Life Tables represent life histories

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Life Tables represent life histories The life
history of a species comprise the timing of its
life cycle events, particularly as relating to
growth, fertility and death. Life histories are
subject to evolution. Thus, species should
optimize life cycle events to maximize fitness in
a given environment.
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R0, the net reproductive rate, is a fitness
estimator
R0 S l(x)b(x)

• Time of first reproduction?
• mature fast and have babies (lay eggs) right away

• Survivorship after reproduction?
• high, robust and strong mature body

• Fecundity in successive years?
• high many offspring every year

• Offspring survivorship?
• high large newborns, fed and protected by their
  parents

• Longevity?
• high become very old and reproduce till the end

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Tradeoffs - a key concept in evolutionary ecology
-
An evolutionary tradeoff between two traits
exists when an increase in fitness due to a
change in one trait is opposed by a decrease in
fitness due to a concomitant change in the second
trait.
Example An increase in growth rate may lower
the risk of mortality at a young age BUT The
necessary allocation of resources to growth
lowers the allocation of resources to
reproduction, thus lowering fecundity.
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Tradeoffs in life history evolution
1) A tradeoff between growth and reproductive
output
Douglas fir the more cones are produced, the
thinner the annual ring.
(Eis et al. 1965)
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Tradeoffs in life history evolution
2) A number-survivorship tradeoff for offspring
Australian copperhead snake babies from larger
clutches die more often that of babies from
smaller clutches. (Both axes corrected for the
mothers size).
(Rohr 2001)
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Tradeoffs in life history evolution
3) A tradeoff between survivorship and
reproduction
Mammals species that breed early, have a shorter
life span. (Both axes corrected for differences
in female body size)
(Harvey and Zammuto 1985)
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Tradeoffs in life history evolution
4) A size-number tradeoff for offspring
64 grassland species species that produce larger
seeds produce fewer.
(Coombs and Grubb 2003)
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Ecological circumstances determine which
strategy is best.
Death valley Hot dry most of the time, but
reliable winter rains!
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Spring in Death Valley
Annuals germinate and spring and can set seeds
within weeks. The drier the desert, the more
annual plants there are.
General Principle When adult survivorship is
low, the species should reproduce only once in a
lifetime (annuals). When adult survivorship is
high, the species should reproduce repeatedly
(shrubs).
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Bald eagle nest
Pheasant nest
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General Principle When newborns are safe, brood
size is small and parental investment is high
(tree-top breeding eagle). When newborns are
unsafe, brood size is large and parental
investment is minimal (ground-breeding pheasant).

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An oak forest
A corn field in spring
Typical weed seeds
Acorns
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General Principle In an unstable environment
(population size ltlt K), organisms breed early and
have many small offspring (thus maximizing r
weeds). In stable environments (population size
K), organisms breed late and have few large
offspring (thus maximizing competitive ability
trees) These strategies are often called
r-selected (weed) versus K-selected (tree).
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Pygmyism
Pygmy elephants of Borneo
Pygmy hippo of West Africa
Pygmy possum of Australia
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Pygmyism
Baka pygmies of Africa
Batak pygmies of the Philippines
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Why are the pygmies so short?
Bamberg Migliano, Vinicius, Mirazon Lahr 2007.
Life history trade-offs explain the evolution of
human pygmies. PNAS 104 20216-20219
non-pygmies
pygmies
Pygmies initially grow at a similar rate but stop
growing around age 14 rather than 18.
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Why are the pygmies so short?
Bamberg Migliano, Vinicius, Mirazon Lahr 2007.
Life history trade-offs explain the evolution of
human pygmies. PNAS 104 20216-20219
non-pygmies
pygmies
chimps
Pygmies have much lower survivorships. Their life
expectancy at birth is between 16 and 24,
compared to 34 to 48 in non-pygmy hunter-gatherers
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Why are the pygmies so short?
Bamberg Migliano, Vinicius, Mirazon Lahr 2007.
Life history trade-offs explain the evolution of
human pygmies. PNAS 104 20216-20219
non-pygmies
pygmies
Pygmy fertility peaks at ages 20-24 compared to
30-34 in non-pygmy hunter-gatherers.