What is Sexual Selection?
Proposed in 1871 by Charles Darwin
Acts on an organism's ability to obtain or copulate
with a mate.
Organisms have traits which will provide
individuals with advantages in gaining access to
Both humans & animals
• Individuals will compete
with their own sex for
access to the opposite sex
compete with each other
• Large teeth, horns, bigger
• Male-male competition
• Members of one sex will
members of the other sex
(usually females, since
they’re the ones that need
to be impressed)
• Brighter colors, plumage,
courtship display in males
• Female choice
If defense were the driving
force in their evolution, why
wouldn’t females evolve
these traits as well?
Sexual Selection and
Do they differ? Debated.
Male weapons, like large horns, are used both to
attract mates and ward off predators. How would
you differentiate between these two?
Male ornamentation tells us that natural selection
and sexual selection are separate.
Theories of Sexual
Runaway selection- Male ornamentation is driven by
female choice for attractive features. Male features are
NOT a direct indicator of genetic advantage.
Natural selection keeps it in check.
Parasite theory- Adornments evolve to show females
that the males are free of parasites. These males will pass
Males are honest signallers- ornaments correlate with
parasite resistance rather than simply appearing to do so.
Handicap hypothesis- Male ornamentation developed
as impediments to show females their ability to survive.
Female Choice and
Parental investment: males provide the sperm; the
smallest cell in the human body
Females have limited reproductive potential… they
will be more choosy.
A research study in 2009 done by Finkel and
Eastwick supported that in humans, males are
choosier than females in certain social situations.
Males that adopt a long term mating strategy can be
choosy because they are limited in the number of
offspring their relationship can produce.
Why Can’t We All Just Be Asexual?
Cons of Sexual Reproduction
Sex is costly (John Maynard-Smith and George Williams):
1. Cost of meiosis- throwing half of your genes away on
(Some males do not reproduce at all)
3. Cost of courtship
4. Sex only wins out when it doubles the number of
Asexual- 100% of genes passed on
Sexual- 50% of genes passed on.
Why Can’t We
All Just Be
Muller’s ratchet- Harmful mutations that
arise in individuals in an asexual
population will be passed on. The
mutations will accumulate. In sexual
populations, only half of the offspring will
inherit the mutation.
Raffle analogy- In a game of survival and
reproduction, sex leads to variation, and
each offspring is like a new ticket with a
new number. Being asexual means giving
each offspring the same number. There is
not much of a chance to win.
Tangled bank hypothesis- In any given
competition for space, food, and resources,
it is advantageous to diversify.
The Red Queen- Parasites and hosts are in
a perpetual ‘genetic arms race’ in which
one must survive. The host must produce
genetically variable offspring in retaliation
to parasites so that some of them will have
resistance through variability.
Humans: Culture & Sexual
Variability in weight, hair
color, height, etc…
Universally, males prefer
good looks in women. Some
traits are large eyes, good
teeth, shiny hair, full lips,
small jaw, and the hourglass
All features are indicators for
youthfulness and fertilitysignals correlate with levels
financial resources and high
social status in men. More
often than not, these men are
Studies show that women
place greater emphasis on
physical attractiveness in
men in parts of the world
where parasites are more
Fashion and Fertility
Li & Haselton, 2008studied the women’s choice
of dress across ovulatory
fashionable clothing during
the fertile window of their
Low fertility (A) and high
fertility outfits (B).
Female Body Shape
Human beings focus their attention on the female body as a source of pleasure,
attraction, fertility, and reproduction. Across cultures, there is a difference in what
is considered an ideal body shape.
The waist-to-hip ratio (WHR) is the ratio of the circumference of the waist to that of
the hips. It is an indicator of health and a measure of attractiveness. 0.7 is preferred
overall, whereas it is 0.6 in China and 0.8 or 0.9 in South America and Africa .
In 1993, Gangstead and Thornhill claimed that both males and
females prefer mates who are symmetrical.
Results showed correlation between attractiveness rating and
1990: Langlos and Roggman examines whether if
mathematical averageness is linked to facial
Computer-processed composites of each image.
Averaged by pixels in 2, 4, 8, 16, and 32-face
Results showed the 32-composite face is rated as the
Virtual Miss Germany 2002
The real Miss Germany 2002
Composite Miss Germany
Is sexual selection silently working in the background and influencing our