Review of the Cohesion Concept of Species

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Review of the Cohesion Concept of Species

• John Collier
• University of KwaZulu-Natal
• www.ukzn.ac.za/undphil/collier

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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

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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.

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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)

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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

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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)

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Some remarks on the phylogenetic and cohesion
approaches

• Wileys approach is more general because it does
  not specify heredity mechanisms
• Templetons 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 Templetons focus on genes alone (and
  assuming implicitly that selection is on genes,
  thus intrinsic to gene flow), and Brooks and
  Wileys focus on information flow.

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• 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.

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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.

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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.

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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)

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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 Templetons mis-construction by
  identifying a species as that which is by what it
  does, specifically defining species as the
  products of speciation.)

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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.

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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

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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 Templetons 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 dont 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

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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.

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• Thank you