Download Sexual Selection Natural Selection

The 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
Natural Selection
Sexual selection
Ecological selection
Sexual Dimorphism
Sexual Dimorphism
2) Pipefish
a)
b)
Frigate bird
a)
b)
2) Jumping spider
a)
b)
3) Eclectus parrot
a)
b)
4) Osedax worm
a)
b)
Whale carcass yields bonedevouring worms!!! “Whale fall”
Sexual Dimorphism
1) Red phalarope
a)
b)
1) Rhinoceros beetle
a)
b)
Discovered in
2002 at ~9000
feet depth
Feeds on whale
bones at bottom
of ocean
50-100 males
inside each
female
Genus Osedax
(Latin for "bone
devourer”)
Evolved at same
time as whales
~42 mya
Ovary
“Roots”
Female ~5 cm
Male: 0.2 mm
“Sex-role reversed” species
Sexual Monomorphism
Cryptic
(Prairie vole)
Pipe fish
Conspicuous
SemiConspicuous
(White-faced Gibbon)
(Scarlet macaw)
Giant Water Bug
(Golden lion tamarin)
Sexual Selection-when is it
important?
Arises because one sex is a limiting
resource for the other sex
Midwife Toad
Types of Sexual Selection
Defn:____________________
Intrasexual
Selection
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?
Eclectus roratus
Australian parrot
Nesting sites are scarce (~1
per km2).
Females fight for nesting
sites (tree hollows) and
protect nestlings from
other females. Can remain
on nest for 11 months
per year.
Males forage and feed both
females and nestlings. Up
to 5 males attend each
nest.
Intra-sexual selection
Types of Sexual Selection
Intersexual
Selection
Red-wing blackbird
Defn:
Habronattus americanus
Jumping spider
Hercules beetle
Intersexual Selection
Guppy
Sexually selected behavior
Swordtail
Fruit fly
Bower bird
http://tolweb.org/tree/eukaryotes/animals/arthropoda/arachnida/araneae/salticidae/++salticidae/movies/spid
erdances.html
Artificial head feather in
zebra finches
Why do these preferences
exist?
1. Direct Benefits
2. Indirect
Benefits
3. Sensory Bias
Zebra finch
Direct benefits: hanging
flies and scorpion flies
Direct Benefits
(Thornhill 1976)
Direct benefits cannot explain
mate choice when one sex
provides nothing except gametes
Direct benefits cannot explain mate
choice when one sex provides nothing
except gametes
Indirect Benefits Models
Good genes: _____________________
Runaway: _________________________
A test
for
“Good
Genes”
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: ______________________
Grey tree frog
Fitness differences?
1995
1996
High Low High Low
Fitness Component Food Food Foo Food
d
Larval growth
LC LC
LC
Development time
Mass at
metamorphosis
Larval survival
Postmetamorphic
growth
LC
LC
LC
LC
LC
Indirect Benefits: Fisher’s Runaway
Genotypes
Females
P1 (no preference) P2
(preference)
Males
T2to(no
trait)
T2 (trait)
•P2 females tend
mate
with T2 males.
•P1 females mate randomly.
•T2 males have higher mating success than T1,
so
T2 increases in frequency.
•Offspring of P2 females inherit both T2 & P2,
==>
“genetic correlation” of preference
& trait.
•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
______________________________________
______________________________________
______________________________________
_______________
Guppies
Swordtails
Zebra finch
Genus Xiphophorus
Natural Selection
Ecological selection
Sexual selection
Male swordtail
Male platy
Tradeoffs between natural
and sexual selection
Guppy Predators
Small
predators
Tungara frog
Large
predators