Download Sexual Selection

Males and females evolved after sex
Isogamy
Anisogamy
Anisogamy was the result of disruptive selection
Further sexual dimorphism
Often we see species that have some bizarre looking traits that don’t
seem to have any logical function. As a matter of fact, in many cases
these traits would even seem to hurt the organisms survival
Peacock
Peahen
Typically, these elaborate traits are seen in the
males of the species, but not the females
A special form of natural selection explains
the evolution of these puzzling traits
Sexual selection is a consequence of anisogamy
Relative Parental Investment (RPI):
Male vs. Female investment in offspring
Female gametes cost more per unit than male gametes
Female reproduction will be limited by access to resources
Male reproduction will be limited by access to mates
In other words, Sperm is cheap!
Mating Strategies and Reproductive Success
Natural selection will favor males that attempt to mate often
•Males should compete for females
A female’s reproduction is limited by the number of eggs she can
make--> a single mating may fertilize all her eggs
•Females should be choosy about which male she mates with
Intrasexual selection… Male-Male Competition
Marine Iguanas
(Galapagos Islands)
Male bighorn sheep
Humans?
Males are
bigger than
survival rates
would predict
Male Iguanas compete for territories
N
Males that
copulated
Average size
(Snout-vent length)
253 401
Males that tried 343 390
to copulate
Larger males with better
territories were more likely
to reproduce
In northern elephant seals, successful
males have “harems” of females. Fewer
than 1/3 of the males mate. Male-male
competition is vicious and often lethal.
In species where not so much is
at stake less lethal forms of
male competition may be
selected for.
Intrasexual selection… Sperm Competition
1. Larger ejaculates
2. Mate Guarding
3. Copulatory Plugs
4. Other specialized
structures
Intrasexual selection… Infanticide
When male lions take over
a pride of females, they kill
any non-weaned cubs
fathered by previous males
Advantage for males: females return to breeding
condition 8months earlier than they would otherwise
The role of female choice
Darwin recognized that females were typically
the choosers, and males competed tremendously
with each other to be chosen.
Victorian science was not very PC.
Darwin often caricatured his observations by
stating that females were coy by nature and
males were eager, or aggressive, when it came
to mating.
Intersexual selection… Female Choice
Females may select for males
with traits that indicate their quality
Male widowbirds with longer
tails may advertise “good genes”
Courtship displays by males
Intersexual selection… Female Choice
Females may select males
that provide her with a reward
Male hanging flies provide
their mate with a “nuptial
gift” of a dead insect treat
Male redback spiders place
themselves in position to be
eaten by the female
Sexual selection can lead to the
evolution of bizarre traits
Selection on stalk length also increases
female preference
Gender roles depend on relative
investments of males vs. females
In birds, about 90% of species show “monogamy”,
but only 20-25% not promiscuous
Females who choose males that will stick around will do better,
but cheating with a flashy male may get your offspring better genes
Sex role reversals
Gender roles are not set in stone
Male Jacana birds incubate eggs, guard offspring
Females mate with several males- “harem”
Are humans the product of sexual selection?
Are we solely the product of our genes?
Social Darwinism
BUT Evolutionary biology tells us:
 that sex differences are not universal
 that sex differences are not inevitable
Our brains tell us:
 that sex differences are not always desirable
 that biological differences do not justify social inequality