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Extra credit question
Sexual selection theory predicts that
___________ pipefish should compete
with each other for access to mates,
and that ___________ will be the
choosy sex.
Darwin’s Problem
If selection favors alleles that increase
survival and reproduction, how does
one explain conspicuous traits such
as bright colors, horns, and displays
of many species that seemingly
lower survival of the bearers?
Darwin’s Solution: Sexual
Selection
Selection that arises from differences in
mating success (# of mates that bear
progeny over a defined time interval).
Natural Selection
Ecological selection
Sexual selection
Sexual Selection
Arises because one sex is a limiting
resource for the other sex
Q: How can one sex be a limiting resource
for the other?
Q: Which sex will be a limiting resource?
Q: Can both sexes be a limiting resource?
Butterflies
Pipe fish
Giant Water Bug
Midwife Toad
Types of Sexual Selection
Intrasexual
Contests between members
of one sex for access to
members of the other sex
Intra-sexual selection
Types of Sexual Selection
Intersexual
Selection via mate choice:
Individuals of one sex
preferentially mate with
individuals of the other sex
based on particular
phenotypes (size, color pattern,
behavior, etc.)
Intersexual Selection
Sexually selected behavior
http://tolweb.org/tree/eukaryotes/animals/arthropoda/arachnida/araneae/salticidae/++salticidae/movies/spiderdances.html
Artificial head feather in zebra
finches
Why do these preferences
exist?
1. Direct Benefits
2. Indirect Benefits
3. Sensory Bias
Direct benefits: hanging flies
and scorpion flies
(Thornhill 1976)
Direct benefits cannot explain mate
choice when one sex provides nothing
except gametes
•
Indirect Benefits Models
•
Good genes: females prefer traits that
indicate genetic quality; high-quality mates
pass on ‘good genes’ to their offspring.
Runaway: females prefer arbitrary
traits; preference for arbitrary traits leads to
‘runaway’ process.
•
Indirect Benefits: Good Genes
Females prefer traits that indicate
genetic quality; high-quality mates
pass on ‘good genes’ to their
offspring.
Females choose based on some trait
that indicates high fitness.
Assumption: both trait and fitness
must be heritable, (h2 > 0).
A test for
“Good
Genes”
Fitness differences?
1995
Fitness Component
Larval growth
High
Food
1996
Low High Low
Food Food Food
LC
LC
Development time
LC
LC
Mass at
metamorphosis
Larval survival
LC
Postmetamorphic
growth
LC
Fitness Component
Larval growth
LC
Development time
LC
Mass at
metamorphosis
Larval survival
LC
LC
Indirect Benefits: Runaway
Females
Males
Genotypes
P1 (no preference)
P2 (preference)
T2 (no trait)
T2 (trait)
P2 females tend to mate with T2 males.
P1 females mate randomly.
Offspring of P2 females inherit both T2 & P2, ==> “genetic
correlation” between preference & trait.
T2 males have higher mating success than T1, so T2
increases in frequency.
P2 increases in freq b/c it is co-inherited with T2.
Elaboration (runaway) of trait & preference until mating
advantage of trait outweighed by survival cost.
Sensory bias
Preference for a trait may be a by-product of the sensory system
that has adapted to be sensitive to certain kinds of signals for
reasons that are unrelated to mating.
Guppies
Swordtails
Zebra finch
Genus Xiphophorus
Male swordtail
Male platy
Natural Selection
Ecological selection
Sexual selection
Tradeoffs between natural and
sexual selection