[Paleopsych] SW: On Life-History Invariance across Animal Species

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Evolution: On Life-History Invariance across Animal Species

    The following points are made by Gerdien de Jong (Science 2005
    1) There is obvious variation in the way different animals live their
    lives -- in their life span, in their age and size at maturity, and in
    their size as full-grown adults, to name a few attributes. But are
    there fundamental similarities in the life history strategies that
    different animals use? Charnov [1] argued that there are: He proposed
    fundamental similarities -- "life history invariants" -- to be a major
    explanatory ingredient of life history evolution. Life history
    invariants generalize a life history model over species boundaries and
    over a wide range of animal sizes, leading to an understanding of
    universal life history strategies. New work [2] calls into question
    the principal method to detect life history invariants: the authors
    have determined that the approach is misleading, throwing the very
    existence of the concept into doubt.
    2) Life history invariants are dimensionless ratios of two life
    history traits -- for instance, age at maturity and average length of
    life. Such a ratio is used to answer questions such as "At what
    relative age do animals first reproduce?" Whether we talk about
    rabbits or whales, we hope the ratio will enable us to forget about
    differences in life span, size, environment, and taxonomy. Thus, life
    history invariants point to common properties of organisms not
    immediately clear from direct observation. As such, they are
    potentially very useful for understanding and modeling life history
    evolution: The models are meant to be general, doing away with the
    need to model each species separately. The existence of life history
    invariants is a major argument for one general theory of life history
    evolution, rather than a theory as a set of recipes for how to make
    species-specific models.
    3) Life history invariants are canonically identified from a log-log
    plot of two life history traits involved in a dimensionless ratio. In
    such a plot, the slope is expected to equal 1. Consider two life
    history traits, a and b, and ask whether their dimensionless ratio a/b
    is a life history invariant. If their ratio is constant (c), a log-log
    plot with ln(b) on the x axis and ln(a) on the y axis would show
    points on a line defined by ln(a) = ln(c) + ln(b), with a slope of 1
    and intercept ln(c) (see the top figure). The line is the regression
    line and the intercept can be used to estimate the life history
    invariant, c. A log-log plot of two traits involved in a life history
    invariant leads to a slope of 1 with all variation in the dependent
    variable on the y axis explained by the variable on the x axis (that
    is, R2 = 1 for an ideal invariant where R2 of the regression is the
    proportion of the variation in the dependent variable a explained by
    the variation in the independent variable b). An empirically
    determined slope of 1 at high explained variance R2 has therefore been
    taken to indicate a life history invariant. This is common
    experimental logic, but treacherous, as it disregards the potential
    existence of other ways to arrive at the predicted slope of 1 and very
    high R2. If a life history invariant is the only way to arrive at a
    slope of 1 and very high R2, then one can conclude from an empirical
    slope of 1 and very high R2 that a life history invariant exists.
    4) Many such log-log plots of traits that indicate potential life
    history invariants exist. Allsop and West [3] presented data on
    relative body size at sex change for animals ranging from a 2-mm
    shrimp to a 1.5-m fish. The log-log plot of body size at sex change
    versus maximum body size showed a slope of 1.05, and all the points
    were near the regression line, with R2 = 0.98. The life invariant
    "relative body size at sex change" was perfectly present. Buston et
    al. [4] then threw a spanner in the works. Commenting on Allsop and
    West's data, Buston et al [4] pointed out that random distributions of
    both total body size and size at sex change lead to identical
    properties in a log-log plot as a life history invariant: a slope of 1
    and an R2 of > 0.95. More null models followed [5], using different
    random distributions of traits. But Nee et al. [2] describe the
    general rationale of how slopes of 1 at high R2 arise in log-log
    plots, independent of the distributions of the traits. The culprit is
    a variable on the y axis that is a fraction of the x-variable: The
    plot is of y = cx, with c < 1. In a log-log plot of cx versus x, a
    slope of 1 follows automatically. A wide range on the x axis -- from
    rabbit to whale -- guarantees a high R2. The evidence for life history
    invariants vanishes as the method of finding them evaporates.
    References (abridged):
    1. E. L. Charnov, Life History Invariants: Some Explorations of
    Symmetry in Evolutionary Ecology (Oxford Univ. Press, Oxford, 1993)
    2. S. Nee, N. Colegrave, S. A. West, A. Grafen, Science 309, 1236
    3. D. J. Allsop, S. A. West, Nature 425, 783 (2003)
    4. P. M. Buston, P. L. Munday, R. R. Warner, Nature 428,
    10.1038/nature02512 (2004)
    5. A. Gardner, D. J. Allsop, E. L. Charnov, S. A. West, Am. Nat. 165,
    551 (2005)
    Science http://www.sciencemag.org
    Related Material:
    The following points are made by G.P. Burness et al (Proc. Nat. Acad.
    Sci. 2001 98:14518):
    1) The size and taxonomic affiliation of the largest locally present
    species ("top species") of terrestrial vertebrate vary greatly among
    faunas, raising many unsolved questions. Why are the top species on
    continents bigger than those on even the largest islands, bigger in
    turn than those on small islands? Why are the top mammals marsupials
    on Australia but placentals on the other continents? Why is the
    world's largest extant lizard (the Komodo dragon) native to a
    modest-sized Indonesian island, of all unlikely places? Why is the top
    herbivore larger than the top carnivore at most sites? Why were the
    largest dinosaurs bigger than any modern terrestrial species?
    2) A useful starting point is the observation of Marquet and Taper
    (1998), based on three data sets (Great Basin mountaintops, Sea of
    Cortez islands, and the continents), that the size of a landmass's top
    mammal increases with the landmass's area. To explain this pattern,
    they noted that populations numbering less than some minimum number of
    individuals are at high risk of extinction, but larger individuals
    require more food and hence larger home ranges, thus only large
    landmasses can support at least the necessary minimum number of
    individuals of larger-bodied species. If this reasoning were correct,
    one might expect body size of the top species also to depend on other
    correlates of food requirements and population densities, such as
    trophic level and metabolic rate. Hence the authors assembled a data
    set consisting of the top terrestrial herbivores and carnivores on 25
    oceanic islands and the 5 continents to test 3 quantitative
    a) Within a trophic level, body mass of the top species will increase
    with land area, with a slope predictable from the slope of the
    relation between body mass and home range area.
    b) For a given land area, the top herbivore will be larger than the
    top carnivore by a factor predictable from the greater amounts of food
    available to herbivores than to carnivores.
    c) Within a trophic level and for a given area of landmass, top
    species that are ectotherms will be larger than ones that are
    endotherms, by a factor predictable from ectotherms' lower food
    3) The authors point out that on reflection, one can think of other
    factors likely to perturb these predictions, such as environmental
    productivity, over-water dispersal, evolutionary times required for
    body size changes, and changing landmass area with geological time.
    Indeed, the database of the authors does suggest effects of these
    other factors. The authors point out they propose their three
    predictions not because they expect them always to be correct, but
    because they expect them to describe broad patterns that must be
    understood in order to be able to detect and interpret deviations from
    those patterns.
    Proc. Nat. Acad. Sci. http://www.pnas.org
    Related Material:
    Notes by ScienceWeek:
    A long view of the evolutionary history of life on Earth suggests that
    living systems tend to evolve into larger and more complex forms.
    However, some of the most successful living systems are relatively
    small and have remained small. Is there a pattern in the evolution of
    size? And if there is a pattern, what are the forces responsible for
    the pattern and how do we explain the exceptions?
    The following points are made by Sean B. Carroll (Nature 2001
    1) For the first 2.5 billion years of life on Earth, most species
    rarely exceeded 1 millimeter in size and were generally much smaller.
    The earliest reported bacterial microfossils from approximately 3.5
    billion years ago averaged approximately 5 microns in diameter. Early
    *eukaryotic microfossils (*acritarchs), while considerably larger,
    still ranged generally from approximately 40 to 200 microns in size
    (with a few larger exceptions) for much of their first 600 to 800
    million year history. Organismal size increased appreciably with the
    evolution of multicellular forms. In bacterial and algal forms with
    cell walls, one of the simplest ways to become multicellular was for
    the products of cell division to remain together to form long
    filaments. Many early multicellular eukaryotes were millimeter-scale,
    linear or branched, filamentous forms.
    2) The size and shape of life did not expand appreciably until the
    late *Proterozoic. Radially symmetric impressions and trace fossils
    indicate the presence of millimeter scale multicellular organisms
    (metazoans) around 550 million years ago. The puzzling *Ediacaran
    fauna comprised of tubular, frond-like, radially symmetric forms
    generally reached several centimeters in size (although some forms
    approached 1 meter in size), as did macroscopic algae. Organismal
    sizes expanded considerably in the *Cambrian, including *bilaterians
    up to 50 centimeters in size, as well as sponges and algae up to 5 to
    10 centimeters. Maximal body lengths of animals increased subsequently
    by another 2 orders of magnitude, as did algal size (e.g., *kelp).
    3) The largest existing organisms, giant fungi and trees, evolved from
    independent small ancestors. Land plants are believed to have evolved
    from *charophyte green algae, and both green algae and plants
    apparently evolved from a unicellular *flagellate ancestor. Fossil
    spores indicating the earliest evidence of plant life date from the
    *mid-Ordovician. The oldest plant-body fossil (Cooksonia) suggests
    that early land plants were small, and on the basis of molecular
    phylogenetic analyses are believed to be comparable in organization
    and life cycle to *liverworts. Many of the principal groups of land
    plants have evolved large (> 10 meters) species at some point in their
    history. Thus, increases in both mean and maximal organismal size
    apparently occurred in the evolution of bacteria, eukaryotes, and
    within the algal, fungal, and animal lineages.
    4) There is a long history of support for the general notion of
    overall evolutionary trends toward increases in size, complexity, and
    diversity. However, there are two fundamentally distinct mechanisms
    that have been proposed to explain these trends. One proposed
    mechanism is a random and passive tendency to evolve away from the
    initial minima of organismal size, complexity, and diversity through
    an overall increase in variance ("there is no where to go but up").
    The second proposed mechanism is a non-random, active or "driven"
    process that biases evolution towards increased size or complexity.
    What must be noted is that there are relationships between size and
    complexity and between complexity and diversity that are intuitive
    apparent. Increases in organismal size through increases in cell
    number create the potential for increases in diversity of cell type,
    and as a result, increases in anatomical complexity. Increases in
    morphological complexity then may lead to expansions into previously
    unoccupied "ecospace" and an accompanying expansion of species
    Nature http://www.nature.com/nature
    Notes by ScienceWeek:
    eukaryotic: Cells (or organisms composed of such cells) containing
    internal membrane-bound organelles such as a nucleus.
    acritarchs: Unicellular microfossils of unknown or uncertain
    biological origin that occur abundantly in strata from the Precambrian
    and Paleozoic (see next note).
    Proterozoic: The complete geological time-scale is as follows:
    Time-Frame Starting Date (Millions of Years Ago)
    ---------- -------------------------------------
    Hadean 4600
    Archaean 4000
    Proterozoic 2500
    Cambrian 570
    Ordovician 510
    Silurian 439
    Devonian 408.5
    Carboniferous 362.5
    Permian 290
    Triassic 245
    Jurassic 208
    Cretaceous 145.6
    Paleocene 65
    Eocene 56.5
    Oligocene 35.4
    Miocene 23.3
    Pliocene 5.2
    Pleistocene 1.64
    Holocene 0.01
    Ediacaran: The term "Ediacaran" refers to an assemblage (until
    recently the oldest) of soft-bodied marine animals, the assemblage
    first discovered in the Ediacara Hills in Australia.
    Cambrian: See time-scale above. The most outstanding aspect of the
    Cambrian was the rather sudden appearance of numerous invertebrate
    fossils, so numerous that some researchers have termed the Cambrian an
    explosion of evolutionary processes. Many of the life forms that
    existed during the Cambrian are long extinct, but their fossils are
    numerous, and through their fossils the various Cambrian species have
    been the subject of much study by paleobiologists. The Cambrian
    explosion of life forms has been a long-standing puzzle for
    paleobiologists, and at present there is apparently no single
    generally accepted explanation.
    bilaterians: The "Bilateria" are a major division of the animal
    kingdom comprising all forms with bilateral symmetry, and the term
    "bilaterians" refers to the first such forms appearing after the
    emergence of protozoa.
    kelp: A group of large brown "seaweeds", actually algae, growing in
    large structures that may be as long as 60 meters.
    charophyte green algae: In general, "green algae" are algae in which
    chlorophyll is not masked by another pigment. Charophyte green algae
    (also known as "stoneworts"), are a type of green algae usually found
    in fresh or brackish water.
    flagellate: Possessing one or more flagella. A flagellum is a long
    threadlike extension providing locomotion for a cell.
    mid-Ordovician: See time-scale above.
    liverworts: (Hepaticopsida) A group of lower plants in which the
    dominant generation is the sexual phase of the plant (gametophyte

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