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Copyright © 2006 by Allyn and Bacon
Chapter 2 Evolution, Genetics, and
ExperienceThinking about the Biology of Behavior
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From Dichotomies to Relations and Interactions
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Is it physiological or psychological?
Descartes
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Is it inherited or is it learned?
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Is the “nature-nurture” debate alive today?
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Problems with Traditional Dichotomies – Mind-brain dualism
Brain damage impacts psychological functioning
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Problems with Traditional Dichotomies – Nature-or-nurture
“
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A model of the biology of behavior
All behavior is the product of the interaction of three factors:GenesExperiencePerception of the current
situation
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Thinking about Human Evolution
Evolution does not proceed in a single line.
Humans have only been around for a brief period of time.
Rapid evolutionary changes do occur. Fewer than 1% of all known species
are still in existence.
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Thinking about Human Evolution
Evolution is a tinkerer, not an architect – results are not perfect.
Not all existing behaviors or structures are adaptive.Spandrels – incidental non-adaptive by-
products (such as the human belly button)
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Thinking about Human Evolution
Not all existing adaptive characteristics evolved to perform their current function.Exaptions – evolved to do one thing, but now
do something else (such as bird wings) Similarities among species do not
necessarily mean that the species have common origins.
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Thinking about Human Evolution
Homologous structures – similar structures due to a common evolutionary origin
Analogous structures – similar structures without a common origin
Convergent evolution – the evolution of similar solutions to the same enivironmental demands by unrelated species
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Evolution of the Human Brain
There is no relationship between brain size and intelligence.
Brain size is generally correlated with body size.
More informative to look at relative size of different brain regions.
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Evolution of the Human Brain
Brain stem regulates reflex activities that are critical for survival.
Cerebrum is involved in complex adaptive processes such as learning, perception, and motivation.
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Evolution of the Human Brain
The human brain has increased in size during evolution.
Most of the increase in size has occurred in the cerebrum.
Increased convolutions in the cerebrum have served to increase the volume of the cerebral cortex.
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Fundamental Genetics
Dichotomous traits
True-breeding lines
Mendel studied dichotomous traits in true-breeding lines of pea plants
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Mendel’s Experiments
Crossed a line bred true for brown seeds with one bred true for white
First generation offspring all had brown seeds
When the first generation were bred, the result was ¾ brown and ¼ white seeds.
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Mendel’s Experiments
True-breeding linesWhite (ww)Brown (BB)
Brown was the dominant trait, appearing in all of the 1st generation offspring (Bw)
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Mendel’s Experiments
Phenotype – Genotype –
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Mendel’s Experiments
gene alleles Homozygous Heterozygous
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Chromosomes, Reproduction, and Linkage Genes Chromosomes Meiosis
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Chromosomes, Reproduction, and Linkage Gametes, eggs and sperm, are produced
by meiosis When egg and sperm combine to form a
fertilized egg (zygote), 23 pairs of chromosomes are again present
Mitosis – a form of cell division that yields daughter cells that have 23 pairs of chromosomes
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Chromosomes, Reproduction, and Linkage Meiosis leads to diversity as the 23 pairs
of chromosomes are randomly sorted into the 2 gametes produced
Linkage – the tendency of traits encoded on the same chromosome to be inherited together
Crossing over – increases diversity, “shuffles the genetic deck”
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Sex Chromosomes and Sex-Linked Traits Sex chromosomes, X and Y, look different and
carry different genes Female = XX Male = XY
Sex-linked traits - influenced by genes on the sex chromosomes
Dominant traits on the X chromosome will be seen more commonly in females, recessive ones in males
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The Interaction of Genetic Factors and ExperienceOntogeny –Phylogeny –Ontogenetic development is a product
of gene-experience