Transcript Genetics and Analysis of Quantitative Traits

Evolutionary Constraints
…any factor within a lineage that prevents an
anticipated course of evolution.
[Source: McKitrick 1993, Ann. Rev. Ecol. & Syst. 24, 307-330]
Evolutionary Constraints
Genetic
Functional
Developmental
[Source: Arnold 1992, American Naturalist 140, S85-S107]
Genetic Constraints
R ih
2
where,
R  response to selection
i  selection differential
h2  heritability
[Source: Falconer 1981, Introduction to Quantitative Genetics, Longman Press]
Genetic Correlations
Ri
Rj
=
βi
βj
*
Gii
Gji
Gij
Gjj
where,
R
response to selection
i 
selection gradient for trait i
Gii  genetic variance for trait
Gij 
i
genetic covariance between traits i and j
[Based on Lande and Arnold 1983, Evolution 37, 1210-1226]
What are genetic correlations made of?
• Linkage
– Traits linked physically or
statistically
• Pleiotropy
– Genes that affact one trait also
affect other traits
[Source: Lynch and Walsch 1998, Genetics and Analysis of Quantitative Traits]
Evolutionary Constraints
Genetic
Functional
Developmental
[Source: Arnold 1992, American Naturalist 140, S85-S107]
Functional Constraints
…result from limitation on the values of traits or
combinations of traits.
Usually, functional constraints involve tradeoffs.
[Source: Arnold 1992, American Naturalist 140, S85-S107]
Functional Constraints
• Allocation tradeoffs
• Acquisition tradeoffs
• Specialist-generalist tradeoffs
Angilletta et al. (2003), Trends Ecol. Evol.
Maintenance
Activity
Production
Tradeoffs in the
Allocation of
Resources
Growth
Reproduction
Storage
Egg weight (mg)
Grasshoppers that
produce large eggs
have smaller clutches.
Field Grasshopper
(Chorthippus biguttulus)
Clutch Size
[Source: Stearns 1992, The Evolution of Life Histories, Oxford University Press]
Tradeoffs between Acquisition and Survival
SC
NS
[Sources: Billerbeck et al. 2001, Evolution 55: 1863-187;
Lankford et al. 2001, Evolution 55: 1873-1881]
Specialist-Generalist Tradeoffs:
Specialist-generalist
An Example
Growth rate
Tradeoff
rature
Temperature
An increment in growth rate at one temperature results in a
decrement in growth rate at another. This tradeoff is caused
by thermal adaptation of enzymatic structure.
Specialist-Generalist Tradeoffs:
Evidence
1.2
Ancestral lines
Evolved lines
0.8
-1
Vmax (h )
1.0
0.6
0.4
0.2
0.0
15
20
25
30
35
40
45
Temperature (C)
Selection of E. coli for growth at 37C for 2000 generations.
[Source: Cooper et al. 2001, Evolution 55: 889-896]
Three Classes of Tradeoffs
• Allocation tradeoffs – a decrement in the resources
allocated to one or more functions resulting from an
increment in resources allocated to another function.
• Acquisition tradeoffs – an increase in the probability of
being killed or parasitized resulting from an increase in
the duration or intensity of foraging.
• Specialist-generalist tradeoffs – a decrement in
performance within one range of environmental
conditions that results from an increment in performance
within another range.
Evolutionary Constraints
Genetic
Functional
Developmental
[Source: Arnold 1992, American Naturalist 140, S85-S107]
Developmental Constraints
…result from limitations on the set of possible
developmental states and their morphological
expressions.
[Source: Arnold 1992, American Naturalist 140, S85-S107]
Evolution of Limb Morphology
• Modification of limb structure are some of the
best examples of adaptation.
• Mutations in limb structures are common
(e.g., polydactyly occurs in 0.1-0.2% of
humans).
• Yet, limb bones in vertebrates are highly
conserved in number. Mortality of mutants is
high.
[Source: Galis et al. 2001, TREE 16, 637-646]
Mechanisms of Losing Digits
Opossum
A reduction in digits occurs through destruction of some
digits after development.
Mechanisms of Adding Digits
European Mole
Extra digit is a modified
sesamoid bone!
Similar structures occur in
turtles, whale, and pandas
[Source: Galis et al. 2001, TREE 16, 637-646]
Development Constrained by Pleiotropy
• Limb development occurs during a highly
conserved stage of embryonic development
• Interactions among somites, notochord,
neural tube, lateral plate mesoderm, and limb
buds is intense.
• Changes in limb development seems to have
a pleiotropic effect on other structures. Mostly,
this effect is deleterious.
[Source: Galis et al. 2001, TREE 16, 637-646]
Summary
•
Evolution is constrained by a lack of genetic
variation, functional relationships between traits,
and developmental mechanisms.
•
These constraints can be incorporated into
optimization theory.
•
Some of these constraints are temporary,
meaning that natural selection, mutation, and
genetic drift can modify constraints over time.