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Power Laws in Biology
Stephanie Forrest Introduc9on to Scien9fic Modeling
CS 365, 2012
On Growth and Form • D’Arcy Thompson “On Growth and Form” (1917)
– Structuralism vs. Survival-‐of-‐the-‐FiUest – Structuralism: Physical laws govern the form of species, in addi9on to
evolu9on – AUempted to account for differences in the forms of related animals
could be described by means of rela9vely simple mathema9cal transforma9ons
– Style was descrip9ve
Transforma9on of Argyropelecus into Sternoptyx diaphana by applying a 20° shear mapping
Allometry: The study of rela9onships between body size and shape
Metabolic Scaling Theory A general theory for the origin of allometric scaling
laws in biology (1997)
In biology, bigger networks are slower when they are centralized"
Kleiber’s Law
Hemmingson 1960
3/4
Observed Metabolic Scaling: ""B ∝ M3/4"
"B is the rate of energy (oxygen) use"B: the master biological rate governs"
"ecological interactions""food webs & ecosystem dynamics""growth and reproduction"
"Mass Specific Scaling"Other biological rates ∝ M-1/4"
Biological times ∝ M1/4"
“There is a unity of the single system of energy, ecology, and economics…
let us here seek common sense overview
which comes from overall energetics.”
H. T. Odum (1973)
Organisms have evolved networks to distribute energy efficiently
Social insects use networks to acquire energy and communicate
Global Shipping Routes Halpern et al Science 2008
Human engineered networks span the globe
Metabolic Scaling Theory
"Bigger organisms require bigger networks"• Pipe lengths (L) are longer"• Cross sectional areas (A) are larger"• # of capillaries increases slower than pipe volume"
"N = cV3/4"
!Metabolism: B = cM3/4"""""Increasing volume (mass) 100 times increases delivery rate 30 times!""Diminishing returns: Network size grows faster than network delivery rate"""
Elements of the Theory Metabolic Ecology: A Scaling Approach (2012)
• All cells need nutrients and oxygen – Delivered by an internal space-‐filling, hierarchical (fractal) distribu9on
network – This assump9on has been adjusted in later versions of the theory
• The final branch of the network (capillary) is constant size, independent of organism size (invariant terminal units) – This assump9on has been adjusted in later versions of the theory
• The energy required to distribute resources is minimized (network design is op9mized)
• Network is area preserving at every level of hierarchy
Network scaling concepts
• Network volume (Vnet) increases faster than number of capillaries (Nc) [1] Vnet ∝ Nc
4/3
Diminishing returns: Network volume grows faster than delivery rate A network 100 9mes bigger delivers only 30 9mes more blood per unit 9me • Each capillary is the same: B ∝ Nc
[2] Vnet ∝ B4/3
• Biological constraint, blood volume is a constant % of mass: Vnet ∝ M
[3] B ∝ M3/4 Controversy, but accepted that centralized distribu9on networks generate
Vnet ∝ NcD+1/D
Network scaling accurately predicts rates and times
• Physiology"• Individual Growth"• Population growth"• Reproduction"• Disease spread"• Lifespan"• Photosynthesis & carbon flux "• …"
Biomass Produc9on: P∝M 3/4
Physiological Rates: B∝M −1/4
-1/4!
Scaling 9mes associated with disease
Time to first symptoms! Time to death!
1/4!
Metabolic scaling determines growth rates
€
Emdmdt
= B0m3 / 4 − Bmm
Rate energy is available for growth = incoming metabolic rate -‐ maintenance metabolic rate
West et al 2001 Moses et al 2008
(m/M
)1/4
T=(at/4M)1/4-‐ln(1-‐(m0/M)1/4)
1-‐e-‐T
Metabolic scaling during growth: B∝M 3/4
Life9me reproduc9ve effort: % of mother’s mass invested in offspring
Predicted LRE = 1/δ = 4/3 (in theory) δ = 0.7 in prac>ce
A female lizard lays 1.4 times her own body mass in eggs (hatchling mass)"A female mammal raises to weaning offspring totaling 1.4 times her body mass"
What about computa9onal networks?
• Modern microprocessors contain ~1 billion transistors
• operate at power densi9es (W/m2) approaching a nuclear blast
• Wire-‐scaling drives increased power on single core chips
Predic9ng & Minimizing Power Requirements of Microprocessors
Scaling in compu9ng (Moses and Forrest)
Summed area of transistors on a chip as a function of die area (observed b = 0.53, predicted b = 1/2, data from www.icknowledge.com/history/history.html
Internet backbone bandwidth as a function of the processing power of Internet hosts (observed b = 0.66, predicted b = 2/3, data from www.isc.org/index.pl?/ops/ds/host-count-history.php and www.zakon.org/robert/ internet/timeline/
Challenges for a MST Theory of Computer Chips
• Distributed networks: Computer networks are not centralized like the vascular system
• Density dependence – MST assumes cells are constant size – Transistor densi9es have increased exponen9ally over 9me from thousands to millions of transistors per sq. mm.
• The last mile: network delivers resource to a service unit and then it is delivered to des9na9on using other methods – E.g., the isochronic region in chips
• Superlinear scaling: The wire scaling problem
Clock trees: Scale like circulatory networks
Hierarchical Space filling Fractal branching Varia9on in length of terminal wires Aclock ∝ NAchip
1/2
Decentralized vs. Centralized Networks
Computer Chips
0.5 1 1.5 2 2.50.5
1
1.5
2
2.5
log10
Predicted Power From Decentralized Modello
g10 O
bse
rve
d P
ow
er
€
P ∝λV 2Ntr1/ 2Achip
1/ 2 fSlope = 0.95!
-4 -3 -2 -1 0 1 2 3 4!1.5
!1
!0.5
0
0.5
1
1.5
2
2.5
log
ob
serv
ed
po
we
r
log predicted power
€
P ∝λV 2NtrAchip1/ 2 f
Slope = 0.5!
Each Wire length depends on radius: N * (A1/2)
Each Wire length depends on distance between nearest components: N * (ρ-‐1/2)
Decentralized vs. Centralized Networks
Centralized Network Prediction " " Decentralized Network Prediction"
Road Networks
Hierarchical Fractal Networks
Aorta: k = 0 Capillaries: k = K
Area preserving branching
€
rkrk+1
= b1/ 2
Space-‐filling branching
€
lklk+1
= b1/ 3
Metabolic rate B~ Nc Nc = bK
€
Nc = bK
lK = lcrK = rc
€
N0 =1r0 ~ rcb
K / 2
l0 ~ lcbK / 3
€
Vnet = πr02l0 b− i / 3
i= 0
K
∑
Vnet ∝ Nc4/3
b is the branching ra9o
LRE =(offspring/year)*(offspring mass at independence)*(adult lifespan) / (adult mass)
dm/dt: Simple growth & produc9on model δ: metabolic exponent R0: Fitness=total # of offspring per life9me α: age at 1st reproduc9on S: prob of living to age α E: adult lifespan Maximize R0: set deriva9ve wrt α = 0 Z: Instantaneous mortality rate Z= 1/E
Calcula9ng Op9mal Life9me Reproduc9ve Effort
__ __ __ __ __
≈ 4/3 Alterna9ve growth model