Exam 1 Results: Mean: 75.7
Std. Dev: 10.3
Histogram
12
Frequency Distribution for Column 1
To (<)
Count
43.500
49.111
1
49.111
54.722
0
54.722
60.333
2
60.333
65.944
4
65.944
71.556
7
71.556
77.167
10
77.167
82.778
11
82.778
88.389
7
88.389
94.000
4
Total
46
10
8
Count
From (!)
6
4
2
0
40
50
60
70
80
Column 1
90
100
Recall: Sexual selection…
• Viability: probability of survival to reproductive age
• Mating Success
Sexual selection: differential reproductive success
due to differential mating success
• Fecundity: viable offspring per female
Differences in energetic costs of reproduction lead to
competition by one sex for the other.
usually, males compete for females
So far, we’ve seen intrasexual selection: females mate with
which ever male gets access to them
Selects for male traits that allow them to dominate, intimidate or
sexually outcompete rival males…
1
Intrasexual selection: competition between males
1. Direct competition for mates
Forms of intrasexual selection…
2. Sperm competition:
e.g., barbed penises
e.g., large testes
3. Infanticide:
2
Intersexual selection: mate choice
Females often mate preferentially with showy males
Enlarged, ornamented traits; coloration; vocalization; other
behavior.
Showy male
Drab female
Like intrasexual selection, leads to evolution of sexual dimorphism
Mate choice: female widowbirds prefer longer-tailed males
Futuyma, Fig. 11.9
3
Intersexual selection and speciation
Leads to trait divergence
among isolated populations:
role in reproductive isolation,
ultimately speciation
Traits favored by mate choice my compromise a male’s viability
e.g., guppy coloration with predators
e.g., Vocalizations in túngara frogs:
Physalaemus pustulosis
4
Rapid spread of silent phenotype following parasite invasion
(2006)
http://www.newsroom.ucr.edu/cgi-bin/display.cgi?flash=1418
On Kauai, > 90% of male crickets now have mutant wing (“flatwing”),
less than 20 generations after introduction of parasitoid fly
Toothy comb (normal male)
>30% parasitized
Also see shift in male behavior:
flatwings move near calling
males to pick up females
Mutant (flatwing) male:
<1% parasitized
5
Mate choice: How and why do female preferences arise?
Explanation 1: Direct benefits. Females choose traits with direct
benefits for her or her offspring
Direct fitness advantage to choosiness, so the preference is
favored by natural selection.
e.g., female birds choose males with best territory for resources
Mate choice: How and why do female preferences arise?
Explanation 1: Direct benefits. Females choose traits with direct
benefits for her or her offspring
Direct fitness advantage to choosiness, so the preference is
favored by natural selection.
e.g., female birds choose males with best territory for resources
e.g., female hangingflies prefer males who provide larger prey during mating.
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Explanation 2: Sensory bias. Intrinsic preference for particular
sensory stimuli. Female’s preference precedes the evolution of the
preferred male trait.
e.g., Prey orientation: lizard courtship movements
Explanation 2: Sensory bias. Intrinsic preference for particular
sensory stimuli. Female’s preference precedes the evolution of the
preferred male trait.
e.g., Prey orientation: lizard courtship movements
e.g., Females of swordfish fish species 1 prefer long tails even though no
males possess them: preference predates the species.
Species 1
Species 2
Female preference for long tails
arose before Species 1 and 2
diverged from their common ancestor
P+: females prefer long tails
T+: males possess long tails
7
Sensory bias and female preference…
Zebra finches females prefer males with an artificial white crest
Explanation 3: Indirect benefits. Females prefer males with traits that
indirectly benefit the female or her offspring.
a) “Sexy son” hypothesis (= Runaway sexual selection)
(=“Fisherian model”)
Indirect fitness benefit to female who chooses attractive male: her sons
will be attractive and so she’ll have more grandchildren
8
Explanation 3: Indirect benefits. Females prefer males with traits that
indirectly benefit the female or her offspring.
a) “Sexy son” hypothesis (= Runaway sexual selection)
(=“Fisherian model”)
Indirect fitness benefit to female who chooses attractive male: her sons
will be attractive and so she’ll have more grandchildren
Future generations will
pass on the alleles for
the trait (expressed in
males) and the alleles
for the trait preference
(expressed in
females).
(Establishes
genetic
correlation
between the trait
(T) and
preference for
the trait (P))
Can lead to fixation of the trait
& preference through positive
frequency-dependent selection
Sexy son hypothesis illustrated: sandfly
Offspring of most vs
least attractive males do
not differ in viability or
fecundity (=no direct
benefits).
Lutzomyia
longipalpis
But sons of attractive
fathers have greatest
mating success.
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Indirect benefits…
b) Indicators of genetic quality: ‘Good genes’
Females evolve preference for male traits associated with high fitness
(viability).
e.g., Female stickleback prefer males with red bellies.
Red-bellies= Males are well nourished and have good immune
systems (from carotenoids). Offspring are resistant to parasites:
Good genes…
Females prefer energetic courtship displays
10
Good genes…
Male gray tree
frogs with longer
calls sire fitter
offspring:
Hyla versicolor
http://video.google.com/videoplay?docid=-1630917301467247878&q=gray+tree+frog&hl=en
Evolutionary Conflict between Sexes
For unrelated mates: the fitness of a mate only matters to
the extent that it ensures the fitness of the offspring…
Intersexual selection can lead to antagonistic coevolution:
selection favors traits in one sex that reduce the fitness of
the other sex
Evolutionary “arms race” between the sexes
11
Antagonistic coevolution…
At the level of gametes:
1. Abalones (e.g., Haliotis)
• Selection on sperm: increased speed at
penetrating eggs (lysin protein activity)
• Selection on eggs: increased resistance,
to prevent polyspermy
Haliotis sp.
Molecular evolution of lysin gene: rapid
change at nonsynonymous sites;
selection to alter protein to overcome
egg barrier
lysin
2. Proteins that make Drosophila sperm resistant to
dislodging are toxic to females.
Antagonistic coevolution…
At the level of individuals:
Copulation leads to injury or death
e.g., traumatic insemination (bedbugs)
e.g., drowning of female ducks
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Antagonistic coevolution…
At the level of individuals:
Copulation leads to injury or death
e.g., traumatic insemination (bedbugs)
e.g., drowning of female ducks
May select for female traits/preferences that confer resistance to
males’ inducements to mate: “Chase-away sexual selection”
e.g., water striders:
Males have evolve flattened abdomens that
facilitate forced copulation; females have
evolved spines to obstruct males’ grip
Futuyma, Fig. 14.14
Chase-away sexual selection may
explain why female preference for a
male trait is sometimes greatest in
species where the trait hasn’t yet
evolved (e.g., swordtails, zebra finches)
Antagonistic coevolution at the level of individuals…
Sexual cannibalism
>60% of the diet of female praying mantises may be males.
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2.
1.
4.
3.
Male praying mantises avoid being eaten when they can.
Males are wary around
hungry females: jump
on her from far away to
avoid opportunties of
being eaten
14
But not all sexual cannibalism is coevolutionary antagonism:
Australian redback spider: males encourage cannibalism
Males that are cannibalized:
• mate >2X longer
• produce 2X as many offspring
• insert genital plug.
Direct benefits
http://www.scar.utoronto.ca/~mandrade/video/somersault_clip_large.mov
15