Review of the Cohesion Concept of Species

1. Review of the Cohesion Concept of Species John Collier University of KwaZulu-Natal www.ukzn.ac.za/undphil/collier

2. Phylogenetic species Phylogenetics: The Theory and Practice of Phylogenetic Systematics, E.O. Wiley, 1981 • Species determined by phylogeny. Historical individuals. • not phenotypic similarity • traits are signs of lineage • diagnostic traits determined by considerations of evolutionary history, derivation of traits, making some traits more reliable for diagnosis than others • evolutionary processes constrained by synchronic and diachronic constraints Wiley called horizontal and vertical cohesion • cohesion determines species identity

3. Information flow Evolution as Entropy, Brooks and Wiley, 1986 • Phylogeny as a process of information flow. • Historical constraints (vertical cohesion) and ecological constraints (horizontal cohesion) plus self organization within information flow govern speciation. • Cohesion determines species identity • Speciation is a disruption of species cohesion • Species cohesion determines species macrostates, with variation within species determining microstates • Entropy of species (as determined by informational macrostate/microstate relation) is typically not maximal • This allows self-organization and disruption of cohesion by self-organizing bifurcation.

4. Information Flow 2 Brooks and Wiley, 1986 (continued) • Focus of information is on genes (and chemical DNA), but this is not required, and some exceptions are noted. • Distinction between internal and external (within and without the flow of genetic information) noted. Sexual selection a case of an internal process, natural selection an external constraint. Developmental constraints are also assumed to be internal, though development is also treated as a case of information flow with vertical and horizontal constraints. External constraints (e.g., selection) said to be purely rate-determining • Restrictions on gene flow and equilibrium with constraints are assumed and treated statistically with the entropy concept. • Kinetics (both internal and external) largely ignored (mistakenly: Harrison)

5. Cohesion concept of Species The meaning of species and speciation, Templeton, 1989 • Explicit use of cohesion to define species • Fully gene centred: species determined by gene flow (“intrinsic cohesion mechanisms”) • Process oriented, but no explicit distinction between vertical and horizontal cohesion. • No theoretical unification, just a collection of mechanisms (next slide) that confine and separate gene flow • Cohesion both separates species (isolation) and unifies species (facilitation of gene flow) • Speciation as disruption of cohesion

6. Cohesion concept of Species 2 Cohesion mechanisms (Templeton 1989) • Genetic exchangeability 1. Promotion (facilitation) 2. Isolation • Demographic exchangeability (constraints from selection and drift) 1. Replaceability (genetic drift promotes genetic identity) 2. Displaceability i. Selective fixation ii. Adaptive transitions (natural selection) a. constraints on the origin of heritable variation b. constraints on the fate of heritable variation ecological, developmental, historical, population genetic (all “extrinsic” – to genes, presumably)

7. Some remarks on the phylogenetic and cohesion approaches • Wiley’s approach is more general because it does not specify heredity mechanisms • Templeton’s approach is more operational, but difficult to generalize • Brooks and Wiley regard sexual selection and developmental constraints as internal, whereas Templeton regards sexual selection as a form of natural selection (intrinsic to gene flow), but developmental constraints are extrinsic • The last is a significant difference following from Templeton’s focus on genes alone (and assuming implicitly that selection is on genes, thus intrinsic to gene flow), and Brooks and Wiley’s focus on information flow.

8. We need a general account of cohesion to give generality to accommodate non-genetic heredity and the formal parity of constraints with information and cohesion in order to evaluate the views and extend them. This requires an analysis of the concept of cohesion.

9. The cohesion concept • First of all, cohesion is an identity concept, so we start there. • Identity, A = B • Logical condition, same for all things • Equivalence relation: symmetric, transitive, closed • A = B implies that B has every property that A has, and vice versa • This tells us virtually nothing, since it is a purely logical relation, but it does put these logical constraints on any concept of autonomy. The next move is to look at what makes parts of something parts of that thing. This is provided by the unity relation.

10. Unity Unity, U(A) • Unity is the relation among the parts of a thing A such that: • If a and b are parts of A, then aUb, and bUa (symmetric) • If a and b are parts of A, then aUb and bUc implies aUb (transitive) • By a. and b., U is an equivalence relation • U(A) is the closure of U, given any initial part. • By a. to d., U(A) contains all and only the parts of A. • It is empirical question what satisfies U(A) for a given A. Typically the type of unity relation will depend on the sort of thing A is.

11. Cohesion, the dividing glue Cohesion, C(A), dynamical unity • Cohesion is the unity relation for dynamical objects, such that: • All parts aCb are dynamical • C is dynamical Simple examples of cohesion: a quartz crystal a gas in a box • Note that in each case the cohesion is not absolute; it is a matter of degree. • We should expect difficult intermediate cases. • Cohesion can differ in strength in different dimensions (factors)

12. The Tautology Problem Gold (2001) • Hey decries this popularly acclaimed definition. He unmasks it as a tautology framed in the ostensibly palpable, yet suffering a lack of utility. Hey urges us to substitute the word “cohesion” with any other word and suggests we will find equal meaning. (Ghiselin later made efforts to repair Templeton’s mis-construction by identifying a species as that which is by what it does, specifically defining species as the products of speciation.)

13. The tautology non-problem • The cohesion concept has alternatives that might be correct, e.g., phenetic concepts or essential properties concepts • The cohesion concept neither endorses or prohibits particular contributions to cohesion. This is an empirical matter. • This empirical matter is testable.

14. Phylogenetic species are cohesion species • Though tautology is a non-problem, if we assume a phylogenetic account of species as historical entities, it is necessary that cohesion is the determiner of species identity. • If we use the cohesion account of species, then the phylogenetic view of species follows • So the accounts are equivalent if not identical

15. Pluralism? • There are many contributors to cohesion, so we might take it that we need a pluralistic account of species to accommodate all of these. • This seems plausible given Templeton’s menu of cohesion factors. • On the Wiley account, however, various factors contribute to overall species cohesion, giving a net effect, as in forces in Newtonian mechanics. • We don’t think that Newtonian systems are pluralistic, so by parity we should not consider species concepts to be pluralistic; there are just different aspects of species identity

16. Pluralism? 2 • However, if we have different loci of action then the argument is not so simple. • The genetic view of Templeton provides a single locus of action, arguably. • But if extra-genetic factors are in play, it is not obvious that there is a single locus of action. • I have not yet worked this out fully. • It would interesting if there are speciation events that do not involve genetic differences.

17. Thank you