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Evolution by Evolution by Natural SelectionNatural Selection
How did life originate? How did life originate?
Why are all the species we Why are all the species we see on earth in existence? see on earth in existence?
3 main ideas3 main ideas
CreationismCreationism
Seeding theorySeeding theory
Evolution by natural selectionEvolution by natural selection
In the distant In the distant future . . . future . . .
Psychology will be Psychology will be based on a new based on a new
foundation, that of foundation, that of the necessary the necessary
acquirement of acquirement of each mental each mental power and power and capacity by capacity by gradation. gradation.
--Charles Darwin, --Charles Darwin, 18591859
Landmarks in the Landmarks in the History of History of
Evolutionary Evolutionary ThinkingThinking
Evolution Before DarwinEvolution Before Darwin
Change over time in organic Change over time in organic structures (evolution)structures (evolution)
Characteristics seemed to Characteristics seemed to have a purpose (porcupines, have a purpose (porcupines, turtles, skunks)turtles, skunks)
Jean Pierre Antoine de Jean Pierre Antoine de Monet de Monet de LamarckLamarck
(1744-1829)(1744-1829)Two causes of Two causes of species change:species change:
1. Progress toward 1. Progress toward a higher forma higher form
2. Inheritance of 2. Inheritance of acquired acquired characteristicscharacteristics
Darwin’s Theory of Darwin’s Theory of Natural SelectionNatural Selection
The explanatory challenge:The explanatory challenge:
1. why change takes place1. why change takes place
2. how new species emerge2. how new species emerge
3. what the functions are of 3. what the functions are of partsparts
One clue: Malthus: more One clue: Malthus: more organisms produced than survive organisms produced than survive
and reproduce, and reproduce, leading to “a struggle for leading to “a struggle for
existence”existence”
More individuals produced than can possibly
survive
Struggle for SurvivalStruggle for Survival
Natural SelectionNatural Selection
VariationVariation InheritanceInheritance Differential ReproductionDifferential Reproduction
The key to natural selection: The key to natural selection: Differential reproductive Differential reproductive
success because of heritable success because of heritable variantsvariants; everyone has ; everyone has
ancestors, but not everyone ancestors, but not everyone leaves descendantsleaves descendants
Natural selection Natural selection provided provided
3 key answers3 key answers Explained change over time (descent Explained change over time (descent
with modification)with modification)
Explained apparent purposive Explained apparent purposive quality of component partsquality of component parts
United all species into one grand United all species into one grand tree of descent (including humans)tree of descent (including humans)
Problems That Troubled Problems That Troubled DarwinDarwin
1. Phenomena that seemed 1. Phenomena that seemed inexplicable on the theory of inexplicable on the theory of “survival selection”“survival selection”
2. The existence of sex differences.2. The existence of sex differences.
Objections to Objections to Natural SelectionNatural Selection
No theory of inheritance at the timeNo theory of inheritance at the time
Hard to imagine utility of intermediate Hard to imagine utility of intermediate stages, and natural selection requires stages, and natural selection requires each step to benefiteach step to benefit
Beliefs at the time that species were Beliefs at the time that species were unchangingunchanging
Three Products of Three Products of Evolutionary ProcessesEvolutionary Processes
1. Adaptations1. Adaptations
2. Byproducts2. Byproducts
3. Noise3. Noise
AdaptationsAdaptations
1. inherited characteristics1. inherited characteristics 2. reliably developing in most or all species members2. reliably developing in most or all species members 3. produced by natural or sexual selection…3. produced by natural or sexual selection… 4. because they solved an adaptive problem—4. because they solved an adaptive problem—
functionalityfunctionality 5. must have contributed to reproductive success, 5. must have contributed to reproductive success,
directly or indirectlydirectly or indirectly 6. need not be present at birth (teeth, breasts, beards, 6. need not be present at birth (teeth, breasts, beards,
desires, etc.)desires, etc.)
One Example of Adaptation: One Example of Adaptation: Umbilical CordUmbilical Cord
ByproductsByproducts
Characteristics that do not solve Characteristics that do not solve adaptive problemsadaptive problems
Do not have functional designDo not have functional design
Are “carried along” with Are “carried along” with characteristics that do have characteristics that do have functional designfunctional design
Examples of Byproducts: Examples of Byproducts: belly buttonbelly button
More Examples of More Examples of ByproductsByproducts
belly buttonbelly button heat from light bulbheat from light bulb white color of boneswhite color of bones Attributing intentionality to Attributing intentionality to
objects that do not have objects that do not have intentionality (sun, clouds)intentionality (sun, clouds)
NoiseNoise: random effects due to : random effects due to mutations or perturbations during mutations or perturbations during
developmentdevelopment Examples: shape of belly button;Examples: shape of belly button;
perturbation in roundness of glass perturbation in roundness of glass
bulbbulb
Adaptations are the Adaptations are the primary products of natural primary products of natural
and sexual selectionand sexual selection
Sexual SelectionSexual Selection
Intrasexual CompetitionIntrasexual Competition Intersexual SelectionIntersexual Selection
““The sight of the peacock The sight of the peacock gives me nightmares” – gives me nightmares” –
Charles Charles DarwinDarwin
Intrasexual CompetitionIntrasexual Competition
competitioncompetition among members of the among members of the same sex for mating same sex for mating
Thought experiment:Thought experiment:
Examples in humans of Examples in humans of intrasexual competition?intrasexual competition?
Intersexual Selection:Intersexual Selection:Preferential Mate ChoicePreferential Mate Choice
a form of sexual selection in which members of one sex are differentially attracted to members of the opposite sex
Intersexual Selection:Intersexual Selection:Preferential Mate ChoicePreferential Mate Choice
Two Kinds Of Causal Two Kinds Of Causal QuestionsQuestions
Proximate:Proximate: HOW a mechanism HOW a mechanism developsdevelops and and operatesoperates
Ultimate:Ultimate: WHY a mechanism WHY a mechanism evolved—evolutionary forces that led evolved—evolutionary forces that led to the creation of the mechanism; the to the creation of the mechanism; the adaptive problem it evolved to solve.adaptive problem it evolved to solve.
Causal QuestionCausal Question
Why are men taller than Why are men taller than women on average?women on average?
Proximate and Ultimate Proximate and Ultimate Answers Answers
to Questionsto Questions Why are men taller than women on Why are men taller than women on
average?average?
Why do people grow calluses?Why do people grow calluses?
Why do people like to eat pizza?Why do people like to eat pizza?
Why do people get jealous?Why do people get jealous?