Sexual Selection

1
Sexual Selection

• Differential reproductive success based on
  sexually dimorphic characters.
• Recognized by intersexual phenotypic differences
  that cant be explained by natural selection
• Selection imposed on males (typically) by other
  males or by females of the same species.
• (1) can produce exaggerated secondary sexual
  characteristics
• (2) sexual dimorphism

2

• In both NS and SS, the investigative challenge is
  determining how the selection improves fitness of
  individuals.
• If reproduction is sexual
• will be an investment asymmetry
• A. energy expended in gamete production
• B. energy expended in offspring i.e.,parental
  investment
• E.g., orangutans
• With exception of monogamous species, general
  pattern
• Males low energy expenditure
• Females high energy expenditure

3

• Different selection pressures on males and
  females.
• Males
• fitness will be limited by number of mates.
• Females
• fitness limited by
• 1. sufficient resources to put into reproduction
• 2. Limits on reproduction imposed by care of
  offspring.

4

• Two kinds of sexual selection
• 1. Intrasexual selection
• With few exceptions, this is male-male
  competition
• Males control access to females
• 2. Intersexual selection
• With few exceptions, females choose mates based
  on variation in male secondary sexual
  characteristics or resources controlled by the
  male.
• Both require heritable, quantitative phenotypic
  variability.

5
Male-male combat
Marine Iguana
6
(No Transcript)
7
(No Transcript)
8
The delimma
9
Territory, body size, and mating success
10
(No Transcript)
11
Sperm Competition

• 1. Number of sperm adjusted for environmental
  condition
• e.g., Mediterranean fruit flies
• Different physiological responses
• 1. Isolated males isolated copulations
• 2. Male male congener male present during
  copulation
• Phenotypic plasticity?

12
Evolved structures enhancing fitness e.g.,
damselflies
13

• Intersexual selection
• Barn swallows

14

• Tail signal modifications
• Controls
• 1. Cut and re-glue (N 11)
• 2. Unaltered (N 11)
• Treatments
• 1. Shorten (N 11)
• 2. Elongate (N 11)

15
(No Transcript)
16
(No Transcript)
17
(No Transcript)