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Adaptation and Evolution
Natural Selection
Proposed by Darwin
Natural Selection
Kin Selection
Reciprocal Altruism
Parental Investment
Sexual Selection
Based on 3 fundamental
observable facts:
Reproductive Strategies of Male and Female Primates
Adaptation and Evolution –
processes of biological of change over
Variation - Lots of variation within and
between species
•Adaptation = short term change
within a species (ex moth wings
change color) (aka microevolution)
•Evolution = long term change that
results in the appearance of new
species (aka macroevolution)
How do species change over
Inheritance - Variation
passed from parents to
By a process called Natural Selection
Natural selection is the primary
mechanism of biological change over
time (adaptation and evolution)
Principles of
inheritance were
not known when
Darwin wrote
Origin of Species
Overproduction – there are
always more offspring born than
can or do survive to adulthood
Survival & Reproduction
Survival alone is not the important thing in
understanding evolution by natural selection
Survival and then reproduction is the key to
understanding adaptation, and evolutionary
change over time
Fitness - The ability to live and reproduce
Overproduction means competition to survive
to adulthood
Some will survive to adulthood
Some won’t
Who survives to adulthood?
Those who happen to have
variation (morphology or
behavior) that helps them
Who doesn’t?
Those who have variation
(morphology or behavior) that
doesn’t help them survive
The environment naturally selects
some variants over others
High fitness =
Low fitness =
Those traits possessed by individuals with high
survival and reproductive success, will be passed
to future generations at higher rates.
The environment naturally selects some variations
(some individuals) over others to survive and
The population, over time, becomes better and better
adapted to local environments.
by Natural Selection
The differential survival and reproductive
success in each parental generation
leads to
A change in the frequency of traits from one
generation to the next
The traits that
• are well suited to the
• give individuals an
advantage in survival
and reproductive
The traits that
• are not well suited to
the environment
• put individuals at a
disadvantage in
survival and
reproductive success
Will appear in increased
frequency in future
Will appear in decreased
frequency in future
Peppered Moth – nicely illustrates the
following 4 points about natural selection and
Evolution (change) operates on the population – no
individual moth ever changed color. Individuals do not
2. Natural selection operates on the individual.
Individuals survive and reproduce or not.
3. Variation had to be there in the first place
4. No such thing as an absolutely better variant – depends
on the environment
by Natural Selection
The differential survival and reproductive
success in each parental generation
leads to
Peppered Moth – example of natural
selection producing adaptation
A change in the frequency of traits in the next
• Applying evolutionary principles,
specifically natural selection, to behavior
• Trying to understand how behavior might
be naturally selected/adapted/evolved
• Asking how behavioral variations give
individuals an advantage in survival and
Developed in the 1960’s and 1970’s with the
development of 3 main ideas:
1. Kin selection
2. Reciprocal Altruism
3. Parental Investment
It all started with the question of altruism –
how could altruism evolve
What is altruism?
Popular definition - helping behavior, selfless behavior,
sharing behavior. Opposite of selfish behavior.
Biological definition - behavior that potentially improves
the survival & reproductive success of the recipient
while potentially endangering the actor
Behavior that benefits the recipient at a cost to the actor
Altruism and Group Selection
Originally, altruistic behavior was thought to
evolve because it was “for the good of the
Group Selection – read in chapter 4 of text
What was the problem with
Group Selection?
Based on what we know about natural selection,
behavior that increases the survival and
reproductive success of the actor will be passed on
at higher rate to next generation (will be favored
by natural selection)
Behavior that decreases the survival and
reproductive success of the actor will not
So how could altruism possibly get passed on?
How could it evolve?
Altruism - examples
The answer to this question was
the origin of sociobiology
Predator alarm calls
Helping another monkey in a fight
Defending a group member from a predator
Feeding/caring for a baby
Altruism evolves not by group selection, but
by kin selection
Kin selection first principle of sociobiology
Hamilton 1963 & 1964
Kin selection based on a
recognition that:
Does altruism ever occur
between non-relatives?
• Each individual shares genetic material with
their relatives
• It is possible to have some of your genes
represented in the next generation even if
you never reproduce, if your relatives are
reproductively successful
Altruism between non-relatives
cannot be explained by kin selection
Important new concepts
introduced with kin selection
Reciprocal Altruism
1. Natural selection can operate on
genes rather than individuals
2. Inclusive fitness – individual fitness
plus effect upon the fitness of relatives
Read about Hamilton’s model in Chapter 4
Food sharing
Most altruism in
primates does
occur between kin
Coalition formation supporting your kin in
agonistic encounters
Alternative explanation proposed:
Reciprocal Altruism
(Trivers 1971)
Helping behavior between non relatives
Individuals help others so that they will get
help in the future when they need it.
Eventually it will pay off – benefits to the
individual will outweigh the costs
Social grooming
Developed in the 1960’s and 1970’s with the
development of 3 main ideas:
1. Kin selection
2. Reciprocal Altruism
3. Parental Investment
It all started with the question of altruism –
how could altruism evolve
Parental Investment
(Trivers 1972)
Sexual Dimorphism
Kin selection and reciprocal altruism are
about explaining altruism
Parental Investment is about explaining sex
differences in behavior (& morphology)
Parental investment is a reformulation of
Darwin’s theory of Sexual Selection
Sexual Dimorphism
Sexual Selection – a mechanism
proposed by Darwin to explain secondary
sexual characteristics
What are secondary sexual
•features of sexual dimorphism
•male female distinctions that appear at puberty
•distinctions between adult males and females
that go beyond basic reproductive differences
Why was another mechanism
needed to explain these features?
Because secondary sexual
characteristics are
1. Not easily explained by
natural selection
•differences in size, coloration, weaponry, shape,
not apparently beneficial to
sometimes even detrimental
to survival
2. Secondary sexual
characteristics don’t appear
until adulthood
According to Darwin, secondary
sexual characteristics evolved due to
Male male competition
Female choice
Male – male competition
(intrasexual selection)
Secondary sexual characteristics give
males a reproductive advantage
through competition with other males.
Natural Selection (NS) vs Sexual
Selection (SS)?
Reproductive success is the bottom line for both, but
• NS emphasizes survival where SS cuts straight to
• NS emphasizes getting to adulthood (the regular
season). Once there, SS kicks in (the playoffs).
• The “environment” is different – for SS it is often a
social environment doing the selecting
• SS operates more on males
Triver’s (1972)
Applied the principles of sexual selection to
male-female behaviour as well as
Identified something he called Parental
Investment. Trivers states that "the relative
parental investment of the sexes in their
young is the key variable controlling sexual
Female choice (intersexual selection)
Secondary sexual
characteristics give
males a reproductive
advantage by
making them more
attractive to females.
Definition of Parental
"Any investment by the parent in an
individual offspring that increases the
offspring's chance of survival (and
hence reproductive success) at the cost
of the parent's ability to invest in other
Any investment that a parent makes in
one offspring decreases the ability to
invest in others
Limited “investment” budget – spending in
one place means not spending in
The different initial and then subsequent levels of
parental investment leads to very different
strategies for males and females (main thrust of
Triver’s famous 1972 paper)
Females – be very choosy – choose males that
will provide assistance, good territory, good
genes etc.
Males – go for as many fertilizations as possible
and move on – little or nothing to lose
Great potential for conflict and
competition between
Variation between and within species
1. parents and
offspring (weaning
2. successive
offspring (sibling
rivalry for
The more similar the level of parental investment
between the sexes, the more similar the sexes
will be in behavior and/or morphology
• Monogamy and monomorphism (owl monkey)
• Polygyny and dimorphism (hamadryas baboons)
• Within species – human mate choice studies
3. Parents (battle of
the sexes)
Important point – it is not about being male or female, per se, but
about the level of PI – this is what determines sex differences
Typical* Male vs Female
PI in Mammals
Females HIGH
• Large nutrient rich egg
• 1 of approx. 400
• Gestation
• Lactation
Males LOW
• Tiny sperm (DNA with tail)
• 1 of approx. 4 billion
• no
• no
3 reasons that male and female
reproductive strategies and behavior
are typically so different:
1. Different levels of parental investment
2. Different variance in reproductive success
(aka lower potential reproductive output
for females)
3. Different limiting factors
* Lots of variation between and within species
Variance in reproductive success
among female primates:
Females typically have similar numbers of babies
They typically have X number of offspring
where X = reproductive lifespan divided by
interbirth interval.
3. Limiting Factors
Females limited by access to energetic
Males limited by access to reproductive
Not that much difference between least and most
successful females (at least not compared with
Variance in reproductive success
among male primates:
Much more variation among males than among females
Males could be big winners or big losers.
One male could theoretically be responsible for 100% of
the offspring born. Or none.
The stakes are much higher for males, and the
reproductive striving therefore much more intense.
Review – 3 factors responsible for
different behavior/reproductive strategies
of male and female primates:
• Less variance/lower
total output
• More parental
• Limited by access to
energetic resources
• Great variance/higher
total output (potentially)
• Less parental investment
• Limited by access to
fertile females
Guinness World Book of Records 1997
Maximum number of offspring produced by a single
human female = 69
Maximum number of offspring produced by a single
human male = 888
Much greater variance in reproductive success for
males than for females (at least in theory)
Much greater potential reproductive output for males
than for males