1) The document discusses various concepts of what constitutes a species and argues that no single existing definition meets the criteria of being generally applicable, empirically adequate, and theoretically consequential. 2) It proposes that the concept of "species" is best understood as a cluster of related meanings and properties rather than a single definition, incorporating phenomena like lineages, genes, and habitats. 3) Templeton's "cohesion conception," which defines a species based on genetic and demographic exchangeability, is presented as one of the more general conceptions.

1. 1
Lateral heredity
and species
JOHN S. WILKINS
Philosophy
University of Sydney, Australia
john@wilkins.id.au
1

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Hybrid Species
22
What is a species?
Where many lineages coincide:

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. 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. 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. 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.