social selection

1. Sexual selection

2. Natural selection tends to produce individuals that
are well adapted to their environment. However,
sexual selection does not adapt the individual to
the environment but does enhance traits involved
in mate acquisition. Moreover, sexual selection
can produce individuals with such elaborate
ornaments that they must be either energetically
costly to develop, costly to maintain, or even lead
to a direct survival cost for the individual that
bears the ornament.Darwin's theory of sexual gave
a plausible explaination for the origin of many
splendid if not bizarre ornaments.

3. Sexual selection
Darwin’s second “major” book:
1871 On the Descent of Man, and
Selection in Relation to Sex
Why a theory of sexual selection?
Darwin needed a theory to explain the many
extravagant traits that seem to reduce survival
e.g. the peacock’s tail
What is sexual selection?
Sexual selection is distinguished from
natural selection by the following criterion:
Sexual selection arises through variance in
mating success

4. Sexual selection
Is sexual selection different from natural selection?
Darwin saw them as distinct - only sexual selection
could produce traits that compromise survival
The basic principles are identical – selection favors
whatever gets more genes into the next generation
In sexual selection, fitness is measured relative to
members of the same sex
Two kinds of sexual selection
Intrasexual selection – mating success determined by within-sex interactions
e.g., male-male combat
Intersexual selection – mating success determined by between-sex interactions
e.g., female choice of males

5. The peacock’s tail
Extravagant male ornaments
The peacock’s tail greatly impairs his mobility…how could such a trait evolve?

6. Male ornaments
Long-tailed widowbird
Males with unnaturally long tails attract
females away from the nests of “normal”
males or males with shortened tails

7. Hypotheses for male ornaments
Fisher’s “runaway” hypothesis
Mate choice originally evolved to facilitate adaptive
choice for traits conferring a survival advantage
Once female preference evolved, any genes that
conferred survival advantage but compromised
attractiveness would not be passed on because
surviving males would fail to mate
Zahavi’s “handicap” hypotheses
Extravagant male traits are costly to develop and maintain
Choosing a mate with “good genes” requires an honest
signal of genetic quality
Only males in good condition (those with good genes)
will be able to fully develop and maintain an ornament
Amotz Zahavi
Ronald Fisher