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

- a key concept in evolutionary ecology - 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? Limiting energy or materials. Increased allocation of energy or material to one function, reduce allocation to another function. 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.

Trait Dimension 1 Trait Dimension 2

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 4) A size-size tradeoff adipose deposits Residual 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).

Bald eagle nest Pheasant 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).

An oak forest A corn field in spring Typical weed seeds 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 non-pygmy 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: Species evolve to make maximal use (fitness) of available resources and metabolic limitations. Different fitness components therefore compete for resources and energy. This generates evolutionary tradeoffs. Between species, one can observe tradeoffs as correlated variation among traits. Species take different positions along the tradeoff axes due to differences in their ecological circumstances (resource abundance, risks, stability).