* Joseph Madison is a microbiologist at Xavier University of Louisiana. He is interested in host-microbiota interactions, microbial evolutionary ecology, and the role of microbes in macroevolutionary processes. (He is also one of those wonderful scientists who enjoys hanging around with philosophers.) He writes…

The grandeur and enigma of macroevolutionary phenomena have long attracted scientific interest. Some look at fossils and see unusual morphologies tinkered together by evolutionary processes in the Cambrian. Others prefer to examine extant forms with nebulous histories such as the bird wing or turtle shell. Do wings and shells represent sudden evolutionary novelties or the gradual selection of adaptive traits? And consider the news-worthy plight of amphibians. Are ongoing amphibian extinction rates part of a larger shift in cladal trends, or maybe even a “sixth mass extinction”? These phenomena provide a great sparring ground for new ideas and concepts.

Ongoing research on these topics is bound to be interesting, and is perhaps even capable of revolutionizing our understanding of ecological and evolutionary processes. Still, I find something missing in this otherwise interesting work. There is a performer that is often neglected in the heat of debates about metazoan evolution. The player was here first and permeates the entire history of life: the microbe.

They’re everywhere and they’re important, even if they’re more often than not out-of-mind

It is understandable that most paleontologists do not look at fossils and ruminate about the bacteria that were cohabiting and shaping the life of those species. Most people do not look at living animals and ask how bacteria are shaping their lives and evolution. Yet while microbes are easy to overlook, they are of the utmost importance for understanding evolution. They are also becoming easier to study. A recent rise in methods for inventorying microbes has made it clear that they are associated with (or a part of) all animals. Much contemporary research has focused on the role of cohabiting microbes in the health and wellness of humans, our pets, and our food. After all, probiotics are all the rage in the (often over-promising) biotech startup bonanza. This emphasis on the now, however, misses the importance of bacteria in another domain of interest: that of the deep past. Here I will discuss the present state of this research program, its history, and its prospects.

Background

Scientific theorizing about the role of microbes in macroevolution is not new. Indeed, the concept of symbiosis has a rich history in evolutionary thought, first centered on work by Russian “symbiologists” in the late 19th and early 20th centuries (Ryan, 2002; Margulis, 1991). These ideas have had varying levels of traction in evolutionary biology ever since; but what no one can dispute is that they have generated some stunning insights along the way.

The idea that microbes may be causal agents of evolutionary novelty gained a foothold in contemporary biology with the endosymbiont theory for organelles (Sagan [Margulis], 1967). This is the idea, put forward by Lynn Margulis, that symbiotic bacteria were the originators of mitochondria and chloroplasts in eukaryotic cells. Accounting for an evolutionary novelty or major transition using serial microbial symbioses remains an important mode of thinking, but it is not the only game in town. When it comes to microbes, the modes of their evolutionary agency may be as diverse as their morphologies and metabolisms.

Visual representation of the endosymbiont theory of the origin of mitochondria and chloroplasts

Recent technological advances in high-throughput nucleic acid sequencing have allowed considerably easier study of microbial community dynamics in metazoan evolution. These community dynamics are increasingly studied using experiments aimed at functional understanding, together with animal and other metazoan gene expression and developmental studies. It is now becoming clear that microbial communities influence the fitness and developmental trajectories of their hosts (Gilbert et al., 2015). Sometimes they may even be “inherited” with their hosts in a vertically transmitted manner. All these considerations have forced evolutionary explanations to include microbes as part of the Darwinian adaptive-selective landscape in their metazoan hosts (Roughgarden, 2023). How these processes occur ecologically and as part of “microevolutionary” processes is currently a robust research program generating many interesting insights.

What is lacking in the contemporary host-microbe research program, however, is a concerted effort to both observe and explain the role of microbes as causal agents of macroevolutionary processes. There are, I think, good reasons for this neglect. One is the feasibility of study. Usually, macroevolution is studied (1) by paleontologists using fossils or (2) by phylogeneticists using genetic (and sometimes phenotypic) information from living organisms. A combination of these approach can also be attempted, such as when phylogenetic ancestral state reconstructions use fossils to calibrate reconstructions (Hsiang et al., 2015). But how can this be done with microbes?

Microbes do not fossilize well. There are fossil microbial colonies such as stromatolites, and we have some ability to visualize fossilized microbial structures with electron microscopy. But microbes such as bacteria do not have the diverse hard-parts that allow detailed morphological descriptions capable of giving insight into function and ecology. The situation becomes even more dire when trying to ascertain host-associated microbes in metazoan fossils. For example, there are fossils of plants that have clear fungal associations (Harper & Krings, 2021). But whether an identified fungal association was a symbiont or pathogen of the plant is very difficult if not impossible to ascertain. Other microbes such as bacteria also have important effects on metazoan development and gene expression, and these effects can occur with very small amounts of bacteria. Again, this is something that is very difficult to see and interpret in fossils. The greatest insight that this so-called palaeomicrobiology might give us is perhaps where certain bacteria, fungi, and other microbes are found on body parts. But even this is a wish that may never be granted with clarity.

A stromatolite from the Biwabik Iron Formation (ca. 2 bya) in northern Minnesota

A more prosaic reason that there isn’t much interest in pursuing a research program into the role of microbes in metazoan macroevolutionary phenomena is the absence of economic incentives. This is clear when comparing this to the more robust research program that examines the role of microbes in human ecology and health. This work has financial drivers associated with human (Tegegne & Kebede, 2022) and animal (Jin Song et al., 2019) health and conservation. Understanding the role of the human gut microbiome in emergent antibiotic resistance is considerably more fundable through biomedical research grants. There also exist a slew of biotech companies focused on “probiotics” that are aiming to use specific microbes for modulation of the human-microbial community to generate specific health outcomes. There are not, however, biotech companies aiming to explain the role microbes might have played in the origin of animal body plans. In a capitalist society, funding opportunities drive research interests.

However, when looking at fossils or thinking about deep time, we know that microbes must have been there. Whether it is dinosaurs in the Jurassic or Dickinsonia in the Ediacaran, we know all metazoans lived and evolved in a microbial world. Dinosaurs almost certainly had diverse gut microbes, and Dickinsonia likely had microbes associated with its “skin,” perhaps even using bacteria as a primary food source while moving (Ivantsov & Zakrevskaya, 2022).

Two specimens of *Dickinsonia,* an enigmatic Ediacaran organism (probably a metazoan)

But if it is right to say that microbes were present (and probably involved) in the most spectacular stories of macroevolution, we mustn’t limit ourselves to speculation. There are testable hypotheses about the role of these humble organisms in large-scale evolution. The pursuit of this research program is now more possible than ever. To give context, let’s look at two examples of this new and exciting perspective.

Amphibian Extinctions

Amphibian extinctions represent a current and at times contentious example of species extinction attributed to a variety of factors. These include habitat loss, climate change, over-exploitation, and disease (Luedtke et al., 2023). They have garnered interest from both biologists and the general public for their use as a heuristic in biodiversity hypotheses, data interpretation, and related conservation efforts (e.g. Dunham, 2023; Bishop et al. 2012; Zippel & Mendelson 2008). Indeed, this extinction heuristic dominates research narratives in popular press on extinction, and research in evolutionary biology, ecology, and amphibian disease ecology in particular.

Beyond the fascination with amphibians as both a flag-bearer for conservation issues and harbinger of mass extinction, there are other highly interesting questions that can and indeed have been asked of this system. The most important for the purposes of this discussion has to do with microbes. Specifically, what is the role of microbes, if any, in channeling differential extinctions between different groups of amphibians resulting in new cladal trends?

Fortunately, it is possible to probe significant questions regarding the role of host-associated microbes in amphibian assemblages exhibiting differential species extinction. This role has been the subject of intense research focused on the emerging amphibian fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). These pathogenic fungi are the causative agent of amphibian chytridiomycosis, a disease that affects amphibian skin and related physiological capacities often resulting in mortality through cardiac arrest (Voyles et al., 2009). They have been implicated in worldwide amphibian declines and extinction over the last three to four decades, especially in tropical regions. In studying the effects of this disease on both populations and species, an odd phenomena was observed. After controlling for factors like climate and bioregion, some species were subject to extreme population declines or even extinctions, while others were essentially unaffected.

In the time since this observation was first recorded, a variety of possible explanations have been proposed including differential immune responses, behavioral variation, and habitat preference. All of these factors may play a role in conferring protection from pathogens, but the additional factor of microbes has also been examined and found to be incredibly important. It seems that the composition of taxon-specific microbial communities plays a role in determining whether a taxon will be unaffected, face declines (and then other stressors like habitat loss), or go extinct. Differences in microbial taxonomy, function, and “core” microbial communities are now subjects of ongoing study.

Visual summary of the frog skin microbiota

Based on these studies, it is now known that amphibian skin microbial communities vary in diversity and function both within and between amphibian assemblages over spatiotemporal gradients. This variation in microbial diversity and function includes differences in anti-fungal capacity affecting susceptibility to Bd and, ultimately, species survival (Mueller et al., 2020; Madison et al., 2017). Specific microbes and microbial communities in particular have been shown to be protective against Bd infection and subsequent chytridiomycosis (Rebollar et al., 2020; Kueneman et al., 2016). Today, scientists are increasingly interested in the complex ways in which microbial communities are assembled through host factors and immune responses, and also the extent to which microbial communities or their assembly capacity are “inherited” between amphibian generations. Bd emergence, coupled with this complex variation in amphibian-associated microbial diversity across species, is therefore a likely exemplar of a species-selection model of macroevolutionary change that will alter future cladal patterns.

How broad these latter cladal changes will be is at present unknown and perhaps unknowable. Certainly genus-level changes have already occurred, with some amphibian genera in decline (or wose) and others poised to diversify. These cladal changes may also extend as differential elimination between the three extant orders of Amphibia: the Caudata (salamanders), Anura (frogs), and Gymnophiona (caecilians). While many species of Caudata and Anura are to some extent impacted by Bd and Bsal, the disease dynamics are much less understood with Gymnophiona. The protective role of microbes in ameliorating Bd- or Bsal-induced chytridiomycosis in Gymniophiona thus needs further investigation, and will perhaps allow us to better understand to what extent cladal changes are occurring at higher taxonomic levels. Nevertheless, the possibility of observing ongoing clade-level selection that is based, at least in-part, on differing microbial communities would be an outstanding development for the microbe-host macroevolutionary research program.

Microbes and the Emergence of a new Evolutionary body plan

Evolutionary novelties are an area of intense focus in contemporary debates about evolutionary theory. Especially contentious is the explanation of evolutionary novelties and subsequent innovations (Erwin, 2021) using concepts of channeling and constraint rather than standard Darwinian selection (Novick, 2023; Gould, 2002). Many of these constraint-based frameworks invoke experimental and comparative findings from evolutionary developmental biology (evo-devo). One important example of an evolutionary novelty explained through an evo-devo mechanism of emergence is the turtle shell. But this may well be an incomplete picture, since in most presentations there is no room in the story for the causal agency of microbes.

All Testudines exhibit a unique body plan featuring inverted rib-scapula development resulting in a shell (Gilbert et al., 2001). Within this body plan there are numerous possibilities for adaptive changes to the shell. Of course, extant turtles had to get to this point from a non-inverted state, with the state transition often considered an exemplar of macroevolutionary novelty (Wagner, 2014). The alteration of the underlying rib structure, with the development of the scapula inside the rib cage resulting in the observed morphological divergence from the typical amniote bilaterian body plan, requires a different developmental-genetic network. Accounting for this change has proven both challenging and controversial, with explanations ranging from Wagner’s character identity networks (ChINs, id. 2014) to recent work invoking nested constraints as an evolutionary mechanism that may be reconcilable with elements of Haeckel’s recapitulation theory (Kuratani et al., 2022; Uesaka et al., 2022). This channeling of carapace formation in turtles is also dependent on genetic networks and their associated mechanisms of regulation and expression.*

[* Indeed, there is some evidence that it is changes in carapacial ridge (CR) gene expression patterns in Sp5, CRABP-I, APCDD1, and LEF-1 — and not gene duplication or co-option — that explains the emergence of the novel feature (Kuraku et al., 2005). This same study also suggested that a shift in trans-acting transcriptional regulators in the TCF/LEF-1 family might be involved in the alteration of CR expression patterns.]

More speculatively, there might also be additional random shifts in expression patterns, resulting in a hypothetical alteration in developmental patterning of multiple ribs. This would be accomplished stepwise over a period of time and would result in serial homologies of rib inversion. A non-stepwise but rather simultaneous pulse resulting in multiple development shifts is also possible and could be described as a “character swarm” (see Wagner, 2015; p. 326 with the canonical case as emergence of feathers in avian evolution). These character swarms can be regarded as an extreme form of channeling where multiple copies of the same part are favored by developmental mechanisms (see Gould 2002). (It is worth noting that these hypothetical scenarios might be tested experimentally, through manipulation of expression patterns or related transcriptional (cis/trans) regulation.)

These developmental-genetic findings have yet to receive fossil corroboration, although the dearth of fossils was somewhat improved by the discovery of Odontochelys (Li et al., 2008), now considered an important transitional fossil in the Testudinata. Odontochelys lacks a carapace, but has a plastron and unusual ribs that follow an intermediary developmental state in their halted growth towards the organism’s midline. This arrangement suggested a “folding theory,” based on the arrest of axial growth of the ribs as seen in Odontochelys (Kuratani et al., 2011). Other accounts push back against the folding theory, however, to include mutational stories in underlying genetic networks. Regardless of the correct explanation, it is clear that further lines of inquiry would be useful in providing an origin story for the turtle carapace.

*Odontochelys* as it might have appeared in life

One line of inquiry that has yet to be seriously pursued concerns the role of microbes in altering gene expression patterns responsible for the rib-scapula inversion. This alternation would have been indirect (so, for example, alteration of microbial produced short-chain fatty acids has been linked to fibroblast growth factor expression and localization, which could alter CR gene expression). For such a process to work, there would have to have been a population-level event wherein gene expression patterns were altered and eventually captured through genetic assimilation. Such an event could have been gradual or sudden, depending on a variety of environmental and host factors.

While the existence of such an event is speculative, it can nonetheless be investigated. If such an event occurred, there would be a clear order in which it happened. First, there would be functional alterations in the relevant microbial communities (e.g. short-chain fatty acid production). Then there would be alterations in gene expression patterns that are subsequently “captured” at the population level. Then, rib-scapula inversion would take place. Finally, the turtle shell proper would emerge and become subject to adaptive selective processes. Because of the significant time involved in this transformation, causal ordering might be used to examine the possibility of microbial involvement in the emergence of the novel feature.

How feasible is this? Microbes are surprisingly well-studied in turtles, largely due to the conservation focus on turtle health. Because of this, we know that turtles harbor a rather conserved core microbial community across species, which is best represented by the Firmicutes and their associated group functions (Hoffbeck et al., 2023). If a large enough sample of extant turtle species is constructed including genetic data from both the turtle and the core microbial community (leaving aside how microbes might be intergenerationally transmitted or recruited), then an ancestral-state reconstruction could be used to predict past microbial community states based on species relatedness (and also non-turtle outgroups). Assuming all extant turtle species do indeed share a conserved core microbiota, outgroups with differing core microbes might be used to predict when microbial communities changed in turtles. If it turns out there was a major change prior to the appearance of the turtle shell in the fossil record, then a microbial role in turtle shell emergence is possible.*

[* Constructing phylosymbioses such as these has been done in the past to some extent, but these are complicated statistical methods under active development (Lim & Bordenstein, 2020). If shown to be practicable, questioning the role of microbes in the historical emergence of evolutionary novelties goes beyond speculation or a limitation of study to present phenomena.]

While speculative, a microbial role in the emergence of the turtle shell fits well in the conceptual framework that regards constraints as causal explanation for certain developmental processes with macroevolutionary consequences (Müller, 2007). Indeed, evolutionary developmental biology is perhaps the most fecund area of biological research currently examining constraints and their effects in evolutionary processes. Yet even with increasing recognition that developmental processes play a role in the production of evolutionary novelty, much work remains. Might the causal agency of microbes emerge as another coordinating theme for evo-devo research?

Other Examples

An additional hypothesis that may implicate microbial agency in macroevolution is the emergence of endothermy. The origin(s) of endothermy is a controversial topic. Often it is given a straightforwardly selective explanation. But it is also possible that the temperature ranges observed in extant endotherms are the result of a constrained but changing possibility space for adaptive changes due to the tolerances of associated microbes. If the bacterial community associated with incipient endotherms had specific temperature requirements, this could have constrained the adaptive space of endothermic metazoan evolution. This space may also have shifted with changing constraints in associated microbial communities in a random manner. This is another example of a testable hypothesis that invokes microbial constraints on evolutionary novelties (emergence of endothermy) and their subsequent innovation (ecologically successful endothermic clades).

Visual representation of an early mammal. (Carl Buell)

A further hypothesis that can be investigated concerns the origin of new host-parasite interactions. In this scenario, the emergence of new parasites is dependent on some initial interaction with a host. For this to happen, there must be the potential for this interaction to take place in a way that elicits a host-protoparasite pairing. And this pairing may be dependent on the microbial communities of the host. Microbial communities may set constraints on the space for initial interaction by their physical presence (biofilms) or by changing the interaction interface (by setting a pH range). Where such constraints exist, certain protoparasites may be prevented from establish an initial pairing that would be necessary for subsequent host-parasite (co)evolution. Metazoan-associated microbial communities would therefore act as a “filter” that only certain proto-parasites would be able to pass through: a filter on possible evolutionary novelties. Like the previous example of endothermy, this is a hypothesis that can be investigated with experimental methods.

The Future of an Old Research Program

The future of the microbe-metazoan macroevolution research program is bright. Of course, I would say this, as I have more or less committed my research program to this area. But even setting aside my personal interests, I believe there are important structural reasons this work will continue, no doubt with ebbs and flows, as an admirable tradition within the biological sciences.

The first reason I have already described. New technologies like high-throughput molecular methods are allowing lines of empirical investigation that were infeasible only a few decades ago. This has allowed taxonomic and functional annotation of entire microbial communities in a timely and cost-efficient manner. Coupling microbial data with metazoan studies of evolution is now possible, using huge datasets. Likewise, increases in computation ability allow data-intensive investigations to be undertaken in a feasible way. One issue that this has brought to light is the complexity of the interactions involved. Assigning causality to microbes in evolutionary experiments will have to be done through careful experimental and statistical design. Expanding data-intensive phylogenetic studies for questions of macroevolutionary import is already starting and will likely expand as the scope of questions and necessary methods becomes better refined.

Another driver for this research program is new explanatory targets. As scientific investigations of new fossil-bearing strata expand, more fossils with strange histories will be discovered (for example the flurry of new transitional fossils of turtles discovered during the last two decades). Explanations will thus be needed to explain new or discordant phenomena as it relates to current explanations. This will likely entail alternative explanations and hypotheses, which may include a role for microbes.

A more abstract yet extremely important impetus for this research program is its ability to challenge traditional models of Darwinian adaptive-selective processes. Here, microbe-mediated macroevolutionary mechanisms present an intriguing challenge to many core assumptions of evolutionary theory. These include evolutionary gradualism and the intrinsic (developmental genetic) basis for evolutionary novelties and related major transitions. Related philosophical problems have received some attention in recent decades; see the flurry of publications about metazoan-microbe units of selection: holobionts, metaorganisms, and superorganisms (O'Malley, 2014). Also garnering attention is the feasibility of inherited or vertically transmitted microbes in metazoans, the possibility of a core microbiome that is predictably assembled from the environment and is metazoan-dependent, and the status of microbial communities as “replicators” and/or “interactors” (Inkpen & Doolittle, 2022; Madison, 2023). However, absent in many of these discussions is a consideration of the role of microbial agency in macroevolution. This includes the role of microbes in determining new body plans in the history of biological evolution, in the advent of novel features, and even in channeling broad evolutionary trajectories over deep time. These are areas of great promise for future work on microbe-mediated macroevolutionary mechanisms of metazoan change.

References

Casadevall, A. (2005). Fungal virulence, vertebrate endothermy, and dinosaur extinction: is there a connection?. Fungal Genetics and Biology, 42(2), 98-106.

Dunham, W. (2023, October 4). Amphibian crisis: 41% of species deemed threatened with extinction. Reuters. https://www.reuters.com/business/environment/amphibian-crisis-41-species-deemed-threatened-with-extinction-2023-10-04/

Erwin DH (2021) A conceptual framework of evolutionary novelty and innovation. Biol Rev 96(1):1—15

Gilbert, S. F., Loredo, G. A., Brukman, A., & Burke, A. C. (2001). Morphogenesis of the turtle shell: the development of a novel structure in tetrapod evolution. Evolution & development, 3(2), 47-58.

Gilbert, S. F., Bosch, T. C., & Ledón-Rettig, C. (2015). Eco-Evo-Devo: developmental symbiosis and developmental plasticity as evolutionary agents. Nature Reviews Genetics, 16(10), 611-622.

Gould, S. J. (2002). The structure of evolutionary theory. Harvard University Press.

Harper, C. J., & Krings, M. (2021). Fungi as parasites: a conspectus of the fossil record. In The Evolution and Fossil Record of Parasitism: Identification and Macroevolution of Parasites (pp. 69-108). Cham: Springer International Publishing.

Hoffbeck, C., Middleton, D. M., Nelson, N. J., & Taylor, M. W. (2023). 16S rRNA gene‐based meta‐analysis of the reptile gut microbiota reveals environmental effects, host influences and a limited core microbiota. Molecular Ecology, 32(22), 6044-6058.

Hsiang, A. Y., Field, D. J., Webster, T. H., Behlke, A. D., Davis, M. B., Racicot, R. A., & Gauthier, J. A. (2015). The origin of snakes: revealing the ecology, behavior, and evolutionary history of early snakes using genomics, phenomics, and the fossil record. BMC evolutionary biology, 15, 1-22.

Inkpen, S. A., & Doolittle, W. F. (2022). Can Microbial Communities Regenerate?: Uniting Ecology and Evolutionary Biology. University of Chicago Press.

Ivantsov, A., & Zakrevskaya, M. (2022). Dickinsonia: mobile and adhered. Geological Magazine, 159(7), 1118-1133.

Jablonski, D. (2022). Evolvability and macroevolution: overview and synthesis. Evolutionary Biology, 49(3), 265-291.

Jablonski, D. (2017). Approaches to macroevolution: 1. General concepts and origin of variation. Evolutionary Biology, 44(4), 427-450.

Jin Song, S., Woodhams, D. C., Martino, C., Allaband, C., Mu, A., Javorschi-Miller-Montgomery, S., ... & Knight, R. (2019). Engineering the microbiome for animal health and conservation. Experimental Biology and Medicine, 244(6), 494-504.

Kueneman, J. G., Woodhams, D. C., Harris, R., Archer, H. M., Knight, R., & McKenzie, V. J. (2016). Probiotic treatment restores protection against lethal fungal infection lost during amphibian captivity. Proceedings of the Royal Society B: Biological Sciences, 283(1839), 20161553.

Lim, S. J., & Bordenstein, S. R. (2020). An introduction to phylosymbiosis. Proceedings of the Royal Society B, 287(1922), 20192900.

Luedtke, J. A., Chanson, J., Neam, K., Hobin, L., Maciel, A. O., Catenazzi, A., ... & Stuart, S. N. (2023). Ongoing declines for the world’s amphibians in the face of emerging threats. Nature, 622(7982), 308-314.

Luo, G., Liu, D., & Yang, H. (2024). Microbes in mass extinction: an accomplice or a savior?. National Science Review, 11(1), nwad291.

Madison, J. D. (2023). Microbial communities as interactors: S. Andrew Inkpen and W. Ford Doolittle: Can microbial communities regenerate? Uniting ecology and evolutionary biology. Chicago: University of Chicago Press, 2022, 182 pp, $20.00 PB.

Madison, J. D., Berg, E. A., Abarca, J. G., Whitfield, S. M., Gorbatenko, O., Pinto, A., & Kerby, J. L. (2017). Characterization of Batrachochytrium dendrobatidis inhibiting bacteria from amphibian populations in Costa Rica. Frontiers in microbiology, 8, 290.

Margulis, L. (1991). Symbiogenesis and symbionticism. Symbiosis as a source of evolutionary innovation, 1-14.

Mueller, E. A., Wisnoski, N. I., Peralta, A. L., & Lennon, J. T. (2020). Microbial rescue effects: how microbiomes can save hosts from extinction. Functional Ecology, 34(10), 2055-2064.

Nee, S. (2006). Birth-death models in macroevolution. Annu. Rev. Ecol. Evol. Syst., 37(1), 1-17.

Novick, R. (2023). Structure and Function. Cambridge University Press.

O'Malley, M. (2014). Philosophy of microbiology. Cambridge University Press.

Rebollar, E. A., Martínez-Ugalde, E., & Orta, A. H. (2020). The amphibian skin microbiome and its protective role against chytridiomycosis. Herpetologica, 76(2), 167-177.

Roughgarden, J. (2023). Holobiont evolution: Population theory for the hologenome. The American Naturalist, 201(6), 763-778.

Ryan, F. (2002). Darwin's blind spot: evolution beyond natural selection. Houghton Mifflin Harcourt.

Sagan, L. (1967). On the origin of mitosing cells. Journal of theoretical biology, 14(3), 225-IN6.

Tegegne, B. A., & Kebede, B. (2022). Probiotics, their prophylactic and therapeutic applications in human health development: A review of the literature. Heliyon, 8(6).

Voyles, J., Young, S., Berger, L., Campbell, C., Voyles, W. F., Dinudom, A., ... & Speare, R. (2009). Pathogenesis of chytridiomycosis, a cause of catastrophic amphibian declines. Science, 326(5952), 582-585.

Wagner, G. P. (2014). Homology, genes, and evolutionary innovation. Princeton University Press.