What is sexual selection?

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What is sexual selection?

• Selection that acts on traits used to attract
  /or keep mates.
• Generally, males need to convince females that
  they have fit genes - females need to pick
  males with fittest genes.
• Recall two different mechanisms
• - Female choice -
• - Male Contest -

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Sexual Selection Females
Females must behave in a way that ensures that
their offspring survive and mate, and that they
have as many offspring as possible If they
behave this way, the genes for this behavior are
passed on. If they dont behave this way, the
genes are not passed on. (remember, over
millions of years all of your ancestors
have successfully mated)
Behavior 1 Pick a mate who will help you raise
the young (if young have long infancy period)
Behavior 2 Pick a mate who has the fittest
genes for your environment and mate with
them (if young have short infancy period or long
infancy period)
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Sexual Selection Males
Males also behave in a way that maximizes the
propagation of their genes. The ones who
possess traits that allow them to mate and have
many surviving offspring pass on those prolific
genes.
Behavior 1 Choose one female to raise the young
with (if young have long infancy period)
Behavior 2 Convince the females that you have
the fittest genes for their environment (if
young have short infancy period or long)
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Behavior 1 male and female identical and raise
young together
(Gibbons, Albatrosses, many seals)
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Behavior 2 Males and females are
differentMales invest more in contest/courtship
Females invest more in parenting
(Elephant Seals, Elk, Lions, Gorillas, Widowbird)
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Sexual Dimorphism

• Males and females of the same species have
  different forms
• and are easily distinguishible

• In occasional cases, dominant males can be
  identified by unique
• sexual characteristics (e.g. coloration of
  certain types of fish)

WHY SHOULD IT BE THIS WAY?
Some of these characteristics allow males to
fight each other more effectively.

• Male sexual characteristics can also reflect the
  fitness of the male,
• and therefore suggest to females that their
  offspring will also be fit

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Males and females of the same species have
different forms and are easily distinguishable
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Male Contest

• Males compete for alpha-male status (in large
  groups) or for territories that attract females.
• Winner mates with many females, losers mate with
  few or none.
• Thus traits that helped males win mates are
  amplified in population

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Beauty is in the eye of the beholder...
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Some of these characteristics allow males to
fight each other more effectively. But what
about the ones that dont help them physically
compete against each other?
Widow Bird
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Do the females really go for it?
1. Catch long-tailed birds and cut off tails.
2. Paste these tails onto short-tailed widowbirds.
3. Catch some long-tailed birds, cut tails and
paste back on.
The Widowbird Study
Findings The long-tailed widowbirds were more
successful at mating and had more
offspring (whether their tails were long
naturally or pasted on)
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How did these characteristics come about if they
advertise to predators?
Theory 1 A bottleneck occurred in the
population. An outward, superficial gene was
linked on the chromosome near a more essential
one. Since males with the essential trait also
had the superficial trait, the females began
choosing the best mate on superficial cues.
The superficial cues have been undergoing
selection ever since.
Theory 2 That dumb characteristic risks the
life of the male. If he can survive with all
those colors and obvious feathers, he must be
some kind of stud.