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Genetic Basis of Behavior

• fru gene in Drosophila: a master regulatory gene that directs the expression and activity of many genes to produce male courtship behavior; mutations to inactive form abolish courtship; females manipulated to express male fru exhibit male-typical courtship toward other females.
• Mechanism: fru programs male courtship by directing male-specific wiring of the central nervous system; differences in behavior can arise from variation in gene product activity, not just gene inactivation.
• Voles as an example of gene expression affecting behavior: prairie voles (Microtus ochrogaster) form pair bonds and show parental care; meadow voles (Microtus pennsylvanicus) are solitary and less parental.
• Vasopressin (ADH) signaling is key: the vasopressin receptor gene is more highly expressed in prairie voles; blocking brain vasopressin receptors inhibits pair-bonding.
• Experimental genetic manipulation: introducing prairie vole vasopressin receptor gene into meadow voles increases receptor levels and induces prairie-like mating behaviors (pair-bonding).
• Takeaway: many behaviors can be shaped by gene expression levels and regulatory networks, not only by gene presence/absence.

Genetic Variation and the Evolution of Behavior

• Behavioral differences can arise between closely related species and within species; population-level variation can reflect natural selection when it correlates with environment.

Case Study: Variation in Prey Selection (Western Garter Snake)

• Coastal populations: diet dominated by banana slugs (Ariolimax californicus); inland populations: diet includes frogs, leeches, fish, rarely banana slugs.
• Experimental cross-fostering: pregnant snakes from coastal and inland populations raised in labs; offspring offered banana slug over 10 days.
• Results: >60% of coastal-mother offspring ate slug on 8+ days; <20% of inland-mother offspring did so.
• Conclusion: banana slug preference has a genetic component; coastal snakes also differ in odor recognition for slug cues.
• Evolutionary story: inland snakes colonized coastal habitats ~10,000+ years ago; those able to recognize slug odors had higher fitness; natural selection increased slug-recognition alleles in coastal populations over generations.

Case Study: Variation in Migratory Patterns (Blackcaps, Sylvia atricapilla)

• Historical pattern: German-breeding blackcaps migrate SW to Spain/Africa for winter.
• 1950s shift: a few began wintering in Britain; over time, westward-migrating individuals increased.
• Experimental approach: breeding in Germany and testing migratory orientation in captivity showed genetic differences underlying migratory direction.
• Results: westward migrants are genetically distinct from southwest-migrants; the shift toward Britain occurred rapidly in recent history.
• Why it happened: changes in migration routes may be reinforced by feeder availability and shorter migration distances.

Altruism and Inclusive Fitness

• Altruism: behavior that reduces the actor’s own fitness but increases the fitness of others; examples include warning calls, care for kin, etc.
• Classic examples: alarm calls in Belding’s ground squirrels increase predator detection but raise caller’s risk.
• Inclusive fitness: total effect of an individual on proliferating its genes by producing offspring plus aiding relatives to produce offspring.
• Hamilton’s Rule: altruism favored when the benefit to the recipient times the coefficient of relatedness exceeds the cost to the altruist.
  • Formula: $rB > C$
  • Definitions:
  • $B$ = average extra offspring produced by the recipient due to altruism
  • $C$ = loss of offspring by the altruist due to altruism
  • $r$ = coefficient of relatedness (fraction of genes shared)
• Example: rescue scenario
  • Suppose a young man’s sister drowns-risk to self; the sister gains two potential offspring if the rescue succeeds; assume cost to sister is $C = 0.5$ offspring; relatedness $r = 0.5$; recipient gains $B = 2$ offspring.
  • Calculation: $rB = 0.5 imes 2 = 1$; since $1 > 0.5 = C$, Hamilton’s Rule is satisfied; altruism favored.
• Kin selection: altruism evolved to increase the reproductive success of relatives; stronger with closer relatedness.
• Relatedness values:
  • Siblings: $r = 0.5$
  • Aunt–niece: $r = 0.25$
  • First cousins: $r = 0.125$
• Examples across species:
  • Belding’s ground squirrels: females more likely to alarm-call to protect close kin; males disperse farther, reducing kin-alarm-call benefits.
  • Honeybees: sterile workers help the queen (their mother) reproduce.
  • Naked mole rats: colonies with one reproducing queen/kings; nonreproductives support kin that share genes.

Reciprocal Altruism

• Altruism toward non-relatives can evolve if there is reciprocity: the helper expects return benefits in the future.
• Limitations: requires stable social groups and likelihood of repeated interactions; risk of cheating.
• Tit-for-tat strategy: cooperate on first encounter; thereafter mimic partner’s previous action; retaliate when cheated but resume cooperation if the partner cooperates again.
• Examples: food sharing among vampire bats; social grooming in primates.
• Game theory context: explains rare reciprocal altruism in animals and more common reciprocal norms in humans.

Evolution and Human Culture

• Humans display substantial behavioral variation; environment strongly shapes the genotype–phenotype path for behavior due to learning and plasticity.
• Sociobiology (E. O. Wilson): genes influence social behaviors; sparked debate about human culture and behavior.
• Humans have constructed complex societies with laws, values, and religions that can constrain or promote behaviors that may not maximize immediate Darwinian fitness.
• Play: varied behaviors (object play, locomotor play, social play) may serve to prepare individuals for unpredictable future events rather than forming specific skills.
• Reciprocal altruism as a key concept in human culture and cooperation; sociobiology framework ties behavioral evolution to cultural evolution.