Hybrid species - talk delivered at Göttingen


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The philosophical implications of hybridism to concepts of species

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Hybrid species - talk delivered at Göttingen

  1. 1. 1 Lateral heredity and species JOHN S. WILKINS Philosophy University of Sydney, Australia john@wilkins.id.au 1
  2. 2. Hybrid Species 2 ‘What’s a philosopher?’ said Brutha. ‘Someone who’s bright enough to find a job with no heavy lifting,’ said a voice in his head. — Terry Pratchett (Small Gods)
  3. 3. Hybrid Species 3 The talk I will ✦ Try to convince you there are too many species definitions ✦ Consider “lateral heredity” in species concepts ✦ Propose some virtues for a scientific concept: Generality, empirical adequacy, theoretical consequence ✦ Argue that none of the existing definitions of “species” meet these requirements ✦ Assert that this is OK, because “species” is a mix-and- match concept based on phenomenal clustering ✦ Deny that lateral heredity is a major problem for trees and if time permits: ✦ Suggest that Templeton’s definition is the most general conception
  4. 4. Hybrid Species 4 Some history Hybridism as a process, and a cause of “new kinds” of organisms, is ancient: ✦ Aristotle, in the Historia Animalium: As a general rule, wild animals are at their wildest in Asia, their boldest in Europe and most diverse in form in Libya [North Africa]; in fact, there is an old saying. ‘Always something fresh in Libya.’ It would appear that in that country animals of diverse species6 meet, on account of the rainless climate, at the watering places, and they pair together; and that such pairs breed if they be nearly of the same size and have periods of gestation of the same length. For they are tamed down in their behaviour towards each other by extremity of thirst. ... Elsewhere also offspring are born to heterogeneous pairs; thus in Cyrene the wolf and the bitch will couple and breed; and the Laconian hound is a cross between the fox and the dog.They say that the Indian dog is a cross between the tiger and the bitch, not the first cross, but a cross in the third generation; for they say that the first cross is a savage creature.They take the bitch to a lonely spot and tie her up—and many are eaten, unless the beast is eager to mate. [History of AnimalsVIII.28 606b16–607a7]
  5. 5. Hybrid Species 5 Some history Repeated by Pliny, read throughout the medieval and renaissance ✦ Repeated by Walter Scott ✦ And one of the KJV translators, George Abbot,Archbishop of Canterbury,1605 ✦ Medievals thought giraffe was a camel-leopard hybrid (“cameleopard”) ✦ Hybridism was thought to be ubiquitous until the Mendelian revolution c1908 ✦ Note also that Mendel does not talk of crossing within species, but of “hybrids” of genetic factors ✦ Reproductive isolation goes back at least to 1770s as a marker of species (Blumenbach), but it gets purchase with the synthesis
  6. 6. Hybrid Species 6 Mayr: A species consists of a group of population which replace each other geographically or ecologically and of which the neighboring ones intergrade or interbreed wherever they are in contact or which are potentially capable of doing so (with one or more of the populations) in those cases where contact is prevented by geographical or ecological barriers. Or shorter: Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. (1942: 120) Dobzhansky: … a species is a group of individuals fully fertile inter se, but barred from interbreeding with other similar groups by its physiological properties (producing either incompatibility of parents, or sterility of the hybrids, or both). … Considered dynamically, the species represents that stage of evolutionary divergence, at which the once actually or potentially interbreeding array of forms becomes segregated into two or more separate arrays which are physiologically incapable of interbreeding. (1935: 353f; cf.1941: 312) Reproductive isolation species conception (RISC):
  7. 7. Hybrid Species 7 RISCs fail for two cases Either an "agamospecies" conception for asexuals, or asexuals do not form species ✦ Mayr thought that asexuals are "aberrant" ✦ But we know now that asexual, or not fully sexual, organisms are the majority of taxa We also know that lateral transfer is ubiquitous in every group (though not universal, maybe) ✦ Lateral heredity includes hybridism, alloploidy, symbiont capture Something is wrong with the RISC ✦ But Coyne and Orr reiterate it RISCs apply only to obligate sexual organisms
  8. 8. Hybrid Species 8 including all reticulation in phylogeny ✦ Of genes ✦ Of populations and organism lineages ✦ Of taxa ✦ In short, of anything that messes up the tree structure of taxonomy ✦ Should we abandon the “tree of life” (Michael Arnold, Mark Ragan)? ✦ I do not think so Hybridisation is a special case of lateral heredity
  9. 9. Hybrid Species 9 Not to propose scientific concepts: that's the scientists' job To engage in analysis of philosophical problems in science (Locke's undergrowth clearing) The philosophical problem here is overgeneralisation of operational conceptions ✦ and more deeply a problem of understanding scientific concepts and theory in general ✦ It is a mistake to think that all concepts in science must come out of a theory I am using history as a guide to the actual role of concepts in science (insert shameless plug for books here) The role of the philosopher here
  10. 10. Hybrid Species 10 Widespread presumptions: ✦ A scientific concept must be general (apply in the same manner in every case) ❖ If not, replace it with a number of distinct, univocal, concepts ✦ Hypothetical concepts that always apply (i.e., are immune from falsification) are unscientific These are not truisms in philosophy of science, and they ought not to be in science itself (and they aren't) Very few truly universal concepts in science (maybe in physics and chemistry, the universal sciences) ✦ Except in very restricted domains or subdisciplines (e.g., species concepts amongst primates?) Concepts in science
  11. 11. Hybrid Species 11 We need not be anarchists, just pragmatists ✦ What do scientists actually do with concepts? ✦ What do they actually need to do? What properties must a scientific concept ideally have? Sometimes scientific concepts are theory-bound Sometimes scientific concepts are observational ✦ And not all observations are bound to the theory the observations test or support: e.g., ❖ You did see that individual orang eat that piece of fruit, even if the binoculars are bound to the theory of optics ❖ The “theory” of frugivory amongst Pongo pygmaeus is decoupled from the theory of optics Theory-boundedness
  12. 12. Hybrid Species 12 Empirical adequacy: it must be consistent with facts ✦ If you have to trim the facts to make the concept work, it is problematic Coverage of entire explanatory domain (generality): partial concepts are faulty Theoretical consequences: a concept that doesn't make theoretical difference is otiose These are the sine quibus non of scientific concepts [Problems? – what facts, how much of a domain?] What should a concept like species achieve?
  13. 13. Hybrid Species 13 One concept, many conceptions ✦ Species as taxa/category confusing Species concept may be ✦ Univocal ✦ Polysemic ❖ Polysemy: a term having a number of related meanings How scientists choose conceptions from related meanings is the core of the Species Problem Species concept
  14. 14. Hybrid Species 14 Four (six?) major alternative species conceptions Weaker RISC (Coyne and Orr): "most", "usually", "occasional" ✦ Fails the generality requirement ✦ Fails the theoretical requirement Ecological SC ✦ Works where intermediates have lowered hybrid fitness ✦ Fails where species are ecologically polytypic or generalists (“mosaics”) ✦ Hence fails generality, and empirical adequacy requirements Evolutionary SC ✦ Tautology (always true): if no shared fate, no species; i.e., post hoc property ✦ Fails to meet empirical adequacy in obvious cases (may not be a failure) ✦ Defeated by lateral heredity Alternatives to the RISC
  15. 15. Hybrid Species 15 Hennigian species ✦ Relies on cladogenesis ✦ Defeated by lateral heredity ✦ Other problems (extinction at cladogenesis) Alternatives to the RISC – Phylogenetic SC 1
  16. 16. Hybrid Species 16 Alternatives to the RISC – Phylogenetic SC 2 Autapomorphic species (Rosen) ✦ Smallest diagnosable group (Cracraft et al., Nelson & Platnick) ✦ Fails for paraspecies/pseudospecies that lack autapomorphies ✦ Circular: you already must know what apomorphies are relevant for species; or else haplotype groups become species, for example (that may be OK for you, or inflation)
  17. 17. Hybrid Species 17 Alternatives to the RISC – Phylogenetic SC 3 PhylogeneticTaxa species ✦ Smallest monophyletic (coalescent) lineage (Mishler, et al., Baum) ✦ Or, lineages maintained by differences in synapomorphies ✦ Problematic in operation: specimen based ✦ Coalescence is not unique, depends on genes and traits chosen (as we have seen) ✦ Monophyly based on between-taxa relations, hence circular (arguably,Wheeler)
  18. 18. Hybrid Species 18 What to do? Abandon SCs? LITUs, ESUs, etc ✦ Tried, and failed Pluralism? Any SC definitions you like ✦ Unacceptable to many; want principles Hidden single solution (monism) waiting to be found? ✦ Unlikely by now
  19. 19. Hybrid Species 19 Overgeneralisation RISC originally defined by an entomologist (Dobzhansky) and an ornithologist (Mayr) ✦ Botanists did not generally find it universal or immediately useful ✦ Nor do virologists, bacteriologists, mycologists etc. ✦ The vast bulk of living things are not "properly" (obligately)sexual, hence not RISC species Other examples of overgeneralisation: ✦ Karyotypic species (MJD White) ✦ Specific mate recognition systems (H Paterson)
  20. 20. Hybrid Species 20 What to do? Why should we expect that species are one kind of things? ✦ Many theoretical terms are abstractions from messy particularity ✦ Placeholders in a way of seeing things ✦ Rough and ready abstractions: ❖ Gene, organism, niche, homolog ❖ Species (arguably the oldest of all general biological terms) ✦ Pre-theoretical terms are either ❖ Abandoned (phlogiston) ❖ Refined (gene) ❖ Redefined (organism) ❖ Some resist all three: species is one
  21. 21. Hybrid Species 21 What is a species? Species problem ✦ Begins with Mendelian genetics ❖ Prior to this we had the species question of origination of species ❖ Attempts to define species in terms of genetic sets ❖ Now in terms of developmental suites and genetic coalescence Rank? ✦ No such “grade” or “level” if multiple conceptions are in play Object of theory? ✦ None I can think of (not even of ecology) Phenomenon? ✦ Consider a mountain – not an “object” of geological theory ✦ Species call for explanations – explananda, not explanans ✦ A clustering of properties: lineages, lifecycles, haplotypes, genes,
  22. 22. Hybrid Species 22 What is a species? Where many lineages coincide:
  23. 23. Hybrid Species 23 Templeton’s Cohesion Conception Defined as “genetic and/or demographic exchangeability” ✦ Same reproductive role, OR ✦ Same ecological/demographic role, OR BOTH Clustering two factors:
  24. 24. Hybrid Species 24 Phenomenal cluster “conception” Works for all organisms Does not restrict the specialist Conceptual delicatessen: ✦ Make your own club sandwich: Ingredients: ✦ Any of the main SCs ✦ Empirical phenomena ✦ Explanations
  25. 25. Hybrid Species 25 Thanks for listening to a philosopher Some of my relevant publications: ✦ 2003. How to be a chaste species pluralist-realist:The origins of species modes and the Synapomorphic Species Concept. Biology and Philosophy 18:621–638. ✦ 2006. Species, Kinds, and Evolution. Reports of the National Center for Science Education 26 (4):36–45. ✦ 2007.The dimensions, modes and definitions of species and speciation. Biology and Philosophy 22 (2):247–266. ✦ 2007.The Concept and Causes of Microbial Species. Studies in History and Philosophy of the Life Sciences 28 (3):389–408. Books ✦ 2009. Defining species: a sourcebook from antiquity to today, American University Studies.V, Philosophy. NewYork: Peter Lang. ✦ 2009. Species: a history of the idea, Species and Systematics. Berkeley: University of California Press.
  26. 26. Hybrid Species 26 Acknowledgements Volkswagen Stiftung Australian Research Council: ✦ Federation Fellowship for Paul Griffiths, FF0457917 ✦ Postdoctoral Fellowship for John Wilkins, DP0984826 Philosophy Department, School of Philosophical and Historical Inquiry, University of Sydney Deutsches Primatenzentrum, Dietmar Zinner, Christian Roos et al.