BIOL 3445 Lecture Notes - Lecture 22: Antagonistic Pleiotropy Hypothesis, Dementia, Fetus
BIOL 3445.001 | Lecture #22 | 4/5/2018
Life History Evolution
OBJECTIVES
― Explain why organisms might experience trade-offs in life history strategies
― Why might selection on a life history trait change over an individual’s lifetime and
in different environments?
― Why and how do parents and offspring adjust their strategies to achieve optimal
fitness
― Why do we grow old?
LIFE HISTORY TRADE-OFFS
― Life History: the pattern of investment in and timing of major “life events”
o Growth rate
o Age at first reproduction
o Reproductive schedule (when do you reproduce)
o Number of offspring produced per breeding season
o Life span
― Selection on life history traits can be understood as a trade-off between costs
and benefits of different strategies
o Is it better to grow fast and reproduce quickly?
o Is it better to invest in a long life and slower reproduction?
― We can also think about the use of finite resources for either survival or
reproduction
o Energy you in invest in yourself is energy you can’t invest in your offspring
and vice versa
o Invest energy in repair of the body (at the cost of current reproduction) or
producing more offspring (at the cost of repair)
SELECTION OVER A LIFETIME
― Selection can favor different traits over the course of an individual’s lifetime
o Mutations that are only deleterious late in life may not be selected against
o Mutations that are beneficial early in life are more strongly selected for
o Generally, traits that are expressed earlier in life experience stronger
selection
― Group Exercise: Name a positive, early-onset trait and a negative, late-onset trait
and explain why they are expressed when they are.
o Early Onset:
▪ Vision – most young are born blind or with poor vision so an
organism who can gain sight quickly will have an advantage
▪ Age of Reproduction – the earlier you reproduce, the better
▪ Lactose Tolerance – being able to breakdown lactose helps you
grow quickly so the longer you keep this trait, the larger you’ll be
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BIOL 3445.001 | Lecture #22 | 4/5/2018
o Late Onset:
▪ Adult onset dementia – losing memory and function is not good but
it doesn’t occur until later in life so it is hard to select against
▪ Huntington’s – genetic disease that affects people after their 30s
▪ Adult onset diabetes – doesn’t appear until later in life, after
people are done having children
― Mutations can also show antagonistic pleiotropy across life stages
o A single trait that is beneficial when young can have detrimental effects
when older
o We can often think of this an extension of the conflict between fertility
and longevity
o For example, producing at a young age means you have a longer time to
reproduce over your lifetime, but it means you’ll wear out and age faster
― Example: Field Mice
o 90% of mice will be eaten before their first birthday
o DNA and protein repair and producing anti-oxidation molecules is costly
o On average, mice will not benefit much from any investment in these
repairs past their first year
o Instead, they gain more fitness by investing that energy into reproduction
o Mostly likely to be eaten in a year anyways, so there is no point in investing
in body repairs because it’s a waste of energy
― Example: Sapelo Island
o Creepy island with a 1950s mansion
▪ Used to belong to a tobacco mogul
▪ Weird pleasure dungeon in the basement
o Completely secluded from the mainland so the animals act strangely
▪ There are no predators on the island, so the animals live without
fear
▪ On the mainland, the possums are eaten by everything but here
they can live freely
o Mainland possums have selected the live fast, die young strategy
▪ Have as many babies as possible in the first two years of life
because their more likely to get eaten
o Island possums aren’t at risk of dying from predation, so they age more
slowly and have longer lifespans
▪ Able to invest energy in body repairs
▪ Able to have fewer babies per litter and more time between litters
― Example: Guppies
o In low predation environments, they grow slower, mate later, have
fewer/larger offspring, and live longer
o In high predation environments, they grow quickly, mate earlier, have
many small offspring, and live quickly
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find more resources at oneclass.com
