appears to have undergone diversification in song syntax without divergence in note parameters. Broadly, we find three different syntactic structures across the eight species in the genus, each occurring in a different geographic region in Asia, with two species apparently exhibiting intermediate syntax. Species within the genus appear to possess similar song notes, but subgroups confined to different geographic regions (e.g., hills south of the Brahmaputra river) arrange these notes according to different syntactic rules to construct songs. Our computational framework to examine the signal structure and diversification across multiple scales of signal organization may help further our understanding of speciation, signal evolution, and, more broadly, fields such as linguistic diversification.

2022

1. Behavioural context shapes vocal sequences in two anuran species with different repertoire sizes

   Ananda Shikhara Bhat, Varun Aniruddha Sane, K.S. Seshadri, and Anand Krishnan

   Animal Behaviour, Sep 2022

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   Acoustic signals in animals serve to convey context-dependent information to receivers. Birds and mammals combine diverse sounds into complex sequences to communicate, but the role of temporal sequencing of signals remains understudied in other taxa. Anuran vocalizations are a prominent feature of their life history, and function in defence of territories and to attract mates. However, there are few data on whether anurans pattern their calls into sequences, and whether temporal sequences convey information about context. Here, we investigated the context-dependent vocal repertoire and the use of vocal sequences by two anuran species belonging to different lineages, comparing frogs vocalizing alone and in the presence of a territorial rival. Using a robust analytical framework, we present evidence that both species modify their vocal sequence structure according to context. Specifically, one species (with a smaller repertoire, from a more basal lineage) appends notes to generate more complex sequences, whereas the other (more recently diverged and with a larger repertoire) shifts to different note types, resulting in different sequences for different contexts. Thus, despite differences in repertoire size, both frog species are capable of adjusting the temporal sequence of vocalizations to communicate in different contexts. Vocal sequences and context-dependent ‘syntax’ may be more common in anurans than previously thought, and our methodology presents a paradigm to study the evolution and function of these complex vocal patterns.

2. A colony-level optimization model provides a potential mechanism for the evolution of novel castes in eusocial ant colonies

   Suryadeepto Nag, and Ananda Shikhara Bhat

   Heliyon, Sep 2022

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   Ant species often have multiple morphologically distinct ‘castes’ within a single colony. Given that most of these castes are involved in non-reproductive tasks, and since such individuals thus never reproduce, the question of how ant castes can evolve is a non-trivial one. Over the years, several models have been proposed in order to explain the evolution of castes in ant colonies. Here, we attempt to answer this question using an economics-based approach, developing an optimization model that implements adaptation and selection at the colony level. We argue that due to the nature of ant colonies, selection is shifted to the group level, and, due to this, individual ants are sheltered from negative selection. We show that our framework can explain the evolution of novel castes in ant colonies, and discuss the novelty of our model with regard to previous models that have been proposed. We also show that our model is consistent with several empirical observations of ant colonies.