Sperm Compet

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Announcements EXAM II next Wednesday (March
14th) Review Monday (March 12th) No Class Friday
(March 16th)
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Why did ansiogamy evolve?
Can lead to conflict among sexes (which will
invest more). Bigger gametes (more parental
investment - be choosy when selecting
mates) More gametes (less parental investment -
mate as many times as possible)
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Sexual Selection occurs in two forms Intrasexual
Male-male competition Intersexual Female
choice Sexual selection the result of a
non-random association between a (secondary
sexual) character and a component of mating
success." Møller (1994)
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secondary sexual character physical or
behavioral characteristics that appear in
sexually mature animals differentiate between
the sexes without having a direct reproductive
function (e.g, horns, color patterns, facial
hair, breasts)
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Some exaggerated ornaments or displays do not
give a competitive advantage (in competition for
mates) but are directly selected for by females.
Many may even incur a cost to the male
with the display!
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Long tailed widowbirds
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Pied flycatchers
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Sexual selection through female choice
what benefits do females get?
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Evolution of female preferences for male
traits The Fisher Process - runaway sexual
selection Indicator Mechanisms - of heath, good
genes Sensory Bias - pre-existing perceptual
mechanism Species Recognition
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• Evolution of female preferences for male traits
• The Fisher Process - two step process
• Trait starts by conferring an advantage (slightly
  longer tail better flight)
• Preference for trait that confers advantage
  evolves
• Females that mate with males with long tails
  produce offspring with advantage (longer tail)
  and with preference for trait!
• This leads to a runaway process in which the
  preference continues even though the exaggerated
  trait no longer confers a fitness advantage.

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Evolution of female preferences for male
traits Indicator Mechanisms - of heath, good
genes
Handicap Principle (Zahavi) signals are
expensive to produce, can be costly, are
therefore accurately convey information about
quality
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Geoffrey Hill and colleagues showed that house
finch color determined by carotenoids gotten from
diet. Better foragers brighter red.
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evidence for healthy mate hypothesis
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evidence for good genes
Marion Petrie 1994
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The Sexy-Son Hypothesis - Weatherhead and
Robertson (1979) Females choose males with
particular traits because their male offspring
will later be preferred (either because they
out compete other males in a dominance hierarchy
or are more attractive to females).
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Evolution of female preferences for male
traits Sensory Bias - pre-existing perceptual
mechanism
Current signal (context A) exploits a sensory
system that evolved in a different context (for
example foraging). Evidence - preference may
precede evolution of a signal!
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chase-away sexual selection
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What is preference surpasses the traits
range? Use the Environment.
Bower birds
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What is preference surpasses the traits
range? Bring in a wing man. In some
animal societies, cooperation occurs in
close-knit family groups and kin selection
explains apparently selfless behavior. Not so for
the lance-tailed manakin. Males of this little
tropical bird cooperate in spectacular courtship
displays with unrelated partners, and the
benefits of lending a helping wing may only come
years down the line.
March 7, 2007
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Additional reading Sexual Selection Malte
Andersson The Red Queen Sex and the Evolution
of Human Nature Matt Ridley Why is sex fun?
The evolution of human sexuality Jared Diamnod