Evolution by Natural
Selection
How did life originate?
Why are all the species we see on
earth in existence?
3 main ideas
Creationism
Seeding theory
Evolution by natural selection
In the distant future . . .
Psychology will be based
on a new foundation,
that of the necessary
acquirement of each
mental power and
capacity by gradation.
--Charles Darwin, 1859
Landmarks in the
History of
Evolutionary Thinking
Evolution Before Darwin
 Change
over time in organic structures
(evolution)
 Characteristics
seemed to have a
purpose (porcupines, turtles, skunks)
Jean Pierre Antoine de Monet de
Lamarck (1744-1829)
Two causes of
species change:
1. Progress toward a
higher form
2. Inheritance of
acquired characteristics
Darwin’s Theory of
Natural Selection
 The
explanatory challenge:
1. why change takes place
2. how new species emerge
3. what the functions are of parts
One clue: Malthus: more organisms produced
than survive and reproduce,
leading to “a struggle for existence”
More individuals produced than can possibly
survive
Struggle for Survival
Natural Selection



Variation
Inheritance
Differential Reproduction
The key to natural selection: Differential
reproductive success because of
heritable variants; everyone has
ancestors, but not everyone leaves
descendants
Natural selection provided
3 key answers

Explained change over time (descent with
modification)

Explained apparent purposive quality of
component parts

United all species into one grand tree of descent
(including humans)
Problems That Troubled Darwin
1. Phenomena that seemed inexplicable on the
theory of “survival selection”
2. The existence of sex differences.
Objections to
Natural Selection

No theory of inheritance at the time

Hard to imagine utility of intermediate stages,
and natural selection requires each step to
benefit

Beliefs at the time that species were unchanging
Three Products of Evolutionary
Processes
 1.
Adaptations
 2.
Byproducts
 3.
Noise
Adaptations






1. inherited characteristics
2. reliably developing in most or all species members
3. produced by natural or sexual selection…
4. because they solved an adaptive problem—
functionality
5. must have contributed to reproductive success,
directly or indirectly
6. need not be present at birth (teeth, breasts, beards,
desires, etc.)
One Example of Adaptation: Umbilical
Cord
Byproducts

Characteristics that do not solve adaptive
problems

Do not have functional design

Are “carried along” with characteristics that do
have functional design
Examples of Byproducts:
belly button
More Examples of Byproducts
belly button
 heat from light bulb
 white color of bones
 Attributing intentionality to objects that do
not have intentionality (sun, clouds)

Noise: random effects due to
mutations or perturbations during
development
Examples: shape of belly button;
perturbation in roundness of glass bulb
Adaptations are the
primary products of natural
and sexual selection
Sexual Selection
 Intrasexual
Competition
 Intersexual Selection
“The sight of the peacock gives
me nightmares” –
Charles Darwin
Intrasexual Competition

competition among members of the same sex
for mating
Thought experiment:
Examples in humans of intrasexual
competition?
Intersexual Selection:
Preferential Mate Choice
a form of sexual selection in which members of
one sex are differentially attracted to members of
the opposite sex
Intersexual Selection:
Preferential Mate Choice
Two Kinds Of Causal Questions
 Proximate: HOW a mechanism develops
and operates
 Ultimate: WHY a mechanism evolved—
evolutionary forces that led to the creation of
the mechanism; the adaptive problem it evolved
to solve.
Causal Question
Why are men taller than women on
average?
Proximate and Ultimate Answers
to Questions

Why are men taller than women on average?

Why do people grow calluses?

Why do people like to eat pizza?

Why do people get jealous?