Transcript (Part 2) Life history evolution

BIOE 109
Summer 2009
Lecture 10-Part II
Life history evolution
What is “life history”?
A typical life history of any animal-
Reproduction
ceases
maturity
Birth
Reproduction
Death
What are “life history” characters?
What are “life history” characters?
• from a “fitness” perspective, there are only two important
events in life: reproduction and death.
What are “life history” characters?
• from a “fitness” perspective, there are only two important
events in life: reproduction and death.
• traits that determine the timing and details of these events
are termed life history characters:
What are “life history” characters?
• from a “fitness” perspective, there are only two important
events in life: reproduction and death.
• traits that determine the timing and details of these events
are termed life history characters:
1. Age at first reproduction (time to maturity)
2. Total life span
3. Mode and frequency of reproduction
4. Fecundity (no. of offspring produced)
5. Parental care
6. Viability of off-springs
Darwinian Demon
• Mature at birth
• Reproduce frequently and in large
numbers
• Produce high quality offspring always
• Live forever (at least live very long)
Do Darwinian Demons exist in nature?
Do Darwinian Demons exist in nature?
Thrips egg mite (Adactylidium sp.)
• Mature at birth- yes!
• Produce frequently and in large amounts-No!
• Live forever-No!
X-ray of female brown kiwi
1/6 its weight!!
• Mature at birth- No!
• Produce high quality offspring frequently- yes!
• Live long-sort of! (20 years)
Remember!
• Amount of energy an organism can harvest is finite and
biological processes take time!
• trade-offs between life history traits are unavoidable!
• variation in life-histories are due to differences in the
allocation of energy.
• Organisms that find “optimal balance” between costs and
benefits are favored by natural selection.
• The “optimal” solution might be different in different
environments.
Some life history questions:
1. Why do individuals age and die?
Some life history questions:
1. Why do individuals age and die?
2. How many offspring should an individual produce in any
given year?
Some life history questions:
1. Why do individuals age and die?
2. How many offspring should an individual produce in any
given year?
3. How large should each offspring be?
Why do organisms age and die?
Why do organisms age and die?
• Aging or senescence, is a late life decline in an individual’s
fertility and probability of survival.
Why do organisms age and die?
• Aging or senescence, is a late life decline in an individual’s
fertility and probability of survival.
Reproduction
Survival
Aging reduces fitness, therefore, should be opposed by natural selection
Why do organisms age and die?
• two main theories that try to explain aging are “rate-ofliving” theory and the evolutionary theory.
Why do organisms age and die?
• two main theories that try to explain aging are “rate-ofliving” theory and the evolutionary theory.
1. The “rate-of-living” theory (ROL)
• aging caused by the accumulation of irreparable damage
to cells and tissues.
Why do organisms age and die?
• two main theories that try to explain aging are “rate-of-
living” theory and the evolutionary theory.
1. The “rate-of-living” theory (ROL)
• aging caused by the accumulation of irreparable damage
to cells and tissues.
• organisms have reached their limit of biologically possible
repair.
Why do organisms age and die?
• two main theories that try to explain aging are “rate-ofliving” theory and the evolutionary theory.
1. The “rate-of-living” theory (ROL)
• aging caused by the accumulation of irreparable damage
to cells and tissues.
• organisms have reached their limit of biologically possible
repair (they have been already selected enough!)
• populations lack the genetic variation that would enable
them to evolve more effective repair mechanisms
The ROL theory makes two important predictions:
The ROL theory makes two important predictions:
1. Lifespan should correlate negatively with metabolic
rate.
The ROL theory makes two important predictions:
1. Lifespan should correlate negatively with metabolic
rate.
Metabolic rate
• predicts that by-products of metabolism cause cell and tissue
damage (and ultimately death).
Lifespan
Testing the rate-of-living theory
amount of energy expended/gram of tissue in lifetime
Wide variation of energy expenditures among mammals!
The ROL theory makes two important predictions:
1. Lifespan should correlate negatively with metabolic
rate.
• predicts that by-products of metabolism cause cell and tissue
damage (and ultimately death).
2. Longevity should not respond to selection.
The ROL theory makes two important predictions:
1. Lifespan should correlate negatively with metabolic
rate.
• predicts that by-products of metabolism cause cell and tissue
damage (and ultimately death).
2. Longevity should not respond to selection.
• predicts no genetic variation should exist for genes
increasing longevity.
Selection for increased life span in Drosophila
Luckinbill et al. 1984
The ROL theory makes two important predictions:
1. Lifespan should correlate negatively with metabolic
rate. NO
• predicts that by-products of metabolism cause cell and tissue
damage (and ultimately death).
2. Longevity should not respond to selection. NO
• predicts no genetic variation should exist for genes
increasing longevity.
2. The evolutionary theory of aging
2. The evolutionary theory of aging
• aging and death caused by incomplete repair of cell and
tissue damage throughout life that leads to gradual decay
and eventually complete collapse of metabolic system.
2. The evolutionary theory of aging
• aging and death caused by incomplete repair of cell and
tissue damage throughout life that leads to gradual decay
and eventually complete collapse of metabolic system.
Why repair is incomplete?
2. The evolutionary theory of aging
• aging and death caused by incomplete repair of cell and
tissue damage throughout life that leads to gradual decay
and eventually complete collapse of metabolic system.
Why repair is incomplete?
1. Late onset of deleterious mutations
2. The evolutionary theory of aging
• aging and death caused by incomplete repair of cell and
tissue damage throughout life that leads to gradual decay
and eventually complete collapse of metabolic system.
Why repair is incomplete?
1. Late onset of deleterious mutations
• many examples known in humans (Huntington’s, some
cancers), Drosophila, houseflies.
2. The evolutionary theory
• aging and death caused by incomplete repair of cell and
tissue damage throughout life that leads to gradual decay
and eventually complete collapse of metabolic system.
Why repair is incomplete?
1. Late onset of deleterious mutations
• many examples known in humans (Huntington’s, some
cancers), Drosophila, houseflies.
2. Trade-offs between fecundity and longevity
2. The evolutionary theory
• aging and death caused by incomplete repair of cell and
tissue damage throughout life that leads to gradual decay
and eventually complete collapse of metabolic system.
Why repair is incomplete?
1. Late onset of deleterious mutations
• many examples known in humans (Huntington’s, some
cancers), Drosphila, houseflies.
2. Trade-offs between fecundity and longevity
• genes that have a beneficial effect early in life but a
detrimental effect later are said to exhibit “antagonistic
pleiotropy”.
Evidence for antagonistic pleiotropy:
the methuselah locus in Drosophila
How many offspring should an individual
produce in a given reproductive attempt?
How many offspring should an individual
produce in a given reproductive attempt?
-produce 10-50 million eggs
-diameter= 50-55 micrometers
-produce 100 eggs
-diameter= 300 micrometers
How many offspring should an individual
produce in a given reproductive attempt?
• individuals of some species reproduce only once and then
die (e.g. pacific salmon).
• this is called semelparity.
How many offspring should an individual
produce in a given reproductive attempt?
• individuals of some species reproduce only once and then
die (e.g. pacific salmon).
• this is called semelparity.
• individuals of most species reproduce several times
throughout life.
• this is termed iteroparity.
How many offspring should an individual
produce in a given reproductive attempt?
• individuals of some species reproduce only once and then
die (e.g. pacific salmon).
• this is called semelparity.
• individuals of most species reproduce several times
throughout life.
• this is termed iteroparity.
• for iteroparous species, how many offspring should an
individual produce in a given attempt?
How many offspring should an individual
produce in a given reproductive attempt?
• individuals of some species reproduce only once and then
die (e.g. pacific salmon).
• this is called semelparity.
• individuals of most species reproduce several times
throughout life.
• this is termed iteroparity.
• for iteroparous species, how many offspring should an
individual produce in a given attempt?
• most intensively studied in birds –
what is the optimal clutch size?
Lack’s hypothesis for the evolution of
clutch size
Lack’s hypothesis for the evolution of
clutch size
• proposed by David Lack in 1947.
David Lack (1910 – 1973)
Lack’s hypothesis for the evolution of
clutch size
• proposed by David Lack in 1947.
• assumes a trade-off between probability of individual survival
and clutch size:
Lack’s hypothesis for the evolution of
clutch size
• proposed by David Lack in 1947.
• assumes a trade-off between probability of individual survival
and clutch size:
Lack’s hypothesis for the evolution of
clutch size
• proposed by David Lack in 1947.
• assumes a trade-off between probability of individual survival
and clutch size:
Selection will favor clutch size that produces max. surviving offspring
One test of Lack’s hypothesis
• Boyce and Perrins (1987) studied 4,489 clutches of great
tits (Parus major) from 1960-1982.
One test of Lack’s hypothesis
• Boyce and Perrins (1987) studied 4,489 clutches of great
tits (Parus major) from 1960-1982.
Range in clutch size
1-17
One test of Lack’s hypothesis
• Boyce and Perrins (1987) studied 4,489 clutches of great
tits (Parus major) from 1960-1982.
Range in clutch size
1-17
Mean clutch size
8.53
One test of Lack’s hypothesis
• Boyce and Perrins (1987) studied 4,489 clutches of great
tits (Parus major) from 1960-1982.
Range in clutch size
1-17
Mean clutch size
8.53
Most productive
clutch size
12
One test of Lack’s hypothesis
Actual mean clutch size= 8.53
Most productive clutch size
= 12
Why such discrepancy?
What other assumptions underlie Lack’s
hypothesis?
What other assumptions underlie Lack’s
hypothesis?
1. No trade-offs between reproductive efforts
across years.
What other assumptions underlie Lack’s
hypothesis?
1. No trade-offs between reproductive efforts
across years.
• many studies have added extra eggs to nests and found
adverse effects on subsequent reproduction in the following
year.
What other assumptions underlie Lack’s
hypothesis?
2. Clutch size only affects viability of offspring.
What other assumptions underlie Lack’s
hypothesis?
2. Clutch size only affects viability of offspring.
• large clutches may reduce reproductive success of
progeny.
What other assumptions underlie Lack’s
hypothesis?
2. Clutch size only affects viability of offspring.
• large clutches may reduce reproductive success of
progeny.
Example: Schluter and Gustafsson’s (1996) study on
collared flycatchers
Clutch size can affect offspring fitness
What other assumptions underlie Lack’s
hypothesis?
3. No year-to-year variation in optimal clutch
size.
What other assumptions underlie Lack’s
hypothesis?
3. No year-to-year variation in optimal clutch
size.
• clutch size is not fixed for given genotype. It is
phenotypically plastic.
What other assumptions underlie Lack’s
hypothesis?
3. No year-to-year variation in optimal clutch
size.
• clutch size is not fixed for given genotype. It is
phenotypically plastic.
• thus, optimal clutch size may vary from year-to-year
depending on availability of resources, etc.
Lack’s hypothesis is a null model!
How big should each offspring be?
How big should each offspring be?
• many studies have established a trade-off between egg
size and egg number.
How big should each offspring be?
• many studies have established a trade-off between egg
size and egg number.
26 species of fish
49 species of fruit flies
A model by Smith and Fretwell (1974)
Assumption 1: A trade-off between number and size of offspring
A model by Smith and Fretwell (1974)
Assumption 2: Individual offspring will have better chance of survival
If they are larger.
A model by Smith and Fretwell (1974)
Result: The optimal offspring size is intermediate
A model by Smith and Fretwell (1974)
this is parentoffspring conflict!
Result: The optimal offspring size is intermediate
Recap
• differences in life-history traits involve differences in the
allocation of energy.
• trade-offs between life history traits are unavoidable!
Questions:
1. Why do individuals age and die?
2. How many offspring should an individual produce in
any given year?
3. How large should each offspring be?