Biological Bases of BehaviorBiological Bases of Behavior

Section 5 – Evolution, Genetics, And Heritability

Natural SelectionNatural Selection

• Evolutionary theory, developed by English naturalist Charles Darwin (1809-1888) in ( )1859, describes the way our bodies and behaviors change across many

tigenerations.• Darwin propose the notion of natural

l ti i h i b hi hselection is a mechanism by which organisms have developed and changed based on the “survival of the fittest ”based on the survival of the fittest.

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Natural SelectionNatural Selection

• The individuals who can adapt to their environment will have an advantage for gsurvival and will eventually produce more offspring. These individuals have been

l t d b t f i lselected by nature for survival.• Over time, an increasing percentage of our

b h i h f b i t ll d bbehavior has gone from being controlled by instincts to more voluntary control by our brainsbrains.

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Natural SelectionNatural Selection

• Thus, biopsychologists used evolutionary theory to study the nervous system to i i t it i fl dpinpoint its influence on our mood,

feelings, drives, thought processes, and behavior.behavior.

• Evolutionary theory provides unified framework for integrating a variety of psychological phenomena, but does not provide all answers for psychological questionsquestions.

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GeneticsGenetics

• Inside each cell, our genes provide the basic physiological building blocks for the h dit t i i f di ti tihereditary transmission of our distinctive characteristics or behavior patterns, called our biological traits. The degree by whichour biological traits. The degree by which genetics plays a role in controlling behavior is a debate that has been going on for

d i th h ti th t thyears and is the research question that the branch of psychology, called behavioral genetics, aims to study.g s, a s o s udy

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Mendel’s PeasMendel s Peas

• The Austrian monk and botanist GregorMendel (1822-1884) is considered the ( )father of the modern genetics due to his breeding experiments on common varieties f th dof the garden pea.

• He found that if true-breeding tall pea l t d ith t b diplants are crossed with true-breeding

dwarf pea plants, the offspring will always be tallbe tall.

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Mendel’s PeasMendel s Peas

• He referred to the stronger attribute that appeared in th fi t ti fthe first generation of offspring as the dominant trait (tallness), and the ( ),weaker trait, a recessive trait (dwarfism).

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Mendel’s PeasMendel s Peas

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Mendel’s PeasMendel s Peas

• Whenever the dominant gene is paired with a recessive gene, Mendel found that g ,the expression of this genotype called the phenotype, is of the dominant trait.

• Therefore we have a 3:1 ratio of tallness versus dwarfism

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Human Genotypes & PhenotypesHuman Genotypes & Phenotypes

• Human genetics are a bit more complex than Mendels peas, since the expression of p , pa single genotype can give rise to a wide range of phenotypes.

• For example, human height is largely genetically controlled, but a number of

i t l f t h t itienvironmental factors, such as nutrition, hormones, and immune system deficiency, can lead to a range of phenotypic heightscan lead to a range of phenotypic heights.

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The Human Genome ProjectThe Human Genome Project

• Efforts are underway to map the entire human genome. The 15 year Human g yGenome Project has recently preliminarily mapped the entire human genome, and it i h d th t thi i ill llis hoped that this mapping will allow scientists to better understand how genes cause or contribute to disease andcause or contribute to disease and behavior.

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HeritabilityHeritability

• Heritability is the technical term that refers to the extent to which variation among gindividuals is due to genetic causes.

• For example, height and hair color are highly heritable because both can be predicted on the basis of inheritance from b th tboth parents.

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HeritabilityHeritability

• Heritability is always estimated with respect to a given trait in a given p g gpopulation at a given time. It does not apply to single individuals.

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Chromosomes Chromosomes

• Genes are parts of chromosomes, rod-shaped bodies that contain many genes that are found i id th l f llinside the nucleus of cells.

• Each species has different numbers of chromosomes; humans have 46 (23 pairs) in eachchromosomes; humans have 46 (23 pairs) in each cell. One of each pair came from each parent at conception.Ch i d f d ib l i• Chromosomes are comprised of deoxyribonucleic acid (DNA)and govern everything from eye color to blood type and sex.yp

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Chromosomes Chromosomes

• Sex is determined by special chromosomes, known as X and Y chromosomes. Females are XX and males XY.

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Selective BreedingSelective Breeding

• Through selective breeding, animals have been bred to possess and express various p ptraits. Experiments have also shown that other, psychological, traits, such as i t lli b i fl d th hintelligence can be influenced through selective breeding.

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Nature and NurtureNature and Nurture

• Genes and the environment interact in various ways to give various results. y gThrough twin studies researchers can begin to uncover the contributions of both, since id ti l t i h i il iti iidentical twins show similarities in intelligence and other characteristics, even if they have been raised separately inif they have been raised separately in different environments.

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Nature and NurtureNature and Nurture

• Our genes do not determine everything about us; our environment plays a definite ; p yrole on who we become, but the relative contribution of each for our traits such as i t lli i till d d b tintelligence is still under debate.

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Mendel’s PeasMendel s Peas

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