[Paleopsych] SW: On Life-History Invariance across Animal Species
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Evolution: On Life-History Invariance across Animal Species
http://scienceweek.com/2005/sw050916-1.htm
The following points are made by Gerdien de Jong (Science 2005
309:1193):
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
(2005)
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:
DINOSAURS, DRAGONS, AND DWARFS: THE EVOLUTION OF MAXIMAL BODY SIZE
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
predictions:
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
requirements.
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:
ON THE EVOLUTION OF SIZE IN LIVING SYSTEMS
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
409:1102):
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
diversity.
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
phase).
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