2nd half Lecture 3
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• The power of parasites. A popular hypothesis: parasites control the behaviour of the host
(“extended phenotype ”) Phenotype can encompass ot her bodies.
• The culture of science (1): The impor tance of addressing alternative hypotheses
• e.g. Evidence to support the parasite manipulation hypothesis over the “illness” and other
hypotheses - - proximate evidence to test an ultimate hypothesis
(2) The decline of effect sizes when studies are repeated (the so called “wearing off of scientific
truth”). Found in scientific endeavours – in some cases – as different as animal behaviour and
tests of drug eff icacy.
Conflict Between and Within the Sexes: Sexual Dimorphism
Male Bowerbirds build a structure (extended phenotype) to attract females.
Spider web (extended phenotype)
Hangf ly/Scorpion f ly: male genetilia looks like scorpion tail. Male makes balls for m end of
abdomen which are powerful pheromones to attract the females. Males catch prey then signal for
the female who struggles for the prey. Male copulates as they struggle for the prey. Female reject
or break up quickly if prey is small. Males break up if have a big prey because can use it to mate
with other females! 20 min -> time for full mating. M ale break up at 20min mark if they can
reach it. Females want to fully feed on prey but t he male does not want her to! Has Darwinian
selection, female choice, and conf lict.
sexual dimorphism -> male and females look different. Females can use to judge males from the
display. Eg) Peacock, elephant seals, spiders, armory beetles, crustacean barnacle larvae,
humans. Males are more diverse than females. But male displays are anti-survival. Maybe the
display is a sign of a good survivor? Females prefer these traits!
Sexual selection is about insemination success, not mate getting ability.
• sexual differences (males typically more showy)?
• do the males of related species differ more greatly than the females?
Viabil ity selection: the selection of individual organisms who can survive until they are able to
reproduce. Against massive male display.
From Gary Larson The hypothesis about strong and rapid selection on male traits presents a
paradox: how can this selection be strong enough to counter viability selection not just on males
but also on females?
Mechanisms of Sexual Selection
• Intra sexual
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