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Rarity has but one happy aspect—given enough time, it gets converted to fair frequency.
(Gould Reference Gould1989, 62)1. Introduction: Evolutionary Individuals Evolve
Although there are general approaches to biological individuality, philosophers often focus on what it takes to be an individual in selection. They cite various features as important markers of evolutionary individuals, such as reproduction, replication, collaboration, integration, immunology, policing, and demarcation mechanisms (Dupré and O’Malley Reference Dupré and O’Malley2009; Godfrey-Smith Reference Godfrey-Smith2009; Pradeu Reference Pradeu2010; Clarke Reference Clarke2013; Ereshefsky and Pedroso Reference Ereshefsky and Pedroso2015). Rather than push for any particular account, I explore how evolutionary individuality is informed by a view about the nature of evolution itself. Specifically, I argue that evolution’s contingency motivates a plurality of types of individuals in selection. Evolutionary contingency is a view about the nature of evolution. Before introducing this view, it is worth noting the following.
There is a call for evolutionary individuality concepts that are consistent with evolutionary change. For example, Haber (Reference Haber, Bouchard and Huneman2013, 213) recommends a notion of individuality that includes how individuals are expressed, maintained, and continue to evolve . Godfrey-Smith’s (Reference Godfrey-Smith2009) account of individuality, although dynamic, is confined to a particular set of parameters ranging from marginal to paradigm individuals. Ultimately, he argues that evolutionary individuals are reproductive units forming lineages that “evolve by natural selection” (Godfrey-Smith Reference Godfrey-Smith2015, 10124; emphasis added). And finally, Ereshefsky and Pedroso (Reference Ereshefsky and Pedroso2015) develop a pluralistic and open-ended account of individuality that captures the vast array of heterogeneous entities including multispecies consortia, such as biofilms. My goal is to provide a framework for thinking about the evolution of individuals in selection. I argue that the Evolutionary Contingency Thesis gives good reason to be individuality pluralists—there are many types of individuals in selection. The details of that framework are what I turn to next. In the final sections, I illustrate contingent individuality and show why pluralism makes sense in that context.
2. The Evolutionary Contingency Thesis
Characterizing the structure of evolution and its contingent nature is a significant research program, so I focus only on those features relevant to evolutionary individuality (Gould Reference Gould1989; Beatty Reference Rieppel, Wolters, Lennox and McLaughlin1995, Reference Lennox and McLaughlin2006; Desjardin Reference Desjardin2011; Turner Reference Turner2011). John Beatty (Reference Beatty1982) argued that evolution’s contingent nature makes sense of the non-lawlike status of biological generalizations as well as the plurality of mechanisms and theories biologists propose. I argue that something analogous occurs for evolutionary individuality.
On the one hand, outcomes of evolution causally depend on prior evolutionary states, events, processes, and so on. This is often referred to as the historical sense of contingency, contingent upon, or contingency-as-causal-dependence. One upshot is that evolutionary history constrains future possibilities—the direction of evolution is shaped by the trajectories of prior evolutionary pathways (Desjardin Reference Desjardin2011, 734). On the other hand, there is another aspect of contingency that tends to be forward-looking—the occurrence of a particular prior state is insufficient to bring about a future outcome (Gould Reference Gould1989, 278; Beatty Reference Lennox and McLaughlin2006, 339). For example, the survival of the primitive vertebrate Pikaia was potentially necessary, but alone provided no guarantee, for the existence of humans. This is because many other factors affected the sequence of evolutionary events that took place between Pikaia and Homo sapiens. Occurrences in the past sometimes do not guarantee an evolutionary outcome owing to a dependence on stochastic processes, such as mutation and mutational order.
According to Beatty (Reference Beatty1982), evolutionary contingency explains the inapplicability of traditional conceptions of scientific laws in the biological domain. Hempel (Reference Hempel1966) argued that universal and exceptionless laws are needed for the deduction involved in scientific explanation. Analogously, philosophers often cite necessary criteria for all evolutionary individuals. For example, a particular form of reproduction may be required of all individuals in selection. I will return to this later. Biological theories were considered problematic for citing laws of nature that do not meet those standards (Hempel and Oppenheim Reference Hempel and Oppenheim1948). Laws of nature are supposed to be universal statements with empirically determined truth-values. They are true necessarily because any exception would be considered physically impossible (Beatty Reference Beatty1982, 398). However, biological generalizations admit of exceptions. Beatty argued that biological generalizations describe evolutionary outcomes—the exceptions one sees to biological “laws” are symptomatic of the biological domain. He draws from the case of meiosis. As we shall see in section 3, evidence that the meiotic mechanism evolves is also relevant for individuality.
2.1. Mendelian Inheritance and Mutational Change: Nondisjunction as a Meiotic Mutation
Meiosis is a process of cellular division that leads to the formation of gametes or germ line cells in sexually reproducing organisms. Mendel’s Law of Segregation states that two alleles in a chromosome pair segregate into different gametes during gamete formation. But there is an exception to this “normal” meiosis: nondisjunction occurs when a homologous chromosome pair fails to separate into different gametes during meiosis. At first glance, “nondisjunction may seem to imply a process in which partner chromosomes that should have segregated failed to disjoin” (Janicke et al. Reference Janicke, Lasko, Oldenbourg and LaFountain2007, 1645). However, this usually occurs when the chromosomes disconnect prematurely and cannot properly orient themselves to separate poles. Nondisjunction or the failure of homologue separation in either the first or second stage of meiosis results in one gamete that receives two of the same type of chromosome while the other gamete receives no copy.
Meiotic mutants, such as nondisjunction, are not just exceptions to Mendel’s Law of Segregation. They help illustrate what it means to be a contingent outcome of evolution, which is important if different types of individuals are outcomes of evolution. Biological generalizations, such as Mendel’s laws, describe contingent outcomes. The agents of evolutionary change—directed and random mutation, natural and sexual selection, random drift, and so on—all have “rule-making and rule-breaking” capabilities (Beatty Reference Rieppel, Wolters, Lennox and McLaughlin1995, 218). Generalizations emerge as certain traits are selected for: “What the agents of evolution render general, they may later render rare” (222). Mutations, such as nondisjunction, demonstrate that Mendelian meiosis has a genetic basis and therefore undergoes evolutionary change. This is no mere philosophical conjecture. Marcy Uyenoyama (Reference Uyenoyama and Moens1987, 21) proposed that the process of genetic transmission itself evolves by natural selection. More recently, Van Leeuwen et al. (Reference Van Leeuwen, Vanholme, Pottleberge, Nieuwenhuyse, Nauen, Tirry and Denholm2008, 5980) investigated how inheritance in mitochondria is non-Mendelian because there is no recombination through meiosis. Non-Mendelian inheritance is not only theoretically possible; it has been identified. Abnormal meiotic behavior can be viewed as a mutant inheritance mechanism and evidence of evolutionary change.
2.2. Evolutionary Outcomes: A Transition to Contingent Individuality
Lewontin’s recipe for evolution by selection is cited as a constraint for evolutionary individuality (Godfrey-Smith Reference Godfrey-Smith2009). Individuals in selection will exhibit varying, heritable traits that make a difference to their fitness. Currently, there is dispute over how evolutionary individuals transmit traits (Godfrey-Smith Reference Godfrey-Smith2009; Ereshefsky and Pedroso Reference Ereshefsky and Pedroso2015). However, in Lewontin’s framework there is room for a plurality of types of evolutionary individuals. This is the case even if types of individuals in selection share common overarching features that distinguish them from other types of biological individuals.Footnote 1 I propose that as outcomes of evolution, different types of evolutionary individuals evolve to satisfy Lewontin’s constraints.
It is often the case that similar environmental problems are adaptively addressed in different ways—the wings of bats, birds, and insects that all yield the ability to fly. Similarly, there are different ways to satisfy the constraint of trait transmission. One may worry that this merely amounts to one type (rather than many types) of individuality realized in multiple ways. But as we shall see, we can distinguish between types of evolutionary individuality because different approaches will not classify biological entities in the same way. For example, one type of individuality might exclude biofilms, whereas another type of individuality includes them. The different types of individuality are distinguished by different trait transmission mechanisms—the types are distinguished by the causal roles in which ancestors and descendants relate to one another.
Inheritance mechanisms matter for individuals in selection because they facilitate a pattern of ancestor-descendant relationships. Analogous to how descendant organisms of a particular ancestor can be classified into different species, types of individuals will also branch off or merge, creating a new type of individual.Footnote 2 How and when an ‘individuality event’ occurs will of course be up for debate. Assuming that philosophers of biology are tracking mechanisms of individuality, there is groundwork for thinking about ways to pass on heritable material. One way to transmit traits is through reproduction. Godfrey-Smith (Reference Godfrey-Smith2009) focuses on the sort of reproduction that includes bottlenecks and germ-soma distinctions, whereas Dupré and O’Malley (Reference Dupré and O’Malley2009) think any mode of reproduction will do. Ereshefsky and Pedroso (Reference Ereshefsky and Pedroso2015) call for other trait transmission mechanisms besides narrow views of reproduction. Heritability as a condition for evolution by selection points to the ongoing parent-offspring relationships of individuals. Trait transmission to offspring can occur vertically, such as through reproduction, division, and replication. It can also occur horizontally in aggregative entities through lateral gene transfer, such as in biofilms (Ereshefsky and Pedroso Reference Ereshefsky and Pedroso2015). In each case, inheritance is caused by different trait transmission mechanisms, and so it is plausible that two types of individuality have been identified. But what does it mean for individuals to be contingent? We should find evidence that the trait transmission mechanisms responsible for individuality types are contingent and liable to evolve. In the next section I introduce some markers of contingency.
3. Contingent Individuality
Individuals are outcomes of evolution because the biological mechanisms responsible for them are products of evolutionary processes. Some examples of evolutionary processes are natural selection, mutation, and random drift. Examples of evolutionary products are the Krebs cycle for metabolism, Mendelian meiosis, and even modes of reproduction, such as sexual reproduction, which will be addressed later (Beatty Reference Rieppel, Wolters, Lennox and McLaughlin1995; Turner Reference Turner2011, 164). In this section, I outline how evolutionary individuals are products of evolution, which are influenced by processes that affect their likelihood as evolutionary outcomes. We know random mutation plays a role in providing the variation needed for selection to occur and that it is a source of evolutionary contingency. On this picture, selection opportunistically draws from what is available (Jacob Reference Jacob1977). Although mutations might be rare, if selected for they can eventually become the norm.Footnote 3 And so, I will be looking for evidence of mutation and evolution of the biological mechanisms responsible for trait transmission. If these mechanisms evolve, then types of evolutionary individuals underpinned by them evolve too. I will introduce two markers of contingency within the context of individuality, namely, (1) a lack of necessity and (2) impermanence due to evolutionary change. This grounds a more concrete analysis of two cases from biology to follow.
3.1. Two Markers of Contingency
First, nothing necessitates the existence of individuality types, evolutionary or otherwise. This is because biological mechanisms responsible for individuals are outcomes of evolution, dependent upon a complicated sequence of evolutionary events and processes. Their existence is owing to many chance-contributing factors that affect the likelihood of their existence. The relevant mutation or order of mutations may or may not occur, for instance. As products of stochastic processes, individuals require complicated evolutionary pathways riddled with contingency. It would have been very difficult to predict the existence of a particular type of individuality at the outset of evolution.
Second, types of evolutionary individuals are impermanent and undergo evolutionary change. And so long as the products of evolution remain the objects of processes that drive evolution, they potentially continue to evolve. The sources of contingency are evolutionary processes that help to drive evolutionary change. For instance, mutations arise and are selected for that may or may not be passed on to uncertain future generations of individuals. The ever- changing nature of evolution underwrites the lack of necessity—exceptions to the rule are not physically impossible; in fact one could argue that they are what needs to be explained (Beatty Reference Beatty1982, 405).
In summary, accounts of individuality describe outcomes of evolution, which are likely to have been significantly affected by stochastic processes. Sets of universal and necessary conditions are, therefore, inapplicable. We can, however, identify criteria for individuality when certain traits responsible for individuals are selected for and become more significant. In other words, specific conditions for evolutionary individuality will emerge as genetically based individuality mechanisms (i.e., trait transmission mechanisms) are selected for. And an adaptive need can often be met in a variety of different ways. However, there is more to evolution than just natural selection alone. Mutation and other evolutionary processes are also responsible for evolutionary change. And what was at one time “normal” may later become rare and vice versa. This means one cannot preclude what initially appear as exceptions to the rule from eventually becoming the norm. Thus, a single account picking out one type of individual in selection does not quite capture this picture of contingency and change.
That a rare evolutionary occurrence can become standard will often trick us: from our limited perspective, whatever is the norm is often seen as if it must be that way all the time, and exceptions to that norm are viewed as mistakes or mere anomalies. For example, meiosis and sexual reproduction comprise one way of mixing up genes and passing on heritable material. This is a familiar form of trait transmission to animals like us. Many paradigm collective reproducers on Godfrey-Smith’s (Reference Godfrey-Smith2009) account reproduce new individuals through these means.Footnote 4 However, Ereshefsky and Pedroso (Reference Ereshefsky and Pedroso2015) argue that some microbes can mix up genes and transfer heritable traits. And this form of trait transmission found among microbes is not rare because microbial life vastly outnumbers other life forms (O’Malley Reference O’Malley2014). With this understanding of individuality within the framework of evolutionary contingency, it is time to move to more concrete examples.
3.2. Two Cases from Biology
In the following I will search for evidence of contingency and evolutionary change in two cases: (1) meiosis and lateral gene transfer and (2) bottleneck versus aggregate means of forming new individuals.
3.2.1. Mixing Up Genes: Meiosis and Lateral Gene Transfer
There are different ways to mix genes into new combinations, thus creating variation that selection works with. So, meiosis and lateral gene transfer (LGT) are ways to facilitate the variation needed for biological entities to be visible to selection.
Meiosis is a precursor to the successful passing on of genetic traits to offspring. After germ cells are generated, through sexual reproduction those traits are transmitted to later generations. Meiosis is one way to facilitate variation; new combinations of genes are generated by the disjoining of chromosome pairs into different gametes. However, consider how Rattray et al. (Reference Rattray, Santoyo, Shafer and Strathern2015) study the mutation rate during meiosis in Saccharomyces cerevisiae. While germ line mutations and meiotic sorting of parent alleles contribute to evolution, the process itself is “inherently mutagenic.” It increases the mutational load when compared to usual rates of mutations (11). This change in mutation-rate allowance contributes to variation generally. I already discussed mutations of the meiotic mechanism itself in section 2.
We know there are other ways of mixing genetic material besides the “normal” mechanisms of reproduction (i.e., meiosis and sexual reproduction). Consider LGT exhibited by prokaryotes and some eukaryotes. For example, in biofilms two mechanisms are responsible for LGT: transformation allows a bacterium in the biofilm to take in extracellular DNA released from other cells, while conjugation transfers genes through bridges between bacteria (Ereshefsky and Pedroso Reference Ereshefsky and Pedroso2015, 10127). LGT is a way for prokaryotes to express new combinations of traits. It is a way to mix up genes in microbes for new traits and to swap out the bad genes. Ragan and Beiko (Reference Ragan and Beiko2009, 2243) state that through this process various types of genetic material are exchanged in addition to whole genes. This includes stretches of noncoding DNA, portions of genes, intact genes, multigene clusters, operons, plasmids and more. They wonder if whole gene transfer would be “selectively most advantageous.” In doing so, they recognize that the mechanisms of LGT could exchange genetic material differently and in ways that might be better than transferring only segments of genetic material. As a viable mechanism for genetic exchange, they also note that “there is a broad consensus that LGT has contributed to genome evolution in prokaryotes” (2243).
And so, evolution by selection can find different ways to achieve the same goal: “Unlike engineers, tinkerers who tackle the same problem are likely to end up with different solutions” (Jacob Reference Jacob1977, 1164). There are different mechanisms by which variation is achieved through mixing of genetic material. This includes both meiosis and lateral gene transfer. Different types of individuals in selection will exhibit varying genotypes and corresponding phenotypes.
Although well documented in prokaryotes and some eukaryotes, it is not known whether the considerable lack of LGT in some individuals is owing to physical constraints, such as “protection of the germ line from intracellular bacteria,” or regulatory constraints, such as problems integrating into that complex regulatory network (Ragan and Beiko Reference Ragan and Beiko2009, 2245). Meiosis and LGT function as facilitators for variation and provide a platform for mutational change. These identified types will lack necessity insofar as they are evolutionary outcomes sourced and affected by processes responsible for evolutionary contingency, that is, random mutation. For example, there might not have been meiosis if prokaryotes never evolved, that is, if features of meiosis were indeed present in prokaryotic ancestors of eukaryotes. Different causal mechanisms that facilitate variation in populations of individuals are in part responsible for the individuation of different types of individuality. Trait transmission does the rest.
3.2.2. Reproducing New Individuals: Bottlenecks and Aggregation
The sexual reproduction of individuals, such as many of the paradigm collective reproducers in Godfrey-Smith’s (Reference Godfrey-Smith2009) account, is one way to create a new token individual that inherits the traits of its ancestor. This creates a lineage of a certain type of individual, in which tokens of that type pass on heritable material vertically in parent-offspring fashion. Alternatively, horizontal modes of trait transmission create complicated parent-offspring networks. But if those individuals transmit heritable traits to new token individuals, then there are grounds for identifying another type of evolutionary individuality. I will consider each in turn.
Mendelian meiosis is an integral part of sexual reproduction because it creates variable genotypes that are later reproduced. Standard sexual reproduction is one way that trait transmission can occur. This is a type of evolutionary individuality such that token individuals reproduce when two conspecific gametes fuse to create a single embryo. We can identify the point at which a new individual emerges because of the narrowing between generations. This is called a ‘reproductive bottleneck’ because the narrowing distinguishes the emergence of a new individual from mere growth of the parent. The parent-offspring relationships of individuals form reproductive lineages in the way that Godfrey-Smith is concerned with. But there is evidence that this type of evolutionary individuality is evolving.
Evolution of sexual reproduction is currently under study (Zimmer and Riffell Reference Zimmer and Riffell2011, 13204). Zimmer and Riffell investigate changes in reproductive development and argue that gametes undergo selection for mechanisms that increase sperm-egg contact (e.g., changes in morphology, physiological size, etc.). It is reasonable to assume, then, that particular mutations in these mechanisms are selected for (or not) depending on how advantageous they are. That is, those mutations are visible to selection. For example, odd gamete shape and methods of fusing to conspecific gametes can arise as reproductive mutants just as nondisjunction is a meiotic mutant. I have also discussed the plausibility that meiotic mechanisms—those that facilitate variation prior to the occurrence of sexual reproduction—evolve. And so, if odd gamete shape and fusion are not advantageous, they will not be selected for. However, each opportunity still remains as one (though unlikely) possible pathway in the evolutionary history of that mechanism. Zimmer and Riffell give reason to believe that evolutionary change in reproduction, as a mode of trait transmission, is under way.
In vertical forms of reproduction, genetic traits are usually dispersed to new germ cells, and through sexual reproduction those traits are inherited by the offspring. However, we know that many prokaryotic cells exchange and mix up genetic material via LGT. But are there parent-offspring lineages through which genotypic and phenotypic traits are transmitted? At minimum, to be another type of evolutionary individuality, there must be a new individual emerging through a different process by which it inherits traits from its predecessor. Ereshefsky and Pedroso (Reference Ereshefsky and Pedroso2015, 10128) make a case for biofilms as another type of evolutionary individuality because biofilm-level traits are transmitted to new biofilms. Because a new biofilm can be formed by cells that may have broken off from another biofilm, through LGT those preliminary colonizers will transfer traits throughout the new biofilm to later colonizers as they attach. By way of aggregation we have a new individual, which consists of cells that contain genetic material previously exchanged through a lateral process among the cells of the ancestor biofilm. Recall that a reproductive bottleneck to a single-celled embryo in sexual reproduction marks the emergence of a new token individual. Analogously, new aggregative individuals also start with a small amount of cells when compared to the previous biofilm they departed from. And so, a new token individual emerges that inherits material from a predecessor biofilm.
If the nature of sexual reproduction evolves as discussed above (Zimmer and Riffell Reference Zimmer and Riffell2011), and if meiosis evolves (Uyenoyama Reference Uyenoyama and Moens1987; Beatty Reference Rieppel, Wolters, Lennox and McLaughlin1995; Van Leeuwen et al. Reference Van Leeuwen, Vanholme, Pottleberge, Nieuwenhuyse, Nauen, Tirry and Denholm2008), then the types of evolutionary individuality that rely on that machinery evolve. If we take the Evolutionary Contingency Thesis (ECT) seriously, the history of these mechanisms will be riddled with contingency and sourced by stochastic processes, such as the generation of random mutations. Natural selection is not the only evolutionary process that drives change (Gould Reference Gould1989, Reference Gould2002; Beatty Reference Beatty and Rieppel1994, 36). If the evolutionary history of trait transmission mechanisms relevant for individuality is affected by stochastic processes, then those types of processes do not yield outcomes that exist necessarily. Types of individuals, in selection or otherwise, are created and maintained by mechanisms subject to stochastic processes that chisel the structure of evolutionary pathways from a background of alternative possibilities. However, evidence pointing to the evolution of individuality mechanisms implies any identified types of individuals in selection will not necessarily remain the way they look to us now. Recall that the processes driving evolution have both rule-making and rule-breaking capacities (Gould Reference Gould2002; Beatty Reference Lennox and McLaughlin2006). What may appear as counterexamples to a view of individuality may just be new types of evolutionary individuals emerging (e.g., through aggregative means), instead of quasi-evolutionary individuals that only minimally satisfy particular conditions. This individuality pluralism makes sense within the purview of evolutionary contingency.
4. Making Sense of Individuality Pluralism
There are good reasons to be individuality pluralists against the setting of evolutionary contingency. The Gouldian view of evolution as contingently structured means that even if convergent evolutionary outcomes exist, they need not: contingency in evolution is far more significant, and any convergence is not inevitable. Diverse and contingent mechanisms that meet Lewontin’s general constraints for evolutionary individuality distinguish different types of individuals in selection. These differences do not underwrite mere instances of trait transmission but are alternative processes that form parent-offspring relationships needed for selection to occur.Footnote 5 The contingency of the evolution of such mechanisms means we cannot ignore the role of stochastic processes, especially when genetically based mechanisms affected by those processes cause individuality types. But how does individuality pluralism follow from this?
Evolutionary contingency gives good reason to reject the idea that there is only one type of evolutionary individuality. This is because any monistic account of individuality—one that cites a single set of conditions that covers all cases of evolutionary individuals in the biological domain—does not make sense within the evolutionary contingency framework. There is either one type of individual in selection or many types. However, any monist account of evolutionary individuality is held hostage by the contingencies of history. New individuals will likely evolve that do not meet those criteria. Therefore, it is reasonable to explore a plurality of evolutionary individuality types when one is compelled to reject the other option. One might even say that the reason pluralism makes sense here is that, ontologically speaking, there are many types of evolutionary individuals. Some types of evolutionary individuality may be more prevalent than others. The upshot, then, is perhaps we tend to notice evolutionary individuals that fit Godfrey-Smith’s account of reproduction more than others; however, Ereshefsky and Pedroso (Reference Ereshefsky and Pedroso2015) make a good case for the relative significance of other types of individuals in selection from a microbial perspective. Mirroring the reasoning of Beatty (Reference Beatty and Rieppel1994, 41), one could say that individuality accounts should be judged on the extent of their applicability in the biological domain or their correctness per se. But how does an ontological plurality of individuality types follow from ECT?Footnote 6
Perhaps ECT makes sense of individuality pluralism because evolution, as depicted by ECT, yields multiple types of individuals in selection. For example, one may concede that there are multiple types of evolutionary individuals by accepting that at least two types of individuals in selection have been identified here. The case of biofilms only gives further credence to the idea that there can be different types of evolutionary individuals. Types of individuals in selection undergo evolutionary change, and the entities that comprise them are affected by processes other than selection. If different types of evolutionary individuals emerge, evolve, and disappear analogous to speciation events, then we have different types of individuality that succeed one another through evolution diachronically. But these different types of individuals also exist during the same evolutionary time-slice, albeit some with more prevalence than others. Regardless, on that picture, different types of individuals exist synchronically as well. To suggest otherwise—that a single (or set of) mechanism(s) is responsible for individuals in selection—is tantamount to viewing the outcomes of evolution as highly constrained such that evolved individuality mechanisms are and will continue to be maintained (Beatty Reference Beatty and Rieppel1994, 52).
