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Chapter 12 Life on Earth: The Big Picture


Chapter 12 Life on Earth: The Big Picture Introduction to Philosophy of Biology: Sex and Death – PowerPoint PPT presentation

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Title: Chapter 12 Life on Earth: The Big Picture

Chapter 12 Life on Earth The Big Picture
  • Introduction to Philosophy of Biology Sex and

History of life directionality
  • Weak
  • Particular taxa evolve at particular points in
  • Strong
  • Particular kinds of taxa evolve at particular
    points in time

Murky Directionality is
  • Progress (12.1 )
  • Changes in Disparity (12.2/12.3)
  • disputed and contingency of lifes history
    emphasized (12.4)
  • possibly cut down by mass extinctions (12.5)

Directionality is progress
  • Possible conceptualizations of progress
  • Progression towards homo sapiens
  • Progressive adaptiveness (Dawkins)
  • Arms races (Dawkins)
  • Progressive increase in complexity (Gould)

Progress is adaptiveness (Dawkins)
  • Weak
  • Organisms of today are better adapted to the
    environment than earlier ones (i.e. comparing two
    organisms of one population)
  • Strong
  • Weak thesis over million of years
    (requiring a general property of adaptiveness)

Progress is visible in arms races (Dawkins)
  • Thesis
  • Arms races between competing lineages define a
    direction of progress
  • Problems
  • Prolonged arms races reconstruct the environment
  • May involve a rock/paper/scissors evolutionary

Progress is Increase in Complexity
  • Intuitive
  • Complexity measures
  • Property status (relative/ objective)
  • Dawkins ascribed though objective complexity
    length of description of an organism at a fixed
    level of description
  • Gould complexity spread of variation

Goulds variations
  • Life starts off simply and usually stays there
  • Complexity increases by passive diffusion from a
    point of origin (undirected, stochastic process)
  • Real change is increase in total variance
    (bias in the direction of complexity)
  • Facts, presumably
  • No mechanism of adaptation/ speciation/
    extinction favours complexity
  • Bacteria dominate
  • gt Complexity drifts upwards undirected

Smith and Szathmary vs. Gould
  • Thesis
  • Series of major transitions and hence inherent
    directionality (RNA, DNA, eucaryotes, (plants,
    animals, fungi), human language)
  • Crux (according to Sterelny Griffith)
  • Different pictures of variation

Gould vs. S.S. Structures of variance
  • Gould
  • lower limit to complexity
  • no upper limit
  • Gradual spread to higher complexity
  • S.S.
  • evolution of evolvability, i.e. dynamic
  • Major transitions movements of points of max.
    complexity (gt min. complexity)

Goulds challenge (12.2)
  • Claims
  • expectation that complexity/diversity of life
    increases gradually over time due to natural
    selection is mistaken
  • Therefore the received view is also mistaken

Goulds case The Burgess Shale fauna
  • Cambrian explosion
  • 7-8 phyla found that are not existent today
  • Therefore
  • Orthodox conception of the shape of tree of life
    is wrong
  • Diversity increased, but disparity DEcreased

The received view
o f l i f e
Goulds view
Goulds conclusions interests
  • overestimation of the role of selection in
  • Selection plays no role in generating/ reducing
  • History of life is contingent
  • Small change (t0) gt big change (t1)
  • Outcomes sensitively dependent on initial
  • gt Survival of the luckiest

The Concept of Disparity
  • Question
  • What is it and how (if at all) has it changed?
  • Model
  • morphosphere space that represents the
    physical forms of all actual and possible
  • Similar forms close together b/c similar sets of
    physical propertied describe them
  • Disparity Size of morphospace for life existing

Disparity and Morphospace
  • Challenges
  • Distances in morphospace (if any) are not
  • - trait choice, weighting
  • The Cladists anti-subjectivist argument
  • Properties important for genealogy ? Properties
    important for disparity
  • Property lability and retrospective fallacy

Contingency (12.4)
  • Contingency hypothesis
  • Important features of life are not
    counterfactually resilient
  • Importance of particular events in shaping
    history of life and unpredictability of
  • Some features of life not predictable by physics
  • No robust process explanations possible

3 Types of Contingency
  • Contingency of specific taxi
  • Implication of the received view
  • 2) Contingency of Adaptive Complexes
  • undercuts idea that traits are robustly explained
    by a selective environment
  • Inconsistent w/ any kind of empirical adaptionism
  • Contingent Explorations of Morphospace
  • It seems that 1 uncontroversial, 2/3 are
    relatives of Goulds anti-adaptionist criticism
    and besides, hard to test

Mass extinction (12.5)
  • Claim (Whose)
  • Major transitions of life are defined by mass
    extinction, not routine or background extinction
  • Disparity of life depends on extirpation of
    dominant groups
  • Challenge
  • Difference b/w mass and background extinction
  • Accept importance of mass distinctions but
    re-evaluate their importance (Sepkowski)

The Importance of Mass Extinction
  • Gould YES, b/c mass extinctions
  • Change the rules of evolution
  • Have a profound effect on biota
  • Make explanations extrapolating from changes in
    local populations into ecological time frames
  • Sterelny NO, b/c mass extinctions
  • Just change the outcome normal operations in an
    abnormal world
  • Is consistent w/ mass extinction fundamentally
    reshaping the tree of life
  • Mass extinction is no threat to the received view

Review The concepts of
  • Directionality
  • Progress
  • Complexity
  • Disparity
  • Contingency
  • Mass extinctions