Developments in genetic engineering may soon allow biologists to clone organisms from extinct species. The process, dubbed “de-extinction,” has been publicized as a means to bring extinct species back to life. For theorists and philosophers of biology, the process also suggests a thought experiment for the ongoing “species problem”: given a species concept, would a clone be classified in the extinct species? Previous analyses have answered this question in the context of specific de-extinction technologies or particular species concepts. The thought experiment is given more comprehensive treatment here. Given the products of three de-extinction technologies, twenty-two species concepts are “tested” to see which are consistent with the idea that species may be resurrected. The ensuing discussion considers whether or not de-extinction is a conceptually coherent research program and, if so, whether or not its development may contribute to a resolution of the species problem. Ultimately, theorists must face a choice: they may revise their commitments to species concepts (if those concepts are inconsistent with de-extinction) or they may recognize de-extinction as a means to make progress in the species problem.
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A second form of genomic transfer—primordial germ cell transplantation (PGCT)—is more appropriate for organisms from which unfertilized eggs are difficult to obtain, such as birds and fish (Simkiss et al. 1987; Shapiro 2015). This process has already produced chickens (Chang et al. 1997) and zebrafish (Saito et al. 2010) and may be an appropriate means of resurrecting extinct fish or fowl; however, it is technically not considered cloning, which is a term reserved for SCNT. One may consider PCGT rather than SCNT in the discussion below without altering the conceptual inferences noted.
There may still be genetic dissimilarities between donors and clones produced by SCNT. Since mitochondria—which carry their own DNA—are situated outside the cellular nucleus, the success of SCNT does not imply that clones will carry the same mitochondrial DNA as their genetic donors. Furthermore, immune cell genes differentiate during ontogeny in response to the individual organism’s environment and so a clone’s leukocyte DNA will differ from its genetic donor’s (Russell et al. 2008, 976–989). Nevertheless, the genetic similarity between donor and clone will be the highest degree achievable by any means other than mitosis or parthenogenesis.
Kato et al. (2009) recovered viable mammoth cell nuclei. This particular case depended on discovery of a frozen partial carcass whose tissues had not degraded before freezing. Such discoveries are certainly extraordinary. In any event, the preservation of intact genetic material from extinct species requires a highly contingent chain of events.
Between the two genomic transfer methods, SCNT seems more appropriate for mammals given the relative ease with which biologists may culture mature unfertilized mammalian egg cells. By contrast, PCGT is designed to overcome distinctive difficulties posed by avian cloning (Shapiro 2015; cf. Wilmut et al. 1999). If necessary, we can imagine Nucleartransferagus as the product of PCGT without changing the outcomes of our thought experiment. The “material” remains the same: a clone carrying genetic material cultured directly from the cells of an organism belonging to an extinct species.
It is unlikely that back-breeding or direct gene editing would actually produce either phenotypic or genotypic identity with extinct species (see note 2). However, such identity is not logically impossible. Assuming near-complete similarity ex hypothesi is a useful control for our thought experiment.
Standards of biological individuality are more stringent than standards of individuality simpliciter. Gracia (1988, 28) elaborates standards of general individuality, but these are consistent with the possibility of spatiotemporally disjointed ‘scattered objects’ (Cartwright 1999). Biological individuals, by contrast, require spatiotemporal continuity (Ghiselin 1974; Hull 1978; Griesemer 2000; cf. Godfrey-Smith 2011; Piotrowska 2018).
Mayr (1982) argues that species identity across time is ‘irrelevant’ given our inability to test conspecificity along that dimension (286). Ring species provide a spatial analog for the potential intransitivity of species membership across time. These species ‘consist of chains of populations’ in which consecutive links satisfy conditions for conspecificity with their neighbors, but populations separated by multiple links do not (Sterelny and Griffiths 1999, 189). If we consider a chain of populations across space to be analogous with a lineage of populations across time, then we may recognize what motivates theorists such as Mayr to deny the transitivity of conspecificity.
The fourth possible category implied by my two dimensions—synchronic individual concepts—is an empty set. Individuals are diachronic by definition given that they are defined by different historical events (Hull 1978).
Delord also describes “functional” senses of the term, wherein a species may have living members that are unable to perpetuate the species’ lineage. I exclude this sense of the term “extinct” from the discussion above because it does not seem relevant to de-extinction per se, which is intended to resurrect species whose members are all currently dead. Efforts to mitigate or reverse the effects of functional extinction are more properly the domain of conservation biology (Tilman et al. 1994).
Another way to distinguish “demographic” and “final” senses of distinction would be to consider “demographic” extinction a biological sense of the term and “final” extinction an informational sense of the term. Again: de-extinction must assume the biological sense of extinction. After all, if a species can be resurrected at all then the information necessary for its resurrection must be intact, i.e., the species cannot be extinct in the informational sense ex hypothesi.
One might argue that synchronic class concepts do not address the issue of de-extinction at all because the concepts’ intended use is operational and the suggested test for is practically impossible for M. primigenius. However, the intentions of theorists are irrelevant to the concepts’ given standards for species membership; since our clones do not coexist with other members of the extinct species in fact, it follows that the clones cannot be classified in the extinct species (cf. Mayr 1962). See further discussion in the Appendix.
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The author would like to thank Massimo Pigliucci, Marc Ereshefsky, and Helena Siipi for their guidance, support, and feedback. Discussion with and commentary from Derek Skillings, P.D. Magnus, Matt Haber, Derek Turner, Adrian Currie, and Joyce Havstad were also invaluable towards the completion of this work. Additional, input from Michael Bell, Alberto Cordero, Jessie Prinz, Peter Godfrey-Smith, and Markku Oksanen proved instrumental in clarifying the author’s thoughts. This work would not have been possible without the support of Jesus Ilundain, Kaarina Beam, and the administration of Linfield College. Finally, the author would like to thank the reviewers of this essay for valuable comments that have improved this work immensely.
Appendix: species concepts and particular results
Appendix: species concepts and particular results
I follow Mayden (1997) by considering the following twenty-one species concepts. My goal is comprehensiveness without irrelevancy or redundancy; I have therefore included only concepts that have distinctive definitions and excluded concepts that apply only to asexual organisms. In particular: I have excluded the Agamospecies Concept, which supplements the Biological concept for clonal and asexual lines; Mayden’s “Non-dimensional” concept, which corresponds with my synchronic class category; and three variations of the Phylogenetic concept that differ in member diagnosis, but not in species definition (Ibid, 403–408).
I have sorted the twenty-one concepts into their appropriate categories for ease of reference. Note that concepts may share the same species taxon criteria, but differ in their species category commitments (e.g., GCC and GSC; BSC and HSC).
As noted in “Results: Which concepts allow resurrection?” section, no diachronic individual concept is consistent with de-extinction via back-breeding or direct gene editing. I therefore consider only the classification of Nucleartransferagus with respect to the concepts below.
Cladistic (ClSC): Species membership is defined by common descent from a single speciation event and, in the case of extinct species, elimination by a shared cause. Given that last stipulation, Nucleartransferagus would not be classifiable as a part of M. primigenius because that species went extinct at the end of the Pleistocene (see discussion of different senses of the term “extinct” in “Methods: a taxonomy of species concepts” section above).
Cohesion (CSC): Membership is determined by cohesion mechanisms intrinsic to the species lineage, such as those that limit gene flow or genetic drift. If a lineage can have material overlap through spatiotemporal gaps, as Piotrowska (2018) argues, then Nucleartransferagus might be classifiable as a part of M. primigenius: the clone may develop as earlier mammoths did and perpetuate their genetic information, which would presumably limit gene flow or genetic drift as with earlier mammoths. If spatiotemporal continuity is necessary for material overlap in a biological lineage, then Nucleartransferagus could not be classified as a part of M. primigenius, since the lineage would have its terminal endpoint before the clone’s creation.
Diagnosable Phylogeny (DPSC): Membership is determined by overall genetic similarity between organisms descended from a single speciation event, forming the smallest individual lineages that participate in evolutionary processes. Nucleartransferagus could be classified as a part of M. primigenius given that her genetic similarity to earlier mammoths is a result of inheritance from nuclear DNA materially connected with the mammoths’ speciation event.
Evolutionary (ESC): Organisms’ shared evolutionary origins, selective pressures, and ultimate fate determine membership. Given that her nuclear DNA was produced as a material result of natural selection on earlier mammoths, Nucleartransferagus might be classifiable as a part of M. primigenius if current selective pressures are similar to those that earlier mammoths faced.
Genealogical Concordance (GCC): Membership is determined by the presence or absence of particular genetic markers. Nucleartransferagus must share the genetic markers relevant to M. primigenius since her nuclear DNA is taken from a member of that species, and so she must be classifiable as a part of the species.
Hennigian (HSC): Membership is determined by interbreeding between organisms, as limited by intrinsic or extrinsic reproductive isolating mechanisms. Nucleartransferagus cannot interbreed with extinct mammoths, and so she would not be classifiable as a part of the species M. primigenius.
Internodal (ISC): Membership is determined by organisms’ position between nodes on a phylogenetic tree. While Nucleartransferagus’ genome was taken from a mammoth, her parentage is in the E. maximus line. Given that this concept’s standards for species membership are the organism’s position on the phylogenetic tree, but not the genome’s position, it should follow that Nucleartransferagus is not classifiable as a part of M. primigenius.
Phylogenetic (PSC): Membership is determined by common descent from a shared ancestor, but excludes members of descendent species. Assuming that Nucleartransferagus bears the appropriate relation of “descent” from ancestral mammoths (see “Discussion and conclusions” section), it would follow that she is classifiable as a part of M. primigenius.
Polythetic (PtSC): Membership is determined by common descent from a shared ancestor, as diagnosed by statistically covariant traits. Nucleartransferagus shares her traits with earlier mammoths and her similarity to those mammoths is explained by a genome materially inherited from earlier mammoths, and so she should be classifiable as a part of the species M. primigenius.
Reproductive Competition (RCC): Membership is determined by organisms’ competition for the same resources. Nucleartransferagus cannot compete for resources with dead mammoths, nor does she compete vicariously for those resources through competition with organisms that did in fact engage in such competition; consequently, she could not be classifiable as a part of the species M. primigenius.
Synchronic class concepts
As noted above, all synchronic class concepts are inconsistent with de-extinction. Again, any of the three clones may satisfy species taxon criteria, but the nature of the species category in these concepts excludes organisms created after the species’ extinction. Such exclusion may seem trivial, but it is in fact a designed feature of synchronic class concepts. These concepts play an operational role in biological theory; each has been suggested as a means of testing species membership (cf. Mayden 1997; Stamos 2003). Although (say) Nucleartransferagus could potentially reproduce with earlier mammoths, ‘unrealized potentialities don’t count’ in operational concepts (Hull 1965, 209–210; see also footnote 8).
Biological (BSC): Membership is determined by interbreeding between organisms, as limited by intrinsic or extrinsic reproductive isolating mechanisms.
Ecological (EcSC): Membership is determined by interbreeding between organisms, as limited by extrinsic reproductive isolating mechanisms or “adaptive zones.”
Genetic (GSC): Membership is determined by the presence or absence of particular genetic markers.
Genotypic Cluster Definition (GCDC): Membership is determined by overall multivariate genetic similarity between organisms.
Recognition (RSC): Membership is determined by the presence or absence of traits associated with reproduction, and particularly those through which organisms in the species recognize intraspecific mates and exclude members of other species.
Diachronic class concepts
Thought experiment results for individual concepts are given for each concept.
Composite (CpSC): Membership is determined by the presence or absence of traits either fixed or lost in the population by a shared speciation event. Nucleartransferagus would qualify as a mammoth by this criterion since her genetic material is materially continuous with earlier mammoths. Neither Backbreedagus nor Geneditagus would be classified as members of M. primigenius: while both share traits with earlier mammoths, neither bears traits caused by a speciation event shared with all and only earlier mammoths. Both are materially descended from Asian elephants, and so any traits that haven’t been directly manipulated by resurrection biologists would be fixed by the evolution of that group.
Evolutionary Significant Unit (ESUC): Membership is determined by interbreeding between organisms, as that interbreeding contributes to the “evolutionary legacy” of the population. If “interbreeding” requires actual interbreeding, then none of the clones meet criteria for classification as members of M. primigenius; if potential for interbreeding is sufficient, then all three would satisfy those criteria.
Morphological (MSC): Membership is determined by overall morphological similarity. All three clones would be classified as members of M. primigenius since each is as morphologically similar as possible to earlier mammoths ex hypothesi.
Phenetic (PhSC): Membership is determined by overall similarity, including morphological, genetic, and ecological traits. All three clones would be classified in M. primigenius since each of the three would be more similar to earlier mammoths than they would be to any other organisms, again ex hypothesi.
Successional (SSC): Membership is determined by interbreeding between organisms, as limited by intrinsic or extrinsic reproductive isolating mechanisms. Nucleartransferagus and Geneditagus would both be classified as members of M. primigenius given that their overall genetic similarity to earlier mammoths would make them capable of interbreeding, if only potentially. Backbreedagus should not be classified as a member M. primigenius: while she might be capable of interbreeding with earlier mammoths, she should also be capable of interbreeding with her immediate ancestors in the species E. maximus. This follows from the fact that her genotype and phenotype should be only incrementally different from those immediate ancestors; after all, Backbreedagus is a product of selection processes. Members of E. maximus are not classifiable as members of M. primigenius, thus implying a reductio ad absurdum.
Taxonomic (TSC): Membership is determined by the diagnosis of well-qualified taxonomists. Depending on the goals, methods, or whims of individual taxonomists, any of the three clones could be classified in M. primigenius; however, that classification would not be a necessary consequence of the concept.
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Finkelman, L. De-extinction and the conception of species. Biol Philos 33, 32 (2018). https://doi.org/10.1007/s10539-018-9639-x
- Species concepts
- Species problem
- Resurrection biology
- Genetic engineering
- Philosophy of biology