2nd half Lecture 7
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• Dimor phism in horn size & concomitant mating tactics in dung beetles
• The “hor n type” of a male is conditional not only on its own body size, but also on a cue of the
level of sexual competition that the larval male is likely to encounter as an adult
• Because dung quality is predictable, being high when fruit is a large part of the monkey diet,
male beetles can vary their development adaptively.
• When the quality of dung eaten is high, male larvae of a certain body size are more likely to
mature hornless because the high-quality food signals that the population-wide body size of r ival
males will on average be higher.
• Quantity of food (random) + quality (predictable) => decides how the male beetles will
• The Stalk-eyed fly male grows eyes on very long stalks to impress the females. Sexual
dimorph ism. Has costs to m ales! Tropical: found in groups on plants.
Genes and individuals can be in conf li ct.
Mate Choice and Levels of Selection
• goods or services -> females want for direct benefits
• good genes -> good for the of fspr ing
compatibility -> no inbreeding
“runaway” issues-> not want male to run away
• Females being duped (sensory exploitation)
Some correlated evolution: can select for males to have longer (benefits) or shorter eye stalks, but
as do so the sisters of males are also affected to a lesser degree even though it is costly to them
and has no advantage! Males and females evolve together through sexual selection even though it
hur ts the females.
Cyrtodiopsis phylogeny. As increase body length, the eye stalk length also increases but does so
for males more strongly than females. This shows sexual dimor phism.
Can get a female only clutch, but rare to get a male only clutch! Clutches are female biased! Get
female only clutch with long eye stalk only! But ration should be 1:1 because it is adaptive to
make the rarer sex!
Females do not mate with the shor ter eye males because they are more likely to car r y the selfish
genetic element on their X chromosome which kills the Y chromosome during meiosis resulting
in female only offspring! Locus of the selfish genetic element is very close to t he locus of the
allele which controls stalk length! It makes eye stalks shor ter! Mating with shor t stalked eye
males lowers female fitness.
Indirect loss: no sons
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