Download ppt - Language Log

Models of genetic and cultural
evolution, part 2
College 002: Biology, Language, Culture
Thursday, April 3, 2001
3/21/01
Modes of genetic change
• Darwin 1:
Random genetic variation + natural selection = adaptation
“survival of the fittest’
Example: food preferences (chemotaxis etc.)
• Darwin 2:
Random genetic variation + sexual selection = ?
“Survival of the sexiest”
Examples: size, symmetry, ornamentation
• may be environmentally adaptive or not
• “runaway sexual selection”
• Genetic drift:
Random genetic variation + random events = random change
Example: “molecular clocks”
• Other?
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Genetic adaptation as learning
• Advantages
– Application of learning is fast
• “instincts” available from birth
– Happens anyhow
• preferences & behaviors have genetic aspects
• natural & sexual selection always operate
• Disadvantages
– Rate of learning is slow
• one “trial” per generation
– Individuals are inflexible
– “Trial and error” is the only method
• Better if individuals also learn from experience
– adjust behavior depending on environment
• biochemical learning
• neural learing in higher animals
– always partnership between genetic & individual learning
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Example: food choice
• Types of sensory input
– chemical, optical, acoustic, etc.
• Types of response
– attraction/avoidance
– feeding
• Strong genetic component
– innate chemical preferences
• from bacteria to humans
• BUT individual learning applies as well
– ubiquitous in higher animals
• Also social learning
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Food preferences in slugs
Limax species eat a wide range of plants. Associated chemicals
naturally attract them and stimulate feeding behavior.
However, many plants are toxic to slugs, who have a complex
system for learning which plants to avoid.
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Slug lip/brain can “learn”
The isolated lips and nervous system of the terrestrial slug Limax
maximus will produce some of the feeding behavior of the intact
animal; i.e., they generate the rhythmic neural activity characteristic
of ingestion in response to food extracts applied to the lips. This
preparation will respond to a variety of food extracts that elicit
feeding in the whole animal. This provides the opportunity for
aversive conditioning experiments involving taste discrimination.
Pairing lip chemostimulation by attractive food extracts with lip
chemostimulation by using bitte plant secondary substances can cause
the isolatd brain to selectively suppress its neural response to one food
extract while remaining responsive to another.
Chang & Gelperin 1980
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Experimental results
• Quick learning from bad experiences
– sickness (e.g. from CO2 poisoning)
– associations of chemicals
• One-trial learning
– on first exposure to the plant type
– like “bait aversion” in rats
• Generalization from “taste” to “smell”
– Different organs (tentacles vs. lips)
• Obvious advantages over purely genetic food
preferences
– “flexible instinct”: a specialized learning system
– advantage in “arms race” with plants
• relative investment in toxins depends on predation
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From individual to social learning
Some problems of individual learning
1. It’s hard to learn from a fatal mistake
2. Some crucial skills are not obvious
3. Distribution of resources in space/time is often
complex and locally variable
Social learning can be helpful
1. Learn from the fatal mistakes of others
2. Easier to imitate than invent
3. Knowledge about resources can accumulate
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Social learning in bears
A mother brown bear teaches her cubs to fish.
Brown bear cubs stay with their mother for 2-3 years,
The survival of brown bear cubs is totally dependent on the
skill of the mother in both protecting them and teaching
them the basics of what to eat, where when and how to get
it, where to den, and how to cope with danger.
Fewer than 50% survive to become independent adults
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Brown bear diet
(Glacier Park)
In different places and times, crucial parts of bear diet can
be from whitebark pine seeds (harvested from squirrel
middens), cutworm moths (as many as 40,000 moths per
day), berries, roots, …
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Learning/culture as evolution
• Random behavioral variation + reinforcement
– analogous to Darwinian natural selection
• operating on stimulus/response associations rather than
on genes
• Imitation of attractive models
– analogous to Darwinian sexual selection
• may or may not be environmentally adaptive
• “Memetics”
– Ideas (“memes”) as analogous to genes
– Culture as the “meme pool”
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Limitations of the analogy
• Genetic evolution has a consistent
physical foundation, while learning and
culture do not
– genes “exist”, memes don’t
• Culture is subject to a wider set of
dynamic forces than the gene pool is
– for example, conformity
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Physical foundations
– DNA/RNA/protein system exists
• Genes are identifiable, replicating units
• Same mechanisms from bacteria to humans
– Physical substrate for individual and social
learning is highly variable
• Many specialized subsystems
– e.g. food preferences vs. face recognition
• Basic units (“ontology”) not well defined
– across different systems
– at all?
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Dynamic forces
• Forces in genetic evolution
–
–
–
–
random mutation (accessible genotypes)
natural selection (survival)
sexual selection (reproductive efficiency)
that’s all!
• Other forces in “memetic” evolution
– non-random new ideas
• e.g. analogy
– higher-level influences on selection
• e.g. conformity
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Language as species
(to be continued…)
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