3/21/01 Models of genetic and cultural evolution, part 1 College 002: Biology, Language, Culture Thursday, March 22, 2001.

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Models of genetic and cultural evolution, part 1

College 002: Biology, Language, CultureThursday, March 22, 2001

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Prospectus

• Analogies between genetic and cultural evolution (part 1)

• Differences between genetic and cultural evolution (part 2)

• Genetic/cultural co-evolution

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Road Map for part 1

• Evolution as variation + selection = adaptation

• Evolution of behavior– genetic – cultural

• Emergent properties of social behavior– as result of evolving strategies– as environment for evolution

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Genetic variation and adaptation• Genetic variation is ubiquitous• Is a variant worse, neutral or better?

– the answer often depends on the environment• In a given environment:

– some variants may be “selected for”• i.e. produce more offspring

– Result: the population adapts to the environment– (though much variation is selectively neutral

• The population is (part of) the environment– so the population adapts to interact with itself– especially

• in social species• mating behavior

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Adaptation in Fundulus Heteroclitus (Mummichog)

• Also known as “killifish”• 5-7 inches, olive green to blue back, white belly• Lives in salt marshes and tidal creeks from Labrador to Mexico

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LDH

• Lactate dehydrogenase

• Enzyme that catalyzes conversion of lactate to pyruvate

• A key step in energy production in living things

• Often the limiting factor in speed or endurance

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LDH variants

• All enzymes have varying effectiveness at different temperatures

• Some variants work better at lower temperatures, some work better at higher temperatures

• Fish are cold-blooded, so their body temperature depends on the water temperature

• Mummichog genetic LDH variants– vary in effectiveness at different temperatures– are selected according to location (I.e. temperature)– result: adaptation to different environments

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Mummichog LDH efficiency

The mummichog B locus for lactate dehydrogenase (LDH) has two common alleles, LDH-Ba and LDH-Bb. Catalytic efficiency varies with temperature: the bb genotype “wins” at lower temperatures, while the aa genotype “wins” at higher temperatures.

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Mummichog LDH geography

In the Gulf of Maine, nearly all mummichogs have thegenotype bb at the LDH-B locus. Off South Carolina, the aa genotype rules. In between, it’s in between.

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Adaptation can be rapid

• In Long Island Sound, the aa (warm-water) genotype has an overall frequency of 15%

• But for Mummichogs living in the thermal effluent of a power plant, aa frequency is 30%.

• Efficient adaptation to environmental variation: a mummichog micro-habitat

• Number of generations: <20• Like beak variation among Darwin’s Finches on

the Galapagos Islands– major adaptation to climate change in <5 generations

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Bacterial taxis

[Bacterial Chemotaxis at the microscopic level][Bacterial chemotaxis at the molecular level]

Taxis: “ability to move in a desired direction”:Phototaxis: towards or away from lightChemotaxis: towards or away from chemicalsMagnetotaxis: with respect to magnetic field

Simple but important behavior:• finding food• avoiding harm

In colonial and multicellular creatures,chemotaxis is crucial in creating structures (“morphogenesis”)

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Basic mechanisms of bacterial taxis

• Alternation of (straight-line) “runs”and (random) “tumbles”– 3D random walk [animation]

– attractants bind a chemoreceptorthat increases time between tumbles

– repellants decrease time between tumbles

• About 40 genes in the basic system– plus additional genes for new sensors

– Can orient towards or away from:• nutrients, poisons, oxygen, light, pH, temperature, etc.

• Categories (which chemicals? which wavelengths? etc.) and responses (towards or away?) evolve adaptively

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Rhodospirillum centenumlikes it warm and dark…

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Tropisms in higher organisms

• Mechanisms are more complex– (and largely unknown)

• Result is similar– attraction towards some things– repulsion from others

• Such preferences may be “learned”– by individuals: what we normally think of as

learning– by (sub-)species: through genetic variation and

selection leading to adaptation

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Natural behavior variation in Drosophila Melanogaster

Fruit flies show natural variation in preferencesPupae hatching in experimental “habitat maze”

– separated themselves on four dimensions of preference• up/down• light/dark• acetalldehyde/ethanol• time of the day of emergence (early/late)

– 16 preference combinationsend up in different places in the maze

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(Un)natural selectionleads to speciation!

• Two “opposite” strains were mated for 25 generations– early/up/dark/acetaldehyde vs.

late/down/light/ethanol

• Result: reproductive isolation and habitat specialization

• Conclusion: behavioral preferences were genetically mediated

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Cultural variation and adaptation• Cultural variation is ubiquitous• Is a variant worse, neutral or better?

– the answer often depends on the environment• In a given environment:

– some variants may be “selected for”• i.e. populations grow or traits are borrowed

– Result: cultures adapt to their environment• (though much cultural change appears to be neutral)

• The population is (part of) the environment• so the population adapts to interact with itself• especially

– in social species– for social behavior

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The culture of military tactics

• From Greek taxis “order, arrangement”

• Two main approaches in the ancient world:

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The two patterns

• Heavy infantry: like the Macedonian phalanx– Associated with city-states in the eastern

Mediterranean

• Light cavalry: like the Mongol hordes– Associated with pastoral nomads on the Eurasian

steppes

• Choice in many dimensions:– weapons, armor, tactics, lifestyle

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Herders vs. farmers

• Foot soldiers• 70 pounds of arms

– helmet, shield– spear, sword

• Success based on shock of mass formation– little training required

• Not very mobile

• Mounted archers• Light bow, no armor• Success based on

mobility– lots of practice in

riding and shooting

• No good at defending fixed positions

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Geography of two military cultures

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Comparison of two failed invasions

• In 514 B.C., the Persian king Darius tried to conquer the Scythian nomads (roughly in the area of present-day Ukraine). He chased them all over their vast domains, and finally had to return home in frustration.

• Darius invaded Greece in 490 B.C. He sacked a few cities, and was defeated by the Athenian infantry (who were outnumbered 3 to 1) at the battle of Marathon. The Athenians caught the Persians unprepared by running a mile in full panoply, which was an unheard-of tactic at the time.

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The Scythians indeed have in one respect, and that the very most important of all those that fall under man's control, shown themselves wiser than any nation upon the face of the earth. Their customs otherwise are not such as I admire. The one thing of which I speakis the contrivance whereby they make it impossible for the enemy who invades them to escape destruction, while they themselves are entirely out of his reach, unless it please them to engage with him. Having neither cities nor forts, and carrying their dwellings with them wherever they go; accustomed, moreover, one and all of them, to shoot from horseback; and living not by husbandry but on their cattle, their waggons the only houses that they possess, how can they fail of being unconquerable, and unassailable even?    

The Greek historian Herodotus describes both invasions, and admires the ability of the Scythian nomads to win withoutpitched battles:

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Summary• Adaptation of weapons, tactics

– Asian steppe vs. Mediterranean coast– Herding vs. farming– Nomadic bands vs. walled cities

• Continuing cultural variation– proliferation and copying of what worked best– result: consistent patterns across thousands of years and

miles• Process was (mainly) cultural

– some genetic effects are possible– ~300 generations

• Adaptation to two ecological niches– fitness depends on the environment– cultural and geographical

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Emergent propertiesof group behavior

• Descriptions at different levels– temperature, sound vs. molecular motions– cell vs. macromolecules– organism vs. cell– ant colony vs. individual ants– marketplace vs. individual merchants

• Some key properties only emerge in group interactions

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Why this matters

• Fitness is often determined by emergent properties– Fitness in multicellular organisms can’t be

determined by looking only at individual cells

– Fitness in social organisms can’t be determined by looking only at individuals

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Ant path selection

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Ant problem solving

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Termite mound

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How social insect behavior evolves

• individual properties are genetically controlled– pheremone chemistry– when to emit which pheremones– effects of sensing pheremones

• intelligent group, stupid individuals – plans and decisions exist only at the group level– dispositions of individuals are like elements of a circuit

or statements in a program

• evolutionary environment is (mainly) the group– like the evolutionary situation of multicelled organisms

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Adam Smith’s “invisible hand”

[A merchant] generally, indeed, neither intends to promote the public interest, nor knows how much he is promoting it.  … [H]e intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention. … By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it.

Wealth of Nations, Book IV

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Summary

• Analogies between genetic and cultural evolution– Variation and adaptation– Outcome depends on environment

• (and chance!)

– Results feed back as part of environment– Emergent phenomena are important

• Next time: some differences!