Leslie Samuel
Issues in Origins and Speciation
Chapter 10
Sexual Selection
Males and females often differ (size, appearance and behavior) – (sexual dimorphism)
10.1 Sexual Dimorphism and Sex
- Sexual dimorphism cannot always be explained by natural selection:
o E.g. Long tail feathers in male widow birds – why only in males, and how
does it benefit the widow birds?
o Sex complicates life
- Sexual selection – differential reproductive success due to variation among
individuals in success at getting mates
Asymmetries in Sexual Reproduction
- Mothers make a larger parental investment (energy and time expended both in
constructing an offspring and in caring for it)
o Parental investment increases reproductive success of the offspring
receiving it and decreases remaining reproductive success of the investing
parent
- A females potential reproductive success is relatively small – access to mate not
limiting resource
- Male’s potential reproductive success is relatively large – access to mate is
limiting resource
- Because of above two factors sexual selection is usually a more potent force in the
evolution of males than in the evolution of females
Asymmetric Limits on Reproductive Success in Newts and Pipefish
Newts:
- Neither parent cares for the offspring
- Entire parental investment is in the production of eggs and sperm
- Thus the investment per offspring is larger for females than for males
- Majority of males failed to mate, but all females mated at least once.
- More mates, for a male, meant more offspring. However, for females, there is no
statistical significance in an increased number of mates
- Sexual selection is more potent force in the evolution of males – fitness is
determined by access to mates
Pipefish:
- Father provides all the parental care (larger parental investment)
- Roles of the sexes are reversed
- Thus the reproductive success depends most strongly on mating success
- Sexual selection is a more potent force in the evolution of females – fitness is
determined by access to mates
Behavioral Consequences of Asymmetric Limits on Fitness
1. Males should be competitive
2. Females should be choosy
-
Intrasexual selection involves males fighting among themselves. Females then
mate with the winners.
Intersexual selection involves advertising for mates. The females then choose the
males with the best display
10.2 Male-Male Competition: Intrasexual Selection
3 Forms:
1. Combat
2. Sperm Competition
3. Infanticide
Combat:
- Can favor morphological traits including large body size, weaponry and armor
- Marine iguanas are sexually dimorphic – males get larger than females
- Males are larger much larger than optimal size – evolutionary puzzle – natural
selection acts against it.
- Darwin invoked the theory of sexual selection to explain these kinds of
phenomena
- Males have to fight for the privilege of copulation
- When compared, the mean size of males that actually got to copulate was
significantly larger than the mean size of all males that tried to copulate  sexual
selection
Sperm Competition:
- Large ejaculates containing many sperm contributes to victory
o The Mediterranean fruit fly: Males raised in the presence of a potential
rival ejaculated more than 2 1/2 times as many sperm as males raised and
mated in isolation.
- Other adaptations:
o Males Guard their mates, prolong copulation, deposit a copulatory plug, or
apply pheromones that reduce the female’s attractiveness.
o In damselflies, males scoop out sperm left by previous mates using a
special structure on his penis.
Infanticide:
- Residence in a pride is the key to reproductive success in lions
- Males in a victorious coalition quickly begin trying to father cubs.
- They kill any cubs in pride that have not been weaned because females do not
return to breeding conditions until after weaning.
- This is a detriment to female’s reproductive success. Two options:
o Defend their cubs
o Spontaneously abort
10.3 Female Choice
Benefits of a choosy female:
- Acquisition of good genes for offspring
- Acquisition of resources offered by males
Female Choice in Barn Swallows
- Barn swallows are sexually dimorphic even though males help care for the young.
o Males are more brightly colored and larger.
o Outermost tail feathers are larger in males
- Males and females may vary in quality
- Many are not monogamous
- Males show off their long outer tail feathers when advertising for mates
Moller’s experiment:
- 4 experimental groups: Shortened tail feathers, mock-altered (control 1), unaltered
(control 2) and elongated tail feathers.
- Results:
o The elongated males, on average, attracted mates more quickly than the
control males, and control males attracted mates more quickly than
shortened males.
o Females who had to settle for less desirable short-tailed males attempted
to compensate by copulating out-of pair with more desirable long-tailed
males.
Gray Tree Frogs experiments
- When a male hears others joining him to make a chorus, he sometimes increases
the length of his calls
- Females go right past one singer to mate with a more distant one.
- 75% preferred long calls to short calls.
- Female choice thought to be the selective force
Explanations for female choice:
- Choosy Females May Get Better Genes for Their Offspring
o Good genes hypothesis: Displays are thought to be indicators of genetic
quality
o Welch and colleagues studied offspring performance related to fitness and
found that the offspring of long-calling males have significantly higher
fitness
- Choosy Females May Benefit Directly Through the Acquisition of Resources
o Choosy female hanging flies get food from their mates
- Choosy Females may have preexisting sensory biases
o Selection for such abilities as avoiding predators, finding food and
identifying members of the same species may result in sensory biases that
make females particularly responsive to certain cues
o This may in turn select on males to display those cues
-
-
o E.g. male leg trembling during courtship in N. Papillator because it
mimics the vibrations produced by copepods and thereby elicits predatory
behavior from the female
Sexy-son hypothesis: Once a particular male advertisement display is favored by
a majority of females, selection on females will automatically reinforce a
preference for the fashionable trait leading to more fashionable sons etc.
All explanations for female choice are mutually compatible
10.4 Diversity in Sex Roles
- When males invest more per offspring than females, access to mates will be a
limiting resource for females.
- Females should then compete over access to mates and males should be choosy
- Female N. ophidion are larger than males and have dark blue stripes and skin
folds on their bellies
o When in captivity, females develop skin fold only when males are present.
- Female S. typhle can change their color to intensify the zigzag pattern on their
sides
- Example with pipefish mentioned earlier
10.5 Sexual selection in plants
- Access to pollinators limits the reproductive success of pollen donors to a greater
extent than seed parents
- Yellow-flowered wild radish got about three-quarters of the pollinator visit and
thus three-quarters of the reproductive success in an experiment conducted by
Stanton and colleagues.
10.6 Sexual Dimorphism in Body Size in Humans
Evolutionary significance of human behavior difficult to study because:
- Human behavior is driven by a complex combination of culture and Biology
- Ethical and practical considerations prohibit most of the kinds of experiments we
might conduct on individuals of other species
Male-male competition is the most obvious kind of sexual selection
- Men kill men at much higher rates than women kill women.
- In several cultures, men committed all of the same-sex homicides.
Study conducted on 3,201 Polish men:
- Married men were shown to be slightly taller than unmarried men
- Men with one or more children were shown to be significantly taller than
childless men
- Evidence is only suggestive