……any factor within a lineage that prevents an any factor within a lineage that prevents an anticipated course of evolution.anticipated course of evolution.

[Source: McKitrick 1993, [Source: McKitrick 1993, Ann. Rev. Ecol. & Syst.Ann. Rev. Ecol. & Syst. 24, 307-330] 24, 307-330]

Evolutionary ConstraintsEvolutionary Constraints

GeneticGenetic

FunctionalFunctional

DevelopmentalDevelopmental

Evolutionary ConstraintsEvolutionary Constraints

[Source: Arnold 1992, [Source: Arnold 1992, American NaturalistAmerican Naturalist 140, S85-S107] 140, S85-S107]

R i h 2

Genetic ConstraintsGenetic Constraints

selection differential

where,where,

Rih

2

response to selection

heritability

[Source: Falconer 1981, [Source: Falconer 1981, Introduction to Quantitative GeneticsIntroduction to Quantitative Genetics, Longman Press], Longman Press]

Genetic CorrelationsGenetic Correlations

where,where,

[Based on Lande and Arnold 1983, [Based on Lande and Arnold 1983, EvolutionEvolution 37, 1210-1226] 37, 1210-1226]

*βi

βj

Gii Gij

Gji Gjj

selection gradient for trait i

R

G

G

i

ii

ij

response to selection

genetic variance for trait i

genetic covariance between traits i and j

Ri

Rj=

• LinkageLinkage– Traits linked physically or statisticallyTraits linked physically or statistically

• PleiotropyPleiotropy– Genes that affact one trait also affect Genes that affact one trait also affect

other traitsother traits

What are genetic correlations made of?What are genetic correlations made of?

[Source: Lynch and Walsch 1998, [Source: Lynch and Walsch 1998, Genetics and Analysis of Quantitative TraitsGenetics and Analysis of Quantitative Traits]]

GeneticGenetic

FunctionalFunctional

DevelopmentalDevelopmental

Evolutionary ConstraintsEvolutionary Constraints

[Source: Arnold 1992, [Source: Arnold 1992, American NaturalistAmerican Naturalist 140, S85-S107] 140, S85-S107]

……result from limitation on the values of traits or result from limitation on the values of traits or combinations of traits.combinations of traits.

Usually, functional constraints involve tradeoffs.Usually, functional constraints involve tradeoffs.

Functional ConstraintsFunctional Constraints

[Source: Arnold 1992, [Source: Arnold 1992, American NaturalistAmerican Naturalist 140, S85-S107] 140, S85-S107]

Angilletta et al. (2003), Angilletta et al. (2003), Trends Ecol. Evol.Trends Ecol. Evol.

Functional ConstraintsFunctional Constraints

• Allocation tradeoffs

• Acquisition tradeoffs

• Specialist-generalist tradeoffs

Tradeoffs in the Tradeoffs in the Allocation ofAllocation ofResourcesResources

Reproduction

Growth

Storage

Grasshoppers that Grasshoppers that produce large eggs produce large eggs have smaller clutches.have smaller clutches.

Field GrasshopperField Grasshopper((Chorthippus biguttulusChorthippus biguttulus))

Clutch Size

Egg

wei

ght

(mg)

[Source: Stearns 1992, [Source: Stearns 1992, The Evolution of Life HistoriesThe Evolution of Life Histories, Oxford University Press], Oxford University Press]

Tradeoffs between Acquisition and SurvivalTradeoffs between Acquisition and Survival

[Sources: Billerbeck et al. 2001, [Sources: Billerbeck et al. 2001, Evolution Evolution 55: 1863-187; 55: 1863-187;

Lankford et al. 2001, Lankford et al. 2001, Evolution Evolution 55: 1873-1881]55: 1873-1881]

SC

NS

Specialist-Generalist Tradeoffs:Specialist-Generalist Tradeoffs:An ExampleAn Example

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:Specialist-Generalist Tradeoffs:EvidenceEvidence

Temperature (C)

15 20 25 30 35 40 45

Vm

ax (

h-1)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Ancestral linesEvolved lines

[Source: Cooper et al. 2001, [Source: Cooper et al. 2001, EvolutionEvolution 55: 889-896] 55: 889-896]

Selection of E. coli for growth at 37C for 2000 generations.

Three Classes of TradeoffsThree Classes of Tradeoffs

• Allocation 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 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 Specialist-generalist tradeoffs –– a decrement in performance within one range of environmental conditions that results from an increment in performance within another range.

GeneticGenetic

FunctionalFunctional

DevelopmentalDevelopmental

Evolutionary ConstraintsEvolutionary Constraints

[Source: Arnold 1992, [Source: Arnold 1992, American NaturalistAmerican Naturalist 140, S85-S107] 140, S85-S107]

……result from limitations on the set of possible result from limitations on the set of possible developmental states and their morphological developmental states and their morphological expressions.expressions.

Developmental ConstraintsDevelopmental Constraints

[Source: Arnold 1992, [Source: Arnold 1992, American NaturalistAmerican Naturalist 140, S85-S107] 140, S85-S107]

Evolution of Limb MorphologyEvolution of Limb Morphology

• Modification of limb structure are some of the Modification of limb structure are some of the best examples of adaptation.best examples of adaptation.

• Mutations in limb structures are common Mutations in limb structures are common (e.g., polydactyly occurs in 0.1-0.2% of (e.g., polydactyly occurs in 0.1-0.2% of humans).humans).

• Yet, limb bones in vertebrates are highly Yet, limb bones in vertebrates are highly conserved in number. Mortality of mutants is conserved in number. Mortality of mutants is high.high.

[Source: Galis et al. 2001, [Source: Galis et al. 2001, TREETREE 16, 637-646] 16, 637-646]

Mechanisms of Losing DigitsMechanisms of Losing Digits

OpossumOpossum

A reduction in digits occurs through destruction of some A reduction in digits occurs through destruction of some digits after development.digits after development.

European MoleEuropean Mole

Extra digit is a modified Extra digit is a modified sesamoid bone!sesamoid bone!

Similar structures occur in Similar structures occur in turtles, whale, and pandasturtles, whale, and pandas

[Source: Galis et al. 2001, [Source: Galis et al. 2001, TREETREE 16, 637-646] 16, 637-646]

Mechanisms of Adding DigitsMechanisms of Adding Digits

Development Constrained by PleiotropyDevelopment Constrained by Pleiotropy

• Limb development occurs during a highly Limb development occurs during a highly conserved stage of embryonic developmentconserved stage of embryonic development

• Interactions among somites, notochord, neural Interactions among somites, notochord, neural tube, lateral plate mesoderm, and limb buds is tube, lateral plate mesoderm, and limb buds is intense.intense.

• Changes in limb development seems to have a Changes in limb development seems to have a pleiotropic effect on other structures. Mostly, pleiotropic effect on other structures. Mostly, this effect is deleterious.this effect is deleterious.

[Source: Galis et al. 2001, [Source: Galis et al. 2001, TREETREE 16, 637-646] 16, 637-646]

SummarySummary

• 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.