Abstract
While philosophers tend to consider a single type of causal history, biologists distinguish between two kinds of causal history: evolutionary history and developmental history. This essay studies the peculiarity of development as a criterion for the individuation of biological traits and its relation to form, function, and evolution. By focusing on examples involving serial homologies and genetic reprogramming, we argue that morphology (form) and function, even when supplemented with evolutionary history, are sometimes insufficient to individuate traits. Developmental mechanisms bring in a novel aspect to the business of classification—identity of process-type—according to which entities are type-identical across individuals and natural kinds in virtue of the fact that they form and develop through similar processes. These considerations bear important metaphysical implications and have potential applications in several areas of philosophy.
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Notes
While philosophers have extensively debated the nature of causation and causal dependence, the notion of causal history has received much less attention. In what follows, we remain neutral with respect to what causes are and how to identify them, and focus instead on distinguishing between different types of causal history.
A popular way to capture these structural differences is to say that birds “fly with their forearms,” while bats “fly with their fingers.” The reason is that bird wings are constituted by their entire forelimb, while bat wings are a prolongation of their digits, which are extremely long and connected by a thin membrane.
Talk about “traits descending from traits” is controversial: some scholars argue that it would be more accurate to talk of lineages constituted by organisms (as opposed to traits). Throughout this article, we remain neutral with respect to this controversial issue regarding the units of selection. For the sake of simplicity, following standard cladistic practice, we shall talk freely of traits lineages; however, the same considerations could be rephrased in terms of organismal lineages.
Biologists define “analogies” as structurally similar traits in organisms which are unrelated from a phylogenetic perspective (e.g. the eyes of vertebrates and arthropods). These structural similarities are typically explained as the result of convergent evolution. “Homologies,” in contrast, are traits whose similarity can be attributed to a common ancestry.
Whether (and when) functional-morphological classifications should be replaced by phylogenetic ones raises substantial issues that lie beyond the scope of this article. The important point, for present purposes, is simply that different biological considerations yield different and incompatible classifications. To emphasize, our claim is not that phylogenetic classifications are necessarily superior to morphological ones, but rather that evolutionary considerations provide an alternative systematization that classifies traits differently from functional and morphological criteria.
To be sure, in the above example, the phylogenetic classification is not necessarily inconsistent with the functional-morphological one. To wit, if biramous limbs characterize a monophyletic group, then, on a cladistic analysis, one could still consider them as a distinct trait from tubular legs. The details of biological systematics need not concern us here. The relevant observation is simply that evolutionary considerations can sometimes lead to a different classification of traits from functional and morphological data.
Indeed, the morphological difference between these plants is so evident that, for a long time, various species of tomato plants used to be considered part of a separate genus and only recently they have come to be regarded as a subgenus of Solanum.
As Stuppy and Kesseler (2008, p. 73) recently expressed it, “Many of the fruits that we have just classed as proper nuts qualify only if nothing but the qualities of the mature ovaries are taken into account. For example, fresh walnuts look more like drupes. They are covered by a freshly green husk that peels off easily when the fruits are ripe. (...) This may seem to be a rather exceptional case but pseudo-drupes are also typical of members of the oleaster family (Elaeocarpaceae) such as sea buckthorn (Hippophae rhamnoides.)”
The relationship between Owen’s definition of homology as “the same organ in different animals under a variety of form and function” (1843, p. 379) and evolution is controversial. According to some authors, in a somewhat Platonic fashion, Owen conceived of two structures as homologous whenever they are concrete and imperfect instantiations of an abstract perfect archetype (Brigandt 2003). Other scholars have challenged the explicitly anti-evolutionary nature of Owen’s argument (Rupke 2009). (See also the introductory essays collected in Owen 2007). Here we need not enter into the dispute. The important point is simply that Owen’s definition predates Darwin’s seminal publication by over a decade, and is independent of Darwinian evolutionary theory.
The expression “historical-phylogenetic” homology is borrowed from Cracraft (2005).
The center of the dispute has focused over whether homologous traits are synapomorphies—traits that derive from a common ancestor—or traits that are developmentally individualized. In short, while Wagner argues in favor of developmental homology, others (e.g. Cracraft 2005) reply that homology is essentially a phylogenetic notion. In a somewhat conciliatory fashion, some philosophers (Brigandt 2007; Griffiths 2007) have defended a pluralistic approach according to which there is no single “correct” homology concept. On this view, far from being mutually exclusive alternatives, “the kind of developmental constraint brought about by the phenomenon of homology and morphological evolvability in a character-by-character fashion are two sides of one coin.” (Brigandt 2007, p. 717).
To wit, consider the following example, borrowed from Wagner (1989). The evolutionary history of amphibians displays a reduction in the number of digits, due to size-related re-patterning of chondrogenic condensations (the conjunction of cartilaginous traits). Imagine comparing two hands with different numbers of phalanxes, and asking whether it was a terminal or preterminal element that got lost. If we focus on shape, we may note that the terminal element (the phalanx that constitutes the tip of the finger) is identical in both the longer and shorter finger. This may lead us to conclude that it was the preterminal element that was lost. However, if we focus on the proximo-distal position of the new terminal element, we may note an analogy with a former preterminal element, suggesting the opposite conclusion, viz. that it is the terminal element that was lost in the descendant.
Providing a precise definition of “developmental individuality” and “developmental homology” is a substantial problem for the philosophy of biology. (Wagner 1989, p. 62) offers the following preliminary definition of developmental homology: “Structures from two individuals or from the same individual are homologous if they share a set of developmental constraints, caused by locally acting self-regulatory mechanisms of organ differentiation. These structures are thus developmentally individualized parts of the phenotype.” To avoid complications that transcend our present concerns, here we simply assume that a trait or organ is developmentally individualized if it reacts to stimuli as an individualized whole.
The “in principle” qualification is required because, at the current state of research, there still are some structural and functional differences between ESC and iPS. For instance, in order to reprogram an adult cell into an iPS, it is not necessary to restore the whole genome, but only to reactivate (at most) four key genes. Consequently, one could distinguish between an iPS and an ESC by identifying certain parts of the genome that are still activated in the former but not the latter. However, in principle these differences could (and perhaps in the future will) be washed away.
We would like to thank an anonymous referee for raising this objection.
Here one may also consider various case studies provided by chemistry. For instance, while vanillin was originally extracted from the fruits of orchids of the genus Vanilla, it may now be synthesized from substances such as clove oil or lignin, or biosynthetically produced from the ferulic acid in rice bran. While sharing the same molecular formula (\(\text{ C }_{8}\text{ H }_{8}\text{ O }_{3}\)), natural, synthetic, and biosynthetic vanillin are regarded as distinct kinds in the food industry. In addition, one may also consider counterfactual or hypothetical scenarios. Suppose that scientists discovered a method for producing gold in a lab through a nuclear bombardment process: even granting that samples of “artificial” gold have the same chemical structure as the “natural” ones, there are good reasons for regarding them as distinct kinds—as witnessed by the fact that a customer buying a “natural gold” ring who discovers that the ring is actually made of “artificial gold” seems, intuitively, entitled to a refund. (We are grateful to an anonymous referee for bringing up this last example.) Of course, the way in which kinds are devised is sometimes accidental (see LaPorte 1996, for further examples from mineralogy).
It is legitimate at this point to wonder about the nature of the causal sequence defining developmental processes. What holds together the different phases of a process? What type of causal link is required by this conception of kinds and individuals? The causal process in question will be identified in terms of two distinct aspects: its qualitative stages as well as its phylogenetic history. Developmental history serves to spell out the former; the latter is taken care of by evolutionary history (see Griffiths 1999, esp. pp. 219–222). Addressing these questions in full requires a solid understanding of the nature of biological traits, an important scientific and philosophical endeavor that, however, transcends the scope of this essay.
For the sake of simplicity, in this example, we are assuming that the genes responsible for pluripotency can be “reactivated” whereas, in a more realistic scenario, they would be reinserted into the cell’s DNA. On this reading, we can interpret \(A^{*}, B^{*}, C^{*}\), and \(D^{*}\) as the genes that are artificially inserted into the genome.
There are of course exceptions, such as Kripke (1980), who famously recognizes the importance of ancestral relations in fixing personal identity or van Inwagen (1990), who sees a crucial ontological divide between living and non-living organisms. However, it is fair to say that none of these arguments is grounded in biological evidence or directly supported by current biological research. In general, metaphysicians seem to regard the biological worldview as methodologically plausible, yet ontologically suspicious.
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Acknowledgments
We would like to express our gratitude to Claudio Calosi, Elena Casetta, Giuseppe Feola, Laura Franklin-Hall, Daniele Fulvi, Giorgio Lando, Patrizia Pedrini, Matthew Slater, Neil Williams, and, especially, to Achille Varzi and Sebastian Watzl, for constructive comments on various versions of this essay. Earlier drafts were delivered at the Conference of the Italian Society for Analytic Philosophy (SIFA) in Padua, at the International Congress of the Italian Society for Logic and Philosophy of Science (SILFS) in Bergamo, at the Conference for Metaphysics and Philosophy of Science hosted by the University of Toronto, and at the Department of Philosophy at the College of the Holy Cross. The audiences at all these venues provided excellent feedback. We are also grateful to two anonymous reviewers for helpful suggestions. In developing some of the ideas discussed in this article, M. J. Nathan benefited from a visiting position at the Laboratory for Stem Cell Biology and Pharmacology of Neurodegenerative Diseases at the University of Milan: many thanks to Elena Cattaneo and her research team for their support.
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M. J. Nathan and A. Borghini contributed equally to this work.
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Nathan, M.J., Borghini, A. Development and natural kinds. Synthese 191, 539–556 (2014). https://doi.org/10.1007/s11229-013-0290-4
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DOI: https://doi.org/10.1007/s11229-013-0290-4