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A pragmatic approach to the species problem from a paleontological perspective ARTICLE in EVOLUTIONARY ANTHROPOLOGY ISSUES NEWS AND REVIEWS · JANUARY 2014 Impact Factor: 3.59 · DOI: 10.1002/evan.21386 · Source: PubMed
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1 AUTHOR: Mary T Silcox University of Toronto 65 PUBLICATIONS 995 CITATIONS SEE PROFILE
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Evolutionary Anthropology 23:24–26 (2014)
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A Pragmatic Approach to the Species Problem from a Paleontological Perspective MARY T. SILCOX
The ideal scenario for paleontologists would be for the species they designate to be equivalent to the species recognized for modern animals, in the sense that they were formed as a result of the same evolutionary processes. This would mean, for example, that we could be confident that in combining extant and extinct taxa in phylogenetic analyses we would be dealing with equivalent operational taxonomic units. Notwithstanding the many thousands of pages that have been spent arguing over species concepts, the only concept that has won widespread acceptance for the designation of modern species is Mayr’s Biological Species Concept (BSC).1 In fact, whenever we complete a cladistic analysis, we assume reproductive isolation of our terminal taxa because otherwise their similarities could be the product of interbreeding rather than common ancestry. Fundamentally, we all behave as though the BSC is true.
Beyond the obvious fact that fossils are no longer capable of reproducing, there are two fundamental problems with trying to apply the BSC to the paleontological record. The first has to do with the relationship between morphological variation and reproductive isolation. Or, more correctly, the lack of any such relationship. It is possible to find a modern example to justify virtually any argument. At one end of the spectrum, cryptic species suggest that valid biological species
Dr. Mary T. Silcox is an Associate Professor of Anthropology at the University of Toronto Scarborough. She is interested in using the fossil record to understand the earliest phases of primate evolution. Her particular speciality is the first primate adaptive radiation, the socalled “plesiadapiform” primates. Email:
[email protected]
Key words: Plesiadapiformes; Paromomyidae; Phenacolemur; Bighorn Basin; Wyoming; Species concepts; paleoprimatology; vertebrate paleontology
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DOI: 10.1002/evan.21386 Published online in Wiley Online Library (wileyonlinelibrary.com).
can be nearly indistinguishable, particularly if one has only hard tissues to work with. Mouse lemur species, for example, often show most of their differentiation in pelage,2 and certainly would not be distinguishable from the types of remains (jaws and teeth) that are often available to paleontologists. At the other end of the spectrum are cases in which reproducing populations show marked morphological differentiation, with the best-documented being the inconveniently interbreeding baboons.3 What’s more, the compelling arguments that support a single species model for Lufengpithecus, in spite of the fact that it exhibits a greater degree of variation than any modern species,4 underscore the fact that the wafer-thin veneer of the present on the totality of evolutionary history may not sample all the possibilities of what represents variation in a reproductively isolated species. The second problem with applying the BSC is that it provides no basis for considering variation through time. Simpson5:153 attempted to extend the BSC into the fourth dimension in his Evolutionary Species Concept (ESC) by
defining a species as “a lineage. . .evolving separately from others and with its own unitary evolutionary role and tendencies.” Although this approach is satisfying from a theoretical perspective, it proves difficult to apply in practice. Simpson5 himself argued that because all life ultimately forms part of one lineage, recognizing species must be fundamentally arbitrary. Ridley6 presented an apparent solution to this problem, suggesting that branching points be used to delimit the boundaries of species. Again, this is a theoretically satisfying approach, but difficult to apply. In most instances, operational taxonomic units need to be defined before determining a branching sequence, leading to inherent circularity. Moreover, even when the fossil record is adequate to recognize what appears to be a single evolving lineage, it is impossible to know whether or not there were any branches diverging from that lineage that remain unsampled. Indeed, demonstrating the absence of such branches would be tantamount to proving a negative. I suggest that paleontologists escape from the tyranny of the present and take a pragmatic approach to defining species that emphasizes what we actually need to do with them. Beyond the satisfaction of doing alpha taxonomy for its own sake, there are two main contexts in which species need to be designated in paleontology. The first is for the purpose of phylogenetic analysis. In this context, what paleontologists require are operational taxonomic units that can be clearly and unambiguously defined as distinct from other units within the context of a given slice of time. This is potentially
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consistent with Cracraft’s7:170 Phylogenetic Species Concept: “the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent.” The problem with this concept rests in the second half. It is very difficult to demonstrate “a parental pattern of ancestry and descent,” particularly if you don’t have operational taxonomic units to work with a priori. From a pragmatic perspective, therefore, it seems prudent to consider species as minimum diagnosable units, without any requirement for a demonstration of how those units came to be. The other critical use of species in paleontology is for biochronological analysis, which introduces the problem of how to deal with change through time. How much change is enough? I suggest that the answer should again focus on the pragmatic. When there is sufficient change that unit A is diagnosably different from unit B, so that A and B will be useful biochronological indicator species, then they should be identified as such, even if there is reason to believe that they represent ends of an evolving lineage. If one is lucky enough to have intermediates, then they can simply be designated as intermediates, which may also have biochronological utility.
A CASE STUDY: THE PAROMOMYID PRIMATES OF THE SOUTHERN BIGHORN BASIN The Bighorn Basin (BHB) of Wyoming has provided some of the most compelling examples of evolutionary lineages evolving gradually through time.8–11 Material from the southern BHB can be placed within a well-defined stratigraphic framework10 for over 600 m of section, which documents nearly the entirety of the Wasatchian Land Mammal Age, from Wa0 to Wa7.11 Very large samples exist for numerous primate groups from the southern BHB. O’Leary,12 for example, studied evolution in a sample of over 3,000 notharctid adapoids. One of the best known cases of gradual evolution in the primate record comes from the rarer anatomorphine omomyoids.
A Pragmatic Approach to the Species Problem 25
Bown and Rose9 documented the evolution of Tetonius matthewi to Pseudotetonius ambiguus through a series of intermediate stages, demonstrating the gradual reproportioning of the anterior premolars through more than 200 m of section. In 2008, my colleagues and I published a study of the paromomyid plesiadapiform primates of the southern Bighorn Basin based on
It is very difficult to demonstrate “a parental pattern of ancestry and descent,” particularly if you don’t have operational taxonomic units to work with a priori. From a pragmatic perspective, therefore, it seems prudent to consider species as minimum diagnosable units, without any requirement for a demonstration of how those units came to be.
more than 570 stratigraphically controlled specimens.13 We hypothesized another case of evolution of one species into another (Fig. 1). Phenacolemur praecox occurs up to the 180-m level, and is apparently succeeded by the similarly sized but diagnosably different Phenacolemur fortior, which first occurs at 210 m. The major difference between these two species is in the form of the p4, which is tall, slender, and sharply pointed in P. praecox (Fig. 1A), but much shorter, fatter, and more bulbous in P. fortior (Fig. 1C). Both species had been named in previous publications,14,15 but the precise nature of their relationship to one another was unknown. We uncovered two specimens from the 182and 190-m levels; these specimens
exhibit an intermediate morphology, with a p4 that is taller than in P. fortior, but fatter than in P. praecox (Fig. 1B). The presence of these intermediates forced us to consider what to do with the original two species. Since they appeared to be ends of an evolving lineage, they could be combined into a single species under the ESC. We made the decision to continue to treat them as separate and to designate the two intermediates as simply “Phenacolemur praecox-fortior intermediates.” The main rationale for this decision is that the two endpoint species are clearly diagnosable and serve as useful biochronological indicators. In particular, P. fortior is restricted to the narrow interval between two previously identified10,16 periods of rapid evolutionary change, Biohorizons A and B, which approximately corresponds to Wa4.12 The other major speciesdesignation problem we had to deal with pertains to Phenacolemur jepseni and P. citatus. Simpson17 suggested that P. jepseni, whose holotype is from the San Jose Formation, may also be present in the Lysite, which corresponds to the upper part of the southern BHB section. We found specimens that were essentially indistinguishable from the holotype of that species in the southern BHB. However, there was a continuous range of morphology extending between those specimens and ones from the same time period that could confidently be referred to P. citatus,14 a species named 40 years earlier15 from BHB material. This meant that it was not possible to diagnose separate taxa out of the paromomyid sample from the upper part of the section. We argued that this sample should be dealt with as one highly variable species, P. citatus, since that represented the only unit that could be meaningfully diagnosed.
IMPLICATIONS OF THE PRAGMATIC APPROACH TO OUR UNDERSTANDING OF PRIMATE DIVERSITY In the case of the southern BHB paromomyids, our approach had
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Figure 1. Paromomyid dentaries from the southern Bighorn Basin, Wyoming, with p4m2. A) Phenacolemur fortior (USGS 3892 reversed; 346 m); B) Phenacolemur praecox-fortior intermediate (USGS 12883; 190 m); C) Phenacolemur praecox (USGS 3899 reversed; 34 m). Note that the intermediate specimen has a p4 that is shorter, fatter, and more bulbous than that in P. praecox, but is taller and more pointed than that in P. fortior. Meter levels are in relation to the base of the Willwood Formation.11,14 Scale 5 1 mm.
somewhat opposing effects in terms of the question of estimating diversity. By giving the ends of an apparent evolving lineage different names, we are potentially inflating species diversity. However, it is important to remember that it is merely a hypothesis that what we were viewing was anagenetic speciation. We do not know, for example, whether or not Phenacolemur praecox may have budded into more than one species. A compelling reason to think that it might have persisted outside the BHB after its apparent evolution into P. fortior rests in the similarity between
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P. praecox and P. citatus. The latter enters the section at 352 m, with no evidence of intermediates between it and P. fortior. It is entirely possible that P. citatus represents an emigrating population of descendants of P. praecox, reinvading the BHB. If this were the case, the apparent anagenetic transformation of P. praecox into P. fortior would actually be one arm of a cladogenetic split rather than a strictly anagenetic event. On the other hand, recognizing a highly variable collection of material as a single species in the case of P. citatus has the potential to reduce our measure of primate diversity. This is ironic, since in most cases it would seem that recognizing minimum diagnosable units would increase the species count. In this case it does not, demonstrating that this approach is not as horribly typological as it may initially appear. Because the minimum diagnosable unit may include specimens that exhibit a range of variation, when that variation is continuous, this approach acknowledges that biological species do exhibit variation. One of the corollaries, however, is that it often becomes more difficult to divide samples into distinct units as sample size increases. Indeed, if there is one general lesson that the BHB provides, it is that when the fossil record is truly excellent, species designation is harder, not easier. In sum, the southern BHB paromomyids provide a practical application of a pragmatic approach to the species problem, emphasizing the needs of the paleontologist over a theoretical framework based on modern biology, which can be impossible to apply in practice. It may not meet the ideal of producing operational taxonomic units that are strictly equivalent to modern species. But what it does provide is the type of data needed to reconstruct evolution and study change through time in the fossil record.
REFERENCES 1 Mayr EW. 1942. Systematics and the origin of species, from the viewpoint of a zoologist. Cambridge: Harvard University Press. 2 Olivieri G, Zimmermann E, Randrianambinia B, et al. 2007. Mol Phylogenet Evol 43:309–327.
3 Jolly CJ. 1993. Species, subspecies, and baboon systematics. In: Kimbel WH, Martin LB, editors. Species, species concepts, and primate evolution. New York: Plenum Press. p 67–107. 4 Kelley J, Plavcan MJ. 1998. A simulation test of hominoid species number at Lufeng, China: implications for the use of the coefficient of variation in paleotaxonomy. J Hum Evol 35: 577–596. 5 Simpson GG. 1961. Principles of animal taxonomy. New York: Columbia University Press. 6 Ridley M. 1989. The cladistic solution to the species problem. Biol Philos 4:1–16. 7 Cracraft J. 1987. Species concepts and the ontology of evolution. Biol Philos 2:329– 346. 8 Gingerich PD, Simons EL. 1977. Systematics, phylogeny, and evolution of early Eocene Adapidae (Mammalia, Primates) in North America. Contrib Mus Paleontol University Michigan 24: 245–279. 9 Bown TM, Rose KD. 1987. Patterns of dental evolution in Early Anatopmorphine primates (Omomyidae) from the Bighorn Basin, Wyoming. Paleontol Soc Memoir 23:1–162. 10 Bown TM, Rose KD, Simons EL, et al. 1994. Distribution and stratigraphic correlation of upper Paleocene and lower Eocene fossil mammal and plant localities of the Fort Union, Willwood, and Tatman Formations, southern Bighorn Basin, Wyoming. US Geol Surv Professional Pap 1540:1–103. 11 Chew AC. 2006. Biostratigraphy, paleoecology and synchronized evolution in the early Eocene mammalian fauna of the Central Bighorn Basin, Wyoming. Unpublished Ph.D. dissertation, Johns Hopkins University School of Medicine. 12 O’Leary MA. 1996. Dental evolution in the early Eocene Notharctinae (Primates, Adapiformes) from the Bighorn Basin, Wyoming: documentation of gradual evolution in the oldest true primates. Unpublished Ph.D. dissertation, Johns Hopkins University School of Medicine. 13 Silcox MT, Rose KD, Bown TM. 2008. Early Eocene Paromomyidae (Mammalia, Primates) from the southern Bighorn Basin, Wyoming: systematics and evolution. J Paleontol 82:1074– 1113. 14 Matthew WD. 1915.Entelonychia, Primates, Insectivora (part IV.). In: Matthew WD, Granger W, editors. A revision of the lower Eocene Wasatch and Wind River faunas. Bull Am Mus Nat Hist 34:429–483. 15 Robinson P, Ivy D. 1994. Paromomyidae (?Dermoptera) from the Powder River Basin, Wyoming, and a discussion of microevolution in closely related species. University of Wyoming Contrib Geol 30:91–116. 16 Schankler D. 1980. Faunal zonation of the Willwood Formation in the central Bighorn Basin, Wyoming, In: Gingerich PD, editor. Early Cenozoic paleontology and stratigraphy of the Bighorn Basin, Wyoming. University of Michigan Mus Paleontol Pap Paleontol 24:99– 144. 17 Simpson GG. 1955. The Phenacolemuridae, new family of early Primates. Bull Am Mus Nat Hist 105:415–441.
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