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Chapter 8 (Overlapping generations)

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Overlapping Generations
Chapter 10
First Thoughts
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Potentially important
Limited theoretical and empirical work
Weak selection / confounding factors /
alternative explanations
Basic Idea
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If standard age distribution is disturbed (by
predictable / unpredictable events) and
there are overlapping generations,
facultative adjustment of the sex ratio may
be adaptive.
RS of males and females vary during their
life (males often peak, females more even)
Two types of Theoretical model
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Stable-age distribution disturbed by:
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Exceptionally high/low mortality (or
recruitment)
Degree of generational overlap varies
cyclically (by season) and differs between the
sexes.
Can lead to temporary difference between
sexes in competition for resources
1. Exceptionally high/low mortality
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A. Impacting one Sex (Werren & Charnov,
1978)
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affected sex
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overlap and fitness benefits needed for bias to
be favoured.
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– alternative explanations?
Exceptionally high/low mortality
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B. Impacting both sexes equally (West, 1997)
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Can occur if reproductive value is distributed
differently in each sex.
After mortality peak, there is selection for the
sex with highest old-age RV.
But: if parent can predict mortality and adjust
sex allocation beforehand, favours sex with
highest young-age RV. (Effect less/negated if
life history is one with RV concentrated in oldage)
No empirical work.
C. Temporary exceptional recruitment
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Affecting a single cohort (generation)
Above average: sex with most evenly
distributed RV favoured.
Below average: sex with most uneven RV?
Bias reverses in the subsequent generations
If females mate once (and store sperm)
male RV more evenly spread.
2. Degree of generational overlap
varies cyclically
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E.g. Spring/Autumn broods. Spring birth:
breed autumn and die in winter. Autumn
birth: overwinter (some die), the breed
spring and some survive to breed autumn
then die. Favours males in autumn, females
in spring.
Generally: sex which has greatest overlap
with future generations favoured
Seger’s models (Seger, 1983)
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Sphecid
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Spring and summer generations. Overlap as
spring males mate with summer females and
some offspring of spring females overwinter.
As overlap increases, bias for males in spring
and females in summer.
Seger’s models (Seger, 1983)
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Halcitine
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Females hibernate, lay eggs in spring, hatch
in summer, second generation emerge in
autumn. No males survive winter. Some
hibernating females delay development until
autumn, some summer males survive to mate
with autumn females.
Empirical evidence mixed. Often males do
not survive to mate with next generation.
Cyclical models and Eusociality
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Need female biased sex ratio (haplodiploidy)
Segler’s models show possible – female bias in
first generation favoured. Offspring can choose
to help mother reproduce
Other explanations: differential diapause, or
variation in cost/benefit of producing sexes (if
offspring from overlapping generations breed
together) or virginity
Concluding remarks
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Theory limited, empirical testing even
more limited
Relevance and prevalence of OG unclear.
Cyclical patterns in sex allocation may be
unrelated to overlapping generations.
Weak selection? assessment mechanism?
Need species-specific models
Very interesting idea, but its unproven.
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