Bio 220 Lectures 2 and 3

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8 Apr 2012
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Lecture 2 and 3 BIO220
(Rowe)
The Evolution of Sexual Differences (Dimorphisms)
The Evolution of Sexual Differences (Dimorphisms)
The puzzles of conspicuous ornaments
Darwin’s answer(hypothesis)
Origin of sexual differences
Male-male competition
Sexual Dimorphism
Conspicuous ornaments
Why should males have these costly ornaments to attract females but that reduce fitness?
Sexual Selection
Limited to the fitness that was approved through mating
Darwin mentioned this
“.....depends on the advantage which certain individuals have other individuals of the same sex
and species, in exclusive relation to reproduction [mating].” –Charles Darwin
Function solely to enhance mating success
A subset of natural selection but instead of survival or number of offspring, it’s
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Why are there 2 sexes?
Unequal-sized gametes
Anisogamy
Sperm
Inexpensive
Many
Small
Egg
Resource-rich
Few
Large (largest cell in the human body)
Parental Investment Theory
Robert Trivers (1972)
Unequal investment between sexes
There will be competition in the sex that invests more, therefore females (because of the
costly production of eggs)
Produces male competition and female choice
Intrasexual Selection
Male-male competition
Pre-copulatory # of copulations
Post-copulatory success of copulation (sperm competition)
Sperm competition
Mate Guarding
Traumatic insemination in bed bugs to stop other males from inseminating
Scenting
Copulation Duration (prolonged)
Sperm removal (damselflies again), organ *horn+ to take out the last guys’ sperm
Intersexual Selection
Female choice
Why do the females favour these odd traits
Why do male widow birds have such long tails?
Experiment
Question: Are long tails preferred by females?
Experiment: Manipulate tail length
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