Fitness: an individual's proportional representation in the gene pool of subsequent generations
Fitness can only be assessed relative to the fitness of other individuals
Two main factors that determine whether an individual has high or low fitness:
o Reproduction
o Survival
Direct Fitness: determined by the number of offspring an organism produces (and that survive to
maturity) over its entire lifetime
o Often involves trade-offs among present and future reproduction
Life History: the timing and duration of key events during a lifetime (age and duration of reproduction,
number and size of offspring produced, lifespan)
Central theme of life history evolution: there are trade-offs among these components of fitness and
the ways they are molded by natural selection
Mate or Wait
Can mate now
Can grow bigger and have more offspring later
Future reproduction is never certain. Waiting to reproduce may result in zero fitness.
This life history trade-off can be represented by:
Current Reproductive Success (i.e. Mating now) = # of offspring
Future Reproductive Success (i.e. Waiting to mate) = # of offspring x likelihood (probability) of
survival
o Example
If you have 90% change of death each year, and are able to produce 5 offspring per year,
and you must die after 5 years (if you have not been already killed)…
(5*1) + (5*.1) + (5*.01) + (5*.001) + (5*.0001) = 5.6 offspring on average
Senescence ("aging"): a decline with age in per capita reproductive performance, physiological function,
or the probability of survival
As organisms age, they decline in some important component of fitness
Selection can strongly favor traits that help organisms survive and reproduce early in life.
Selection is weaker on traits that favor later life survival and reproduction. Selection is absent or
very weak on post-reproductive traits.
o Ex) humans
Antagonistic Pleiotropy: a genetic variant with a beneficial effect on one trait has a detrimental effect
on some other trait
Ex) p53 tumor suppressor - beneficial early in life, but can lead to intrinsic mortality later in life
Guppies in Trinidad Example
Above waterfalls: low predation, high survival
o Have fewer offspring per brood, devoting more energy to growth
o Offspring have high birth weights, because parents can invest large amounts of energy in each
offspring
o Less energy into reproduction, more energy into growth
Below waterfalls: high predation, low survival
o Have a lot of offspring per brood and reproduce quickly
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Document Summary

Fitness: an individual"s proportional representation in the gene pool of subsequent generations. Fitness can only be assessed relative to the fitness of other individuals. Mate or wait: can mate now, can grow bigger and have more offspring later. Waiting to reproduce may result in zero fitness. This life history trade-off can be represented by: current reproductive success (i. e. mating now) = # of offspring. Future reproductive success (i. e. waiting to mate) = # of offspring x likelihood (probability) of survival: example. Senescence (aging): a decline with age in per capita reproductive performance, physiological function, or the probability of survival: as organisms age, they decline in some important component of fitness. Selection can strongly favor traits that help organisms survive and reproduce early in life. Selection is weaker on traits that favor later life survival and reproduction. Selection is absent or very weak on post-reproductive traits: ex) humans.

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