Offspring Number/Size
Offspring Number/Size
• Trade-offs
• Conflicts of interest
http://eebweb.arizona.edu/courses/ecol335/Spring%202006%20active/Lectures/Lecture%205(20)%202006_4.pdf#search=%22offspring%20size%20number%20lecture%22
“Perfect Organism”
- Sexually mature at birth
- Produces infinite number of offspring of same quality as
itself
- Reproduces constantly
- Immortal
But no such organism exists
Mass and energy are not infinite
Biological processes are not instantaneous
Physiological and evolutionary TRADE-OFFS
Ecological context (competition, density-dependence)
Energy and Time
“Perfect Organism”
- Sexually mature at birth
- Produces infinite number of offspring of same quality as
itself
- Reproduces constantly
- Immortal
But no such organism exists
Mass and energy are not infinite
Biological processes are not instantaneous
Physiological and evolutionary TRADE-OFFS
Ecological context (competition, density-dependence)
Energy and Time
What can change within life history to
maximize fitness:
1) Higher survival (lx) through
reproductive ages
2) Higher fecundity (mx) at each
reproductive age
3) Higher fecundity (mx) earlier in life
4) Longer reproductive lifespan
5) Earlier age of first reproduction
Types of Constraints
• Phylogenetic – Evolutionary history
• Genetic/Genetic variation; VT = VG x VE x VGxE
• Antagonistic pleiotropy – one gene multiple effects, one positive, one
negative
• Physiological: Trade-offs - Bioenergetics
Offspring Number/Size
How many offspring should an individual produce in a given year?
• David Lack first placed life histories in an evolutionary context
(for animals with parental care)
Lack’s (1947) Hypothesis – Natural selection will favor the clutch size that produces
(birds)
the most surviving offspring
• Explicit assumptions of Lack’s hypothesis
(1) Offspring are all the same size (identical energy investment)
(2) Probability of individual offspring’s survival decreases with increasing
clutch size
Supported by experiments in which researchers
added eggs to nests
www.unc.edu/~welcha/bio665/lifehistory.ppt
Lack’s model
Lack’s 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)
Number of
Surviving
Offspring
(CS x P)
Probability
of survival
for individual
offspring (P)
Clutch Size (CS)
Clutch Size (CS)
Prediction: natural selection will select for intermediate clutch size
www.unc.edu/~welcha/bio665/lifehistory.ppt
Lack’s model
– Increasing clutch size would increase reproductive success unless
survivorship of offspring was reduced
– Hypothesis: amount of food large broods require may exceed food
parents can gather, thereby reducing survivorship of offspring
• Lack suggested that one could artificially increase the number of eggs
per clutch to show that the number of offspring is limited by food
supply.
• This proposal has been tested repeatedly
– Eg. European magpies.
– Magpies have a clutch size that
corresponds to the maximum number
of young they can rear
http://www.biology.ualberta.ca/courses/biol208/uploads/fall05/lectures/Lecture_8%20_6_slides_per_page.pdf#search=%22age%20and%20size%20at%20maturity%20lecture%22
Lack’s model
Lack’s Hypothesis, continued
• Boyce and Perrins (1987) tested Lack’s hypothesis in Great Tits
• 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?
CS = clutch size; P= prob. Surv. Indiv. offspring
www.unc.edu/~welcha/bio665/lifehistory.ppt
Lack’s model
Implicit assumptions of Lack’s hypothesis:
(1) No tradeoff between a parent’s reproductive effort in one year and its
survival or reproductive success in future years
Frequently invalid – if given an extra egg in their first year, the clutch
size of female collared flycatchers in future years is lower than in
control females
http://www.biology.usu.edu/courses/biol5250-miller/pdfs/Lecture%2021-22%20Aging%20and%20life%20history%20attributes.pdf#search=%22life%20history%20aging%20lecture%22
www.unc.edu/~welcha/bio665/lifehistory.ppt
Lack’s model
(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 and the size of her clutch
Clutch size may also affect reproductive performance of the offspring
http://www.biology.usu.edu/courses/biol5250-miller/pdfs/Lecture%2021-22%20Aging%20and%20life%20history%20attributes.pdf#search=%22life%20history%20aging%20lecture%22
www.unc.edu/~welcha/bio665/lifehistory.ppt
http://www.biology.usu.edu/courses/biol5250-miller/pdfs/Lecture%2021-22%20Aging%20and%20life%20history%20attributes.pdf#search=%22life%20history%20aging%20lecture%22
Number of inflorescences/plant in 2nd season
Reproductive effort in Poa annua in consecutive years
Survival of adult Parus major after breeding
males
females
No. of inflorescences/plant in 1st season
Law et al 1979
http://www.bchs.uh.edu/~biol3306/fall/EBlect18a.ppt
Nur 1984
Organisms with no parental care
Lack 1947, Smith and Fretwell 1974
http://ic.ucsc.edu/~whs68/bio150/07LifeTableLifeHistory.pdf#search=%22life%20history%20strategies%20age%20size%20at%20maturity%22
Offspring size
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:
Few large offspring
Many intermediate-sized offspring
Thousands or millions of tiny offspring
2 mm
www.unc.edu/~welcha/bio665/lifehistory.ppt
Offspring size vs number trade-off
What is the ideal compromise between offspring size and the number produced?
• Two assumptions of analysis by Smith and Fretwell 1974:
(1) Tradeoff between offspring size and the number produced
(good experimental evidence)
(2) Individual offspring will have a better chance of survival if they are large
Potential conflict between fecundity selection and survival selection
Model applies under varying conditions of parental or sibling care, but
best for organisms with no parental care and large clutch sizes.
www.unc.edu/~welcha/bio665/lifehistory.ppt
Fitness functions:
lines of equal parental
fitness
Smith & Fretwell 1974
Optimal fitness
function: line of
highest slope
Fitness set
Smith & Fretwell 1974
26 fish families
Species in three orders of flies
Chorthippus parallelus
Stearns 1991
Charnov 2002
A study of hatchery-raised chinook salmon by
Heath et al. (2003)
http://www.biology.usu.edu/courses/biol5250-miller/pdfs/Lecture%2021-22%20Aging%20and%20life%20history%20attributes.pdf#search=%22life%20history%20aging%20lecture%22
Effects of juvenile “environmental
quality” on offspring size
http://www.biology.usu.edu/courses/biol5250-miller/pdfs/Lecture%2021-22%20Aging%20and%20life%20history%20attributes.pdf#search=%22life%20history%20aging%20lecture%22
Effects of juvenile “environmental
quality” on offspring size
http://www.biology.usu.edu/courses/biol5250-miller/pdfs/Lecture%2021-22%20Aging%20and%20life%20history%20attributes.pdf#search=%22life%20history%20aging%20lecture%22
Offspring size vs number trade-off
What is the ideal compromise between offspring size and the number produced?
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
www.unc.edu/~welcha/bio665/lifehistory.ppt
Offspring size vs number trade-off
Parental
fitness
achieved
with this
clutch
Predicted
offspring
size
Sinervo et al. 1992
Testing the prediction: Egg size in the side-blotched lizard
Size of individual offspring
Side-blotched lizards:
• desert dwellers
• females lay 1 to 9 eggs
• wild populations show heritable variation in egg size
Testing assumption (1): Tradeoff between offspring size and the number produced
Assumption (1)
Valid
1989 Season
Number of
offspring
1990 Season
Egg size
www.unc.edu/~welcha/bio665/lifehistory.ppt
Offspring size vs number trade-off
Parental
fitness
achieved
with this
clutch
Sinervo et al. 1992
Predicted
offspring
size
Testing the prediction: Egg size in the side-blotched lizard
Size of individual offspring
Side-blotched lizards:
• desert dwellers
• females lay 1 to 9 eggs
• wild populations show heritable variation in egg size
The experimenters manipulated egg size:
• made unusually small eggs by withdrawing yolk in
late-term females
• made unusually large eggs by destroying all but 2 or 3
eggs in early-term females
Testing assumption (2): Individual offspring will have a better chance of survival if they are large
Assumption (2)
Valid for 1989 Probability
of offspring
Season
1989 Season
1989 Season
Mother’s
fitness
Prediction of
Intermediate size
supported
survival
Egg size
Egg size
www.unc.edu/~welcha/bio665/lifehistory.ppt
Offspring size vs number trade-off
Sinervo et al. 1992
Unexpected result of Smith and Fretwell’s analysis seen in lizard experiments:
1989 Season
Selection on Mother
Selection on Offspring
Probability
of offspring
survival
Natural selection on parental fitness often favors
offspring smaller than the size favored by selection
on offspring fitness
Mother’s
fitness
Egg size
May lead to conflict of interest
between parents and offspring
Conflict between fecundity selection and survival selection
www.unc.edu/~welcha/bio665/lifehistory.ppt
Fractional Offspring
•
•
Period of time over which energy is collected for a reproductive
effort not included in Smith & Fretwell (1974) model
How does selection respond when
Erep
OptEoff
•
•
•
•
= fraction
Problem is greatest with smaller clutches
If optimal clutch is 1.5, 33% of the reproductive effort is unused with
a real clutch of 1
If optimal clutch is 20.5, 3% of the reproductive effort is unused with
a real clutch of 20
Increasing developmental rate may allow “rounding” up the #
offspring at small clutches by changing time over which energy is
gathered
Search time of oviposition site
Growth and offspring size