10/7/02

Road Map• Evolution:

variation + selection = adaptationexample: LDH in killifish

• Evolved behaviors– Tropisms in bacteria & fruit flies– Gibbon songs

• Instinctive learning– Zebra finches, humpback whales

• Culture as evolution?– “memetics” vs. genetics– arguments pro and con

• Evolution of group behavior– Genetic– Cultural

10/7/02

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

– often this depends on the environment– Including the attitudes of conspecifics

• 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

10/7/02

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

10/7/02

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

10/7/02

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.

10/7/02

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.

10/7/02

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

10/7/02

Bacterial taxis

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”)

10/7/02

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

10/7/02

Rhodospirillum centenumlikes it warm and dark…

10/7/02

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

10/7/02

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

10/7/02

(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

10/7/02

Gibbons

• Arboreal apes– tropical rain forests of southeast asia– 12 species in four (sub-)genera

• subgenera are somewhat more different than humans and chimps

– brachiation– monogamy

• like 3% of mammal species• 90% of bird species

10/7/02

Gibbons and us:Primate Phylogeny

Among the apes, only gibbons and humans have pair bonding.Also, only gibbons and humans sing…

10/7/02

Gibbon duetting

All species of gibbons are known to produce elaborate, species-specific and sex-specific patterns of vocalisation often referred to as "songs" (Haimoff, 1984; Marshall & Marshall, 1976). Songs are loud and complex and are mainly uttered at specifically established times of day. In most species, mated pairs may characteristically combine their songs in a relatively rigid pattern to produce coordinated duet songs. Several functions have been attributed to gibbon songs, most of which emphasise a role in territorial advertisement, mate attraction and maintenance of pair and family bonds (Geissmann, 1999; Geissmann & Orgeldinger in press; Haimoff, 1984; Leighton, 1987).

10/7/02

The female “great call”

The most prominent song contribution of female gibbons consists of a loud, stereotyped phrase, the great call. Depending on species, great calls typically comprise between 6-100 notes, have a duration of 6-30 s. The shape of individual great call notes and the intervals between the notes follow a species-specific pattern.

. A female song bout is usually introduced by a variable but simple series of notes termed the introductory sequence; it is produced only once in a song bout. Thereafter, great calls are produced with an interval of about 2 min. In the intervals, [are] so-called interlude sequences consisting of shorter, more variable phrases … The typical female song bout hence follows the sequential course ABCBCBCBC…,

10/7/02

Male duet contributions

As a rule, adult males do not produce great calls, but "male short phrases" only. Whereas female great calls remain essentially unchanged throughout a song bout, males gradually build up their phrases, beginning with single, simple notes. As less simple notes are introduced, these notes are combined to increasingly complex phrases, reaching the fully developed form only after several minutes of singing …

During duet songs, mated males and females combine their song contributions to produce complex, but relatively stereotyped vocal interactions… Both pair partners contribute to an introductory sequence at the beginning of the song bout (A). Thereafter, interlude sequences (B) and great call sequences (C) are produced in successive alternation…

During great call sequences the male becomes silent and does not resume calling until near or shortly after the end of the female's great call, when he will produce a coda.

10/7/02

Gibbon song samples

• Hylobates Lar– white-handed gibbon

– Female “great call” with male “coda”

• Hylobates Muelleri– gray gibbon

– Female “great call”

with male “coda”

10/7/02

H. Muelleri x H. Lar:

H. Lar

H. Lar x H. Muelleri:

H. Muelleri:

Hybrid Songs

10/7/02

Phylogeny of singing in primates

Singing is rare in mammals. It occurs in members of 26 species in four primate genera: Indri, Tarsius, Callicebus, Hylobates. These are 11% of primate species and 4% of primate genera. Since the four singing genera are widely separated, they are thought to have evolved singing independently.

In all singing primates, both males and females sing, and duetting usually if not always occurs. All singing primates are monogamous (with the possible exception of humans).

Most bird species sing; often bird song is mostly male; duetting bird species are also usually monogamous.

10/7/02

Are humans monogamous?In most mammalian species, sexual access is either determined by rank… and results in polygyny; or else … two individuals become “attached” to one another and then isolate themselves from other members of their species…

[In humans] what is common is… cooperative, mixed-sex social groups, with significant male care and provisioning of offspring, and relatively stable patterns of reproductive exclusion, mostly in the form of monogamous relationships.

Reproductive pairing is not found in exactly this pattern in any other species.

--Terence Deacon, “The Symbolic Species”

Are humans monogamous?

10/7/02

Gular sac

Some gibbons have developed a large “gular sac” apparently involved with breath control and/or resonance. Gular sac size and song complexity seem to correlate across species.

Symphalangus syndactylus(siamang):“the [siamang] duet is probably the

most complicated opus sung by a land vertebrate other than man…”

--Marshall and Sugardjito (1986)

10/7/02

Zebra Finch(Taeniopygia guttata)

• Small songbird (Australia and Timor)• Highly social (colonies of 20-1000)• Pair bonding (with frequent “cheating”)

– Male display, females choose– bond marked by “clumping” and preening

• Males sing, females do not– part of sexual and territorial displays

10/7/02

Typical Zebra Finch song

• Not intrinsically pleasing to most humans

• nasal quality, repetitive rhythm

• Production requires difficult motor control and large expenditure of energy

• Female Zebra Finches (and competing males) are willing to be impressed

original

slowed x 4

10/7/02

Song learningin Zebra Finches and other oscines

• Song patterns are species-specific• However, exposure to adult patterns is necessary for

normal sing development– deafened birds develop highly degraded song– birds reared without adult male models

develop degraded but species-typical songs

• Sensitive/critical period for exposure– 20-35 days after hatching

• Active song develops later– 60-90 days after hatching

10/7/02

Why learned songs?

• Some sub-oscine species have completely programmed song– deafened or isolated birds sing normally

• Suggested advantages of learned song:– more complex or varied song via cultural rather than genetic

development• females prefer constrained novelty• promotes exogamy in large colonies

– intra-species varients of song, called dialects, may serve to segregate populations of the same species

• promotes endogamy in microhabitats

– more rapid adaptation of the song to different acoustic environments

10/7/02

Humpback Whale Songs

• Samples:

• Variation and change:– At any one time, all whales sing similar

songs– Over time, songs change rapidly

10/7/02

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

– often this depends on the environment– including the attitudes of conspecifics

• In a given environment:– some genetic variants may be “selected for”

(i.e. produce more offspring)– producing genetic adaptation to the environment

(though much genetic change seems selectively neutral)

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

• in social species• In mating behavior

10/7/02

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

– often this depends on the environment– including the attitudes of conspecifics

• In a given environment:– some cultural variants may be “selected for”

(i.e. produce more offspringproduce more offspring grow or get borrowed)– producing cultural adaptation to the environment

(though much cultural change seems selectively neutral)

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

• in social species• in mating behavior

10/7/02

The culture of military tactics

• From Greek taxis “order, arrangement”

• Two main approaches in the ancient world:

10/7/02

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

10/7/02

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

10/7/02

Geography of two military cultures

10/7/02

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.

10/7/02

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 speak is 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:

10/7/02

Summary of the example• Adaptation of weapons, military 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 (entirely?) cultural

– some genetic effects are possible– ~300 generations

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

10/7/02

Genetics vs. memetics

• Many similarities:evolution as “descent with modification”

• Differences:– Mechanisms of “descent”

sexual reproduction vs.imitation and borrowing

– Mechanisms of innovationrandom mutation/recombination

vs. explicit problem-solving

– Rate of change

10/7/02

How do behaviors and abilities evolve?

• Algorithm development by “descent with modification”

• Some convincing (modeled) examples

• General principles could apply to individual or cultural learning as well as genetic “learning”

• The mapping from genes to behavior is almost completely mysterious(but the mapping from genes to anatomy is almost as mysterious…)

10/7/02

Example:Strategies for the “Iterated Prisoner’s Dilemma”

• The “prisoner’s dilemma” paradox

• The “iterated prisoner’s dilemma”– Proposed in 1984 by Robert Axelrod

• The IPD tournaments

• Genetic algorithms

• Applications of GA to the IPD

10/7/02

The “prisoner’s dilemma” paradox

Origin: situation of captured thieves– if everybody keeps silent, all go free– if one confesses

• (s)he gets a reward• everyone else gets a heavy sentence

– if everyone confesses• everyone gets a moderate sentence

If you analyze the options objectively, your best bet is to confess. But if everyone confesses, everyone is worse off than if everyone kept silent.

Generically: total cooperation is better than total non-cooperation; but any individual can then better his or her situation by “defecting”.

10/7/02

PD payoff matrix(payoff to me)

Temptation >> Reward >> Punishment >> Sucker’s payoff

[Also: (Temptation + Sucker)/2 <= Reward]

10/7/02

Consider the options…

• If you defect– if I cooperate I pay $100– if I defect I pay $10– so my best bet is to defect…

• If you cooperate– if I cooperate I get $300– if I defect I get $500– so my best bet is to defect…

10/7/02

PD without money or cops:the “furry critter’s dilemma”

10/7/02

Conclusion: nice guys finish last

• PD arguments were used to “prove” that cooperation could never be an evolutionarily stable strategy, except perhaps among kin

• “Every man for himself, and the devil take the hindmost…”

• A depressing conclusion in the context of the cold war, where nuclear standoff looks very much like a PD situation.

10/7/02

Axelrod’s innovation(1984)

• Treat PD as a game with repeating turns– Endless, or at least players don’t know when the

end will come

• Add up the scores across turns• Play different strategies against one another

– Human game-playing– Better, let the computer do it

10/7/02

Payoff matrix for Axelrod’s game

10/7/02

First try

• Axelrod advertised for strategies– 14 were submitted, some very complicated

– Axelrod added one: Random

• Run a tournament: every strategy plays against every other strategy 200 times

• The winner:– “Tit for tat” (submitted by Anatol Rapoport)

– “Cooperate with strangers, and otherwise do whatever the opponent did last time around”

• If we define a “nice” strategy as one that is never the first to defect, then the 8 top scoring strategies (out of 15) were “nice”.

• “Forgiving” strategies do better than those that bear grudges– in fact, “tit for two tats” would have won if it had been entered

10/7/02

Second try

• Analysis of first tournament was published

• Second tournament was opened to any new entrants– 62 entrants this time

– number of rounds was left open

• Result: “tit for tat” wins again– including against “tit for two tats”

10/7/02

More results

• Many other analyses and competitions• Two attempts at evolutionary simulations

– (1) Evolutionary competition among fixed set of 63 strategies

• “tit for tat” won 5 out of 6 rounds• A similar “nice, forgiving” strategy won the 6th• However, no such strategy is “evolutionarily stable”, in

the sense that a uniform population can always be successfully invaded by an alternative.

– (2) “Genetic algorithm” to evolve new strategies and let them compete

10/7/02

Evolving PD strategies

• Strategies as pseudochromosomes– 4 possible outcomes at each stage of the game– 4 x 4 x 4 = 64 possible 3-move “histories”– To determine how to act in each of these 64 cases

requires 64 specification of “C” (cooperate) or “D” (defect)

– Thus a PD “gene” is a string of 64 C’s or D’s– Add 6 more to cover the first 3 moves– Total of 70 letters make up a pseudochromosome

10/7/02

Mutations and mating

• Mutation: just change one of the 70 letters from C to D or from D to C, with some small probability

• Mating: combine two “genotypes” by selecting a random crossover point– crossover point of 9 combines 1-9 from

one parent with 10-70 from the other, and vice versa

10/7/02

Natural selection

• Run a mini-tournament and score each genotype

• Mate genotypes (with random mutations) to produce offspring in proportion to their score on the previous round of the tournament

10/7/02

Results: overall population fitness improves

10/7/02

Gene pool analysis

• Five “alleles” evolved in the vast majority of individuals:

1. Don’t rock the boat: if RRR then C

2. Be provocable: if RRS then D

3. Accept apologies: if TSR then C

4. Forget: if SRR then C

5. Accept a rut: if PPP then D

– Most of the resulting “individuals” beat tit-for-tat

10/7/02

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

10/7/02

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

10/7/02

Ant path selection

10/7/02

Ant problem solving

10/7/02

Termite mound

10/7/02

How social insect behavior evolves

• individual properties are genetically controlled– pheremone chemistry– when to emit which pheremones– effects of sensing pheremones– the “Termite Machine” is like a Turing Machine

that reads and writes smells rather than symbols

• 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

10/7/02

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