Life History Evolution

1
Life History Evolution

• Outline
• What is a Life history?
• Differences in natural selection over the life
  cycle
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The zero-sum game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu
2
Life History Evolution

• Outline
• What is a Life history?
• Differences in natural selection over the life
  cycle
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The zero-sum game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu
3
Life History Evolution

• What is a life history?
• Period from fertilization (in sexual
  reproducers) through death
• includes embryogenesis, birth, larval/juvenile
  period, sexual maturation,
• breeding and rearing of offspring

Abalone (Gastropod Mollusc)
4
Life History Evolution

• What is a life history?
• Period from fertilization (in sexual
  reproducers) through death
• includes embryogenesis, birth, larval/juvenile
  period, sexual maturation,
• breeding and rearing of offspring

• Diversity in life histories
• Variation in
• number of offspring produced and frequency of
  breeding
• duration of development, larval/juvenile
  period, lifespan
• habitat, food sources, predator types

5
Life History Evolution

• Studying the Evolution of Life Histories Involves
  Measuring Reproductive Success
• Why focus on reproductive success?

• The Perfect Life History
• Predictions
• mature at birth
• continuously produce high-quality offspring in
  large numbers
• live forever
• Reality
• none of the above. Why not?

6
Life History Evolution

• Outline
• What is a Life history?
• Differences in natural selection over the life
  cycle
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The Zero-Sum Game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu
7
Team Spawns Rare White Abalone It's a Girl!
It's a Boy! It's 6 Million Baby Mollusks! USGS
News Release, April 30, 2001
Female
Male
Trochophore larva

0.5 mm
Adult
Veliger larva
1 mm
15 mm
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A Traits Effect on Fitness May Change Over the
Life Cycle
TRAIT Ciliary Bands
0.5 mm
Trochophore larva
Veliger larva
1 mm
Adult
Natural selection on a trait may change during
the life cycle
15 mm
9
Life History Evolution

• Outline
• What is a Life history?
• Differences in natural selection over the life
  cycle
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The zero-sum game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu
10
Tradeoffs in Life History Life-time reproductive
success vs. single season success

• How many offspring should an individual produce
  in a given year?
• Lacks Hypothesis Natural selection will
  favor the clutch size that produces
• (birds) the most surviving
  offspring
• Explicit assumptions of Lacks hypothesis
• (1) Offspring are all the same size (identical
  energy investment)
• (2) Probability of individual offsprings
  survival decreases with increasing
• clutch size

11
Tradeoffs in Life History Life-time reproductive
success vs. single season success

• Lacks Hypothesis Natural selection will favor
  the clutch size that produces
• the most surviving offspring
• The number of surviving offspring is the product
  of clutch size and the probability
• of offspring survival (CS x P)

12
Tradeoffs in Life History Life-time reproductive
success vs. single season success

• Lacks Hypothesis, continued
• Boyce and Perrins tested Lacks hypothesis in
  Great Tits in Wytham Wood (U.K.)
• Observations
• (1) Mean clutch size over 40 year period 8.5
  offspring
• (2) But, for average number of surviving
  offspring from clutches of each egg size
• the highest survival was for clutches of 12
  eggs

Natural selection favors larger clutches than the
Great Tits in Wytham Wood actually produce. Why?
13
Tradeoffs in Life History Life-time reproductive
success vs. single season success

• Lacks Hypothesis, continued
• Implicit assumptions of Lacks hypothesis
• (1) No tradeoff between a parents reproductive
  effort in one year and its
• survival or reproductive success in future
  years
• (2) The only effect of clutch size on offspring
  is in determining whether the
• offspring survive

Frequently invalid in collared flycatchers,
inverse relationship between size of clutch the
female was raised in the size of her clutch
Clutch size may also affect reproductive
performance of the offspring
14
Life History Evolution

• Outline
• Life history in a nutshell
• Age-related differences in natural selection
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The Zero-Sum Game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu
15
Tradeoffs in Life History Offspring Size vs.
Number
If offspring size can vary (over evolutionary
time), what is the ideal size of offspring?
Tradeoff between offspring size and the number
that can be produced
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Tradeoffs in Life History Offspring Size vs.
Number

• What is the ideal compromise between offspring
  size and the number produced?
• Two assumptions of analysis by Smith and
  Fretwell

17
Tradeoffs in Life History Offspring Size vs.
Number
What is the ideal compromise between offspring
size and the number produced?
Number of offspring
Survival probability for an individual offspring
Size of individual offspring
Size of individual offspring
The expected fitness of a parent that produces
offspring of a particular size is, number of
offspring parent can make x probability than an
offspring will survive
18
Tradeoffs in Life History Offspring Size vs.
Number
What is the ideal compromise between offspring
size and the number produced?
Predicted offspring size
Parental fitness achieved with this clutch
Testing the prediction Egg size in the
side-blotched lizard
Size of individual offspring
Assumption (1) Valid
19
Tradeoffs in Life History Offspring Size vs.
Number
What is the ideal compromise between offspring
size and the number produced?
Predicted offspring size
Parental fitness achieved with this clutch
Testing the prediction Egg size in the
side-blotched lizard
Size of individual offspring
20
Tradeoffs in Life History Offspring Size vs.
Number
What is the ideal compromise between offspring
size and the number produced?
Unexpected result of Smith and Fretwells
analysis seen in lizard experiments
1989 Season
Probability of offspring survival
Natural selection on parental fitness often
favors offspring smaller than the size favored
by selection on offspring fitness
Mothers fitness
Egg size
21
Life History Evolution

• Outline
• Life history in a nutshell
• Age-related differences in natural selection
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The Zero-Sum Game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu
22
Sub-optimal Life Histories

• Life histories are not perfect and need not be
  optimal. Why not?
• organisms lack the time or variation to evolve
  toward an optimum
• fundamental limits on the ability of populations
  to
• evolve an optimal life history

• Deceive pollinator because they offer no nectar
  reward in return for
• pollination!!

• Only naïve pollinators (newly emerged queen
  bumble bees) visit
• the orchids
• These bees visit the plant only once!

23
Sub-optimal Life Histories
Life histories are not perfect and need not be
optimal. Why not?

• Deceive pollinator because they offer no nectar
  reward in return for
• pollination!!

• Only naïve pollinators (newly emerged queen
• bumble bees) visit the orchids
• These bees visit the plant only once!

• Result of life history strategy (Study by Gill)
• 1. In 10 years, a population of orchids produced
• 895 flowers
• Of these, only 20 (2) produced a fruit capsule!

Several life history mutations could increase
fruiting success in these orchids 1.
Self-pollination (the flowers are self-fertile,
although they do not self-pollinate) 2. Offer
nectar rewards (to attract repeated visits by
bees)
Similar life histories (self-pollination, nectar
rewards) have evolved in other orchids. Life
history of Pink Lady Slipper is sub-optimal. Why
does it persist?
24
Life History Evolution

• Outline
• What is a Life history?
• Differences in natural selection over the life
  cycle
• ? a traits effect on fitness may change
  during the life cycle
• Tradeoffs in life histories (The zero-sum game)
• ? Reproductive success (lifetime vs.
  single season)
• ? Offspring size vs. number
• -- parent-offspring conflicts
• Sub-optimal life histories
• ? Pink ladys slipper orchids

Joe Thompson joethomp_at_email.unc.edu