Biological Processes
Genetics & Brain Evolution
Thought Paper #1
(Due 9/20/01)
In class, we discussed three "modern" conceptions
of the mind-brain problem: Epiphenomenalism, Interactionism,
and Monism. Which of these theories do you think is most likely
to be correct? Do you think science will ever be able to determine
the relation between mind and body? In an ideal world, which
of these theories would you hope would be true? Please explain
and defend your answers to each question.
(Must be typed, double-spaced, 2-3 pages)
Nature vs. Nurture
To what extent is behavior inherited and to
what extent is it learned?
Genetic Principles
- Genetic message resides within chromosomes.
- Human cells have 23 pairs of chromosomes;
half of each pair is from mother, half from father.
- Genes are segments of chromosomes containing
"instructions" for hereditary characteristics (e.g.,
hair & eye color; blood type).
- We have two genes for most traits.
- Dominant gene masks effects of recessive
gene.
Genetic Principles
- We all have genetic material that is not
expressed physically.
- Genotype vs. phenotype
- Genotype: Actual genetic message.
- Phenotype: Observable characteristics.
- Phenotype determined by genotype
& environment.
- Even traits that are highly heritable are
not completely controlled by genetics.
How Heritable is Behavior?
- Intelligence
- Personality
- Mental Disorder
Heritability
- A quantitative index of the extent to which
trait differences in a population can be accounted for by genetic
factors.
- Ranges from 0 to 100%.
- Estimates of heritability apply to groups,
not to individuals.
Twin Studies
- Identical twins share 100% of genes.
- Fraternal twins and normal siblings share,
on the average, 50% of genes.
- Behavior of identical twins can be compared
to that of fraternal twins and siblings.
- Particularly informative when researchers
can find twins who were reared together and twins who were reared
in separate families.
Intelligence
(Nairne, p. 409)
Intelligence
- Estimates of the heritability of intelligence
average around 50%.
- This means that approximately half of the
variability in intelligence in the population is determined by
genes.
Personality
Personality consists of the enduring traits,
dispositions, or characteristics within an individual that give
some measure of consistency to the person’s behavior.
Personality
- Estimates of the heritability of personality
characteristics average around 50%.
Mental Disorder:
Schizophrenia
- Fundamental disturbance to thought, emotion,
or behavior.
- May suffer from delusions, hallucinations,
disorganized speech, "flat" affect.
- Affects 1-2% of population.
- Onset usually occurs in early adulthood.
- Profoundly debilitating.
- No cure.
Schizophrenia
(Nairne, p. 569)
Schizophrenia
- Estimates of the heritability of schizophrenia
average around 50%.
- Heritability of bi-polar disorder may be
somewhat higher.
Interim Summary
- Estimates of the heritability of intelligence,
personality characteristics, and mental disorder seem to hover
around 50%.
- This means that 50% of variability of these
traits is caused by genes.
- This also implies that 50% of the variability
is NOT caused by genes.
Brain Size & Experience
In the 18th Century, Michele Gaetano obtained
2 blackbirds, 2 parrots, 2 goldfinches, and 2 dogs from the same
litter, and trained one member of each pair extensively; the
other was not trained. Trained animals had more folds in cerebellum.
Brain Size & Experience
- Rosenzweig, Bennett, & Diamond (1972):
3 rats from the same litter were raised in three different environments:
Normal, impoverished, enriched.
- Rats raised in enriched environment had greater
thickness of cortex, more weight to cortex, and greater levels
of acetycholinesterase.
Take Home Message
Both nature and nurture matter, and are probably
equally important.
Brain Evolution
- How does the human brain compare to that
of other primates?
- Why is the human brain so big?
- How and why did human cognitive abilities,
such as language, evolve?
Human evolution
- Closest relatives of humans are the African
great apes: Chimpanzees & gorillas.
- Common ancester approx. 5-7 million years
ago.
- Human evolution was not a systematic, linear
progression, but a piecemeal process that included many human-like
forms (overhead).
Brain size
- Human brains are much bigger than those of
great apes.
- Chimpanzees: 385 cc.
- Humans: 1350 cc.
- Most of the increase occurred
late in hominid history.
Distinctive Human Characteristics
- Language
- Components present in great apes, but language is fully expressed
only in humans.
- Right-hand dominance
- May be related to language development.
- Theory of mind
- Our behavior toward others is based on inferences about what
others know about us and the situation.
How did these distinctive characteristics
evolve?
- Charles Darwin
- Early humans were feeble, with few defenses.
- Relied on weapons & tools, & social cooperation.
- Use of weapons & tools led to bipedalism, which freed
hands for making better tools.
- Tool making & social cooperation led to intellectual
development & brain enlargement.
- "Synergistic."
How did these distinctive characteristics
evolve?
- Newer evidence
- Bipedalism developed very early; tool use
& social cooperation came later.
- Earliest hominids had small brains.
- Evidence of tool use dates to about 2.5 myr,
time of first hominids.
- Scavenging probably more common than hunting.
How did these distinctive characteristics
evolve?
- Newer evidence
- Flowering of human cognitive abilities, as
expressed in art, ritual, finely crafted tools, occurred relatively
recently; within the past 100,000 years. This is long after brains
reached their current size.
- Large brains may depend on diet!
Brain Size & Diet
- Most of increase in brain size occurred at
about the time hominids started using stone tools to remove meat
from bone.
- Brain & gut tissue very expensive metabolically.
- Meat is high in calories and easy to digest.
- Eating meat allowed gut to be smaller, freeing
up calories for larger brains.
Summary
- Genotype vs. phenotype.
- Estimates of the heritability of various
behaviors average around 50%.
- Experience has a profound effect on brain
development.
- Genes provide the blueprint; environment
& experience build the house.
- Humans have big brains that support complex
cognitive activities.
- Evidence of big brains arises earlier than
evidence of many of the complex abilities.