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
Tradeoffs
- a key concept in evolutionary ecology -
+
= false
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.
Why should there be tradeoffs?
1) Limiting energy or materials. Increased allocation of energy
or material to one function, reduce allocation to another
function.
2) Traits evolved by natural selection of the fittest
phenotype. Traits are already organized to give organisms
peak fitness, maximizing the use of energy and materials.
Tradeoffs in life history evolution
1) 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)
Tradeoffs in life history evolution
2) A size-number tradeoff for offspring
Across 64 grassland species, species that produce larger seeds produce
fewer seeds.
(Coombs and Grubb 2003)
Tradeoffs in life history evolution
3) A size-growth tradeoff
Among mammals, excluding humans, species with larger brains relative to body mass,
have lower population growth rates (rmax ). The relationship is weaker in species with
cooperative breeding (precocials)
(Isler&Sckaik, 2012)
Tradeoffs in life history evolution
Residual
adipose deposits
4) A size-size tradeoff
Residual brain mass
Among mammals, but excluding humans, species with more fat tissues
have smaller brains relative to body size.
(Navarrete et al. 2011)
Implication for human evolution
of large brains
Brains are “expensive tissues”. In many animals they
cannot grow bigger without decreasing the size of other
organs (gut, gonads, etc).
= gray ceiling
However, some groups of animals overcome the
prohibitive cost of larger brains by
sharing costs among group members
reducing metabolic expenditures (upright walk)
Ecological circumstances determine
which strategy is best.
Death valley
Hot & dry most of the time, but reliable winter rains!
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 risks to adults are high, species
should invest less in traits that
increase survivorship (annuals).
When risks to adults are few, species
should invest in surviving and
reproducing repeatedly (shrubs).
Pheasant nest
Bald eagle nest
General Principle:
When newborns are safe, species can invest
more in parental care, at the cost of reduced
brood size (tree-top breeding eagle).
When newborns are unsafe, species should
invest more in spreading the risk by increasing
brood size, at the cost of reduced parental care
(ground-breeding pheasant).
A corn field in spring
Typical weed seeds
An oak forest
Acorns
General Principle:
In an unstable environment (population size << K), organisms invest in
breeding early and many small offspring (thus maximizing r: weeds).
In stable environments (population size ≈ K), organisms invest in
provisioning for their offspring (large seeds), at the cost of producing fewer
offspring (thus maximizing competitive ability: trees)
These strategies are often called r-selected (weed) versus K-selected
(tree).
Pygmyism
Pygmy elephants of Borneo
Pygmy hippo of West Africa
Pygmy possum of Australia
Pygmyism
Baka pygmies of Africa
Batak pygmies of the Philippines
Why are the pygmies 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.
Why are the pygmies short?
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
Why are the pygmies short?
non-pygmies
pygmies
Pygmy fertility peaks at ages 20-24 compared to 30-34 in nonpygmy hunter-gatherers.
5) A size-growth tradeoff
Pygmies are short because they live in a
dangerous environment. To ensure that enough
females live to reproduction, pygmies reach
sexual maturity earlier, at the cost of reduced
allocation of limited resources to growth.
Short stature is therefore not the goal of adaptive
evolution, hastened maturity is, short stature is the
price paid.
Summary:
1. Species evolve to make maximal use (fitness) of available
resources and metabolic limitations.
2. Different fitness components therefore compete for resources and
energy. This generates evolutionary tradeoffs.
3. Between species, one can observe tradeoffs as correlated
variation among traits.
4. Species take different positions along the tradeoff axes due to
differences in their ecological circumstances (resource
abundance, risks, stability).