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BEHAVIORAL ECOLOGY
(BIOL 627)
Dr. Greg Farley Fort Hays State U. Biology
Lecture 11: Sexual Selection
SEXUAL SELECTION:
Darwin’s attempt to explain evolution of characters,
especially in male animals,
that are:
but that:
“ depends, not on a struggle for existence, but on a
struggle between the [males] for possession for the
[females]; the result is not death to the unsuccessful
competitors, but few or no offspring.”
[ my addition ]
Origin of Species
- Traits persist despite:
energetic cost of producing them
increased risk and exposure to attack
presumably because:
Examples:
1.
2.
3.
4.
5.
Note: high degree of sexual dimorphism in signal
Darwin:
Natural Selection
Sexual Selection
-
phenomena
Modern View:
NATURAL SELECTION 
 SEXUAL SELECTION

ORGANISM
- social environment part of overall environment
- cannot separate phenomena
- POWERFUL
- can overwhelm :
1) INTRASEXUAL -
male:male
female:female
competition among individuals of one sex for matings
Benefit:
Cost:
Leads to evolution of:
- signals:
- various forms of :
- behaviors that are:
ALL of which allow for -
ALL tend to reduce costly battles
Example) positive feedback of dominance
and hormonal control in chickens
2) EPIGAMIC or INTERSEXUAL
- choice of mates
active assessment by sex with:
benefit cost (usually low ? )
majority of time
have greater investment
Based on meiosis
Fundamental difference between the sexes:
size and
number of gametes produced
over given time period
Costs of assessment are in addition to
the direct costs of reproduction
e.g.,
NOTE: these direct costs also
are unequal between the sexes
Ex) Drosophila Bateman
FIGURE 6
unequal costs 
reproductive success
Females:
Males:
SEXUAL SELECTION MODELS
1) Runaway Selection
Fisher (1930s)
selection favors [females] who choose
[males] with particular traits
- initially arbitrary
process continues until
- form of
selection
Important to realize advantage confined
to
- no direct advantage to [females]
Example) Fisher scheme for bird’s tail


- female/male traits covary
- steep slopes unstable
-
ALSO read flow chart (handout)
FIGURE