Sexual selection leads to sexual dimorphism as males and females evolve different traits to obtain mates. For example, male widowbirds have exceptionally long tail feathers while females lack tails. Studies show that female widowbirds prefer males with artificially longer tails, demonstrating they have stronger and more vigorous genes. However, traits that make males more attractive to females, like bright spots on male guppies, can also make them more visible to predators, creating conflict between sexual selection pressures and predation.

1. Sexual Selection

2. Sexual Selection Basics
Sexual selection – differential reproduction as a
result of variation in the ability to obtain mates
(Futuyma 2004)
Sexual dimorphism –
males and females of the
same species have a
different appearance.
Can you think of an
example of sexually
dimorphic animals?
(hint think Virginia)

3. Sexual Selection Examples
• Male widow-birds grow
exceptional long tail feathers,
females lack tail feathers
• Males make low flights over
grasslands making loops
Do females prefer longer tailed
males? WHY?

4. Sexual Selection Basics
Malte Anderson –
Tails were cut and
glued to birds
with naturally
short tails
Artificially
longer tails had
significant
more matings
than normal
males

5. Why Long Tails?
It is very costly to grow and maintain a long tail
Indicates strong
and vigorous
genes
What would it
take to grow a
long tail?

6. Sexual Selection vs. Predators
Being attractive can attract
predators
Male guppy
(Poecilia reticulata)
Females prefer males with
bright spots Far Side 1992

7. Sexual Selection vs. Predators
More spots = More mating = Spreading your genes
Crenicichla (cichlid) preys on guppies
How do predators and females both influence male
phenotypes?

8. Sexual Selection vs. Predators
Endler (1980) –
Classic experiment
Results =
Predators keep
number of spots
lower
(Futuyma 2004)