Transcript Aspidoscelis tesselata

Evolution of Life Histories
Life Histories
• Concerned with
– 1. Size at reproductive maturity
– 2. Age at reproductive maturity
– 3. Number of offspring produced
– 4. Size of offspring produced
• Variation from two sources:
– 1. Epigenetic factors
• Phenotypic plasticity
– 2. Adaptations
• Set by genotypes
Conceptual Problems
• 1. Life history characteristics have low
heritability.
– Relationship between life history characteristics and
fitness
– But, fitness usually has a large, variable, environmental
component.
– Genetic variation for life history characteristics is
maintained by shifting selection pressures.
• 2. Principles summarizing the diversity of life
history strategies are rare.
Life histories and reproductive mode
• Two principles:
• 1. Small genetic component in life history
variation.
– Flexibility is important
• 2. Life history characteristics have not evolved in
order to perpetuate a species.
– Shaped by natural selection increasing fitness of
individuals.
Energy allocation principle
• Energy available to an individual is finite
• A constraint on r (per capita rate of increase)
• Energy shunts:
– 1. maintenance
– 2. growth
– 3. reproduction
• Energy constraints on reproduction results in two
fundamental strategies:
– 1. Large number of small young
– 2. Small number of large young
• LH strategy example: Kiwis
New Zealand
LS: 20 years
Huge reproductive
investment in a
few individuals
Chicken-size
6 lbs
Proportionately
largest eggs of
any bird (1 lb)
Incubation by
male (c. 11 wks.)
Loses 20% BW
Chicks not fed by
adult
Self reliant
• LH example: Thrip egg mites
LS: 4 days
Life History Principles
• Generally begin with birds
• Reproductive output is accessible.
• Reproductive output can be easily manipulated
and adjusted.
• Individuals can be marked for identification.
The evolution of clutch size
• Optimal clutch size
• ? How much energy should an individual allocate
to an episode of reproduction; e.g., how many
eggs?
• Trade-off: The more offspring produced, the fewer
resources available for each individual.
• Lack’s prediction: Selection should favor a clutch
size that maximizes the number of surviving
offspring.
• Clutch size should be a reproductive strategy.
Starting hypotheses
Tradeoff:
Probability of
individual survival
< with > clutch size
Prediction:
Number of surviving
offspring = clutch size
x probability of
individual survival
Assumptions:
1. eggs are all the
same size
2. current
reproductive
effort does not
affect
subsequent
performance
Optimal clutch
size = 5
A test of the prediction: 1960-1982
Number of
Clutches
N = 4489
Mean clutch size = 8.5
Number
surviving
as a function
of clutch size
Parental lifetime fitness can decrease from care necessitated by large broods.
Future effects of clutch size on daughters’ performance
Collared Flycatchers
Effect of age at first reproduction on size of subsequent
clutches
• e.g. Collared Flycatchers
Begin at
different ages
Begin with extra eggs
How large should offspring be?
• Trade-off between number and size of offspring.
• Produce many small OR few large?
• The determining factor can be based on the size of
the individuals produced; a size with adaptive
value.
A fitness enigma
Aspidoscelis tesselata (2n)
SVL = 92.1 mm
Clutch size = 3.9 eggs
A. neotesselata (3n)
SVL = 84.8 mm
Clutch size = 2.6 eggs
A. sexlineata (2n)
SVL = 67.6 mmk
Clutch size = 2.8 eggs
Taylor, H. L., B. A. Droll, and J. M. Walker. 2006. Proximate causes of
a phylogenetic constraint on clutch size in parthenogenetic
Aspidoscelis neotesselata (Squamata: Teiidae) and range expansion
opportunities provided by hybridity. Journal of Herpetology 40:294-304.
• Intraspecific divergence in life histories in A.
tesselata (a parthenogenetic species)
Fort Sumner
Aspidoscelis tesselata
Pattern class E
Pattern class C
Taylor, H. L., J. M. Walker, J. E. Cordes, and G. J. Manning. 2005.
Application of the evolutionary species concept to parthenogenetic
entities: comparison of postformational divergence in two clones of
Aspidoscelis tesselata and between Aspidoscelis cozumela and
Aspidoscelis maslini (Squamata: Teiidae). Journal of Herpetology
39:266-277.
8
Nongravid
Gravid
Sumner C
Number
6
4
2
0
53
8
59
65
71
Nongravid
Gravid
77
83
89
95
101
89
95
101
Sumner E
Number
6
4
2
0
53
59
65
71
77
83
SVL (mm)