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 place2. how new species emerge2. how new species emerge3. what the functions are of parts3. what the functions are of parts

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 quality Explained apparent purposive quality of component partsof component parts

United all species into one grand tree United all species into one grand tree of descent (including humans)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 Hard to imagine utility of intermediate stages, and natural intermediate stages, and natural selection requires each step to benefitselection requires each 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?