Zoology 445
Fall 2006
Reading: Chapter 12
Life Histories: Outline
I: Introduction
Life History = characteristics of growth, survivorship and reproduction of an organism.
Life history is a group of traits that are all under selection.
Life history options are limited by historical (evolutionary) constraints and trade-offs.
Figure 12.2
The evolution of a life history can viewed as a series of trade-offs that can affect fitness.
Three basic questions of life history evolution:
1. Why do organisms senesce & die?
2. How many offspring should an individual produce in a given year?
3. How big should each offspring be?
AM Jarosz
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I. Why do organisms senesce and die?
Three hypotheses that have been proposed to explain death.
A. Rate of living hypothesis
B. Deleterious mutations hypothesis
C. Antagonostic Pleiotropy Hypothesis
Pleiotropy = a single gene influences multiple phenotypic traits
A. Rate of living hypothesis on aging: Aging & death are caused by the accumulation of
irreparable damage to cells & tissues. Damage caused by errors in replication, transcription &
translation.
Selection acts to resist damage to the extent physiologically possible.
Two predictions:
a. Life spans should be tied to metabolic rate.
b. It is not possible to evolve longer life spans.
a. Life span tied to metabolic rate
For mammals, metabolic rate is correlated with size. Smaller animals have higher metabolic
rates than larger animals.
Consequently, small animals should have shorter lifespans than larger animals.
If lifespan was limited by metabolic rate, then all animals should have similar lifetime energy
expenditures per gram of tissue.
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b. It is not possible to evolve longer life spans.
Experiment for early and late reproduction in Drosophila
melanogaster.
Life span increased by about 70% from 35 days in the initial
population to 60 days after selection.
B. Deleterious mutations hypothesis = Mutations that reduce early survivorship will be more
strongly selected against than mutations reducing survivorship only at older ages.
1. Life Tables = the schedule of survivorship and reproduction of females within a population
Say we have a hypothetical organism with the following life table:
a. The probability of survivorship from one year to the next is 80% (0.80),
b. Individuals reach reproductive maturity at age three.
c. Produce one offspring every year it survives beyond maturity
d. The maximum lifespan is 16 years
Life Table:
Wildtype Matures at age 3, 80% survival per year; 1 offspring per
year
Expected
Fraction
RS for
Age
Surviving
RS of Survivors
individuals
0
1.000
0
1
0.800
0
2
0.640
0
3
0.512
1
4
0.410
1
5
0.328
1
6
0.262
1
7
0.210
1
8
0.168
1
9
0.134
1
10
0.107
1
11
0.086
1
12
0.069
1
13
0.055
1
14
0.044
1
15
0.035
1
Expected lifetime
RS =
(Absolute
fitness)
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Now say we have two deleterious mutations enter the population.
One causes organisms to die at 5 while the second causes organisms to die at age 14.
Mutation that causes individuals to dies at age 5
Expected
Fraction
RS for
Age
Surviving RS of Survivors
individuals
0
1.000
0
0.000
1
0.800
0
0.000
2
0.640
0
0.000
3
0.512
1
0.512
4
0.410
1
0.410
5
0.000
1
0.000
6
0.000
1
0.000
7
0.000
1
0.000
8
0.000
1
0.000
9
0.000
1
0.000
10
0.000
1
0.000
11
0.000
1
0.000
12
0.000
1
0.000
13
0.000
1
0.000
14
0.000
1
0.000
15
0.000
1
0.000
Expected lifetime
RS =
Mutation that causes individuals to die at age 14
Expected
Fraction
RS for
Age
Surviving
RS of Survivors
individuals
0
1.000
0
0.000
1
0.800
0
0.000
2
0.640
0
0.000
3
0.512
1
0.512
4
0.410
1
0.410
5
0.328
1
0.328
6
0.262
1
0.262
7
0.210
1
0.210
8
0.168
1
0.168
9
0.134
1
0.134
10
0.107
1
0.107
11
0.086
1
0.086
12
0.069
1
0.069
13
0.055
1
0.055
14
0.000
1
0.000
15
0.000
1
0.000
Expected lifetime
RS =
4
(Absolute
fitness)
(Absolute
fitness)
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C. Antagonostic Pleiotropy Hypothesis = A mutation that simultaneously increases reproductive
success early in life and causes early death late in life could be favored by selection.
Mutation that causes individuals to mature at age 2, but die at age 10
Expected
Fraction
RS for
Age
Surviving RS of Survivors
individuals
0
1.000
0
0.000
1
0.800
0
0.000
2
0.640
1
0.640
3
0.512
1
0.512
4
0.410
1
0.410
5
0.328
1
0.328
6
0.262
1
0.262
7
0.210
1
0.210
8
0.168
1
0.168
9
0.134
1
0.134
10
0.000
1
0.000
11
0.000
1
0.000
12
0.000
1
0.000
13
0.000
1
0.000
14
0.000
1
0.000
15
0.000
1
0.000
Expected lifetime
(Absolute
RS =
fitness)
Text on pages 467 – 474 has examples of data that test these hypotheses.
Scorecard:
A. Rate of living hypothesis ------ Proven to be false
B. Deleterious mutations hypothesis ---- Some data support this hypothesis
C. Antagonostic Pleiotropy Hypothesis --- Other data support this hypothesis
II. How many offspring should be produced?
Fig 12.16
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Test of Lack’s hypothesis. Parus major
In reality clutch sizes seem to be lower than the optimum predicted by the Lack hypothesis.
Reasons:
1. Reproduction this year appears to negatively affect the ability to reproduce young in the next
year. Fig 19.5
2. Clutch size (number of siblings in the nest) appears to affect a female offspring's ability to
produce eggs. (Fig 12.18)
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III. How big should each offspring be?
Fig 12.21b. Relationship between egg volume and egg number. Data are for 26 different fish
families.
Say we have the relationships depicted in Fig 12.22 a & b shown below.
Part c is obtained by multiplying values in a
with values in b.
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In reality, multiple life history traits respond to changes in the environment.
Conclusion:
I. Life history traits appear to be shaped by a series of trade-offs imposed by having to choose
how a limited amount of energy can be utilized for growth, maintenance, and &
reproduction.
II. These trade-offs must be optimized to maximize fitness within a particular environment.
\Evolution Zol 445\2006\Life History handout.doc
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