Sexual Selection

Question 1

Natural Selection

Answer 1

• Differences in phenotype that have consequences for survival and reproductive
success (fitness) are the raw materials for selection

• Adaptations are those features which enhance
relative fitness and thus lead to greater reproductive success

Differential reproduction is critical to natural selection

Question 2

Sexual selection

Answer 2

Selection arising from differences in mating success.

A component of natural selection that explains
differences in mating success.

Question 3

Differential Reproduction

Answer 3

• Mate choice
– Does mating happen?
– Fitness of offspring
– # of offspring

• Asexual reproduction

Question 4

2 basic kinds of reproductive modes

Answer 4

• Copulation

* Spawning

Question 5

Copulation requires

Answer 5

• Individuals recognize and encounter conspecifics of
the appropriate sex.

• Means of transmitting
gametes.

• Requires time & energy searching for mates.

Number of potential
encounters may be limited, so each copulation potentially limits future choices or opportunities.

Question 6

Spawning requires…

Answer 6

Coordination of members of a population with respect to timing of spawning.

• Typically prevents extensive
parental care.

• Concentration and density of
gametes appropriate to
environment.
– Part of coordination equation
– Gamete mortality high

• Gamete recognition
mechanisms.

Question 7

Selective “opportunities” depend on life history

Answer 7

Organisms that do not
“personally” encounter their
mate cannot directly select for
certain features.

Organisms that mate with
one partner cannot select
gametes.

(anemones don’t select)

Question 8

polyandry

Answer 8

Oldest and largest individual is a female; multiple (smaller) males compete for access.

-better guarantee of successful reproduction
-promote competition among males (and among their sperm— more variation
possible!)
-multiple males can procure more resources for the female

Question 9

polygyny

Answer 9

Oldest and largest individual is a male; he maintains a harem of females all of whom he fertilizes
-males maximize mating opportunities

Question 10

Sexual selection

Answer 10

Selection arising from competition between or within sexes in a single population.

Question 11

Demonstration of mate preference violates the

“random mating” tenet of Hardy-Weinberg.

Answer 11

Hardy-Weinberg requirements:
• Random mating
• All offspring have an equal chance of survival and reproduction

If mate choice is based on heritable phenotype, then some offspring will have a greater/lesser chance of reproducing.

Question 12

Random mating

Answer 12

• There’s an equal chance of
every male/female pair
– NO demonstrable preference for copulators

• NO geographic barriers or clines

• NO advantage in fertilization
– NO demonstrable preference for spawners

Question 13

Assortative Mating

Answer 13

• Contrast to random (non-assortative) mating
• Positive assortative mating: Preference for similar
phenotype (may lead to inbreeding depression)
• Negative assortative mating: Preference for different
phenotype (may make it hard to maintain species
boundaries

Question 14

Positive assortative mating

Answer 14

Preference for similar

phenotype (may lead to inbreeding depression)

Question 15

Negative assortative mating

Answer 15

Preference for different

phenotype (may make it hard to maintain species boundaries)

Question 16

Components of (reproductive) fitness

Answer 16

• Viability:
Probability that individual bearing genotype will survive.

• Mating Success:
Number and quality of mates

• Fecundity:
Number of gametes per individual

• Segregation Advantage: Alleles wind up in the gametes more/less frequently

• Gamete viability:
Alleles impact the longevity or quality of the egg or sperm

• Fertilization Success
Alleles impact the probability that fertilization will take place.

Question 17

Fertilization Success

Answer 17

Alleles impact the probability that fertilization will take place.

Question 18

Gamete viability

Answer 18

Alleles impact the longevity or quality of the egg or sperm

Question 19

Fecundity

Answer 19

Number of gametes per individual.

Question 20

Mating Success

Answer 20

Number and quality of mates

Question 21

Viability

Answer 21

Probability that individual bearing genotype will survive

Question 22

Figuring out what is being chosen can be hard

Answer 22

Is it male size, pattern, color, or
swords that females key into?

Do females prefer males with longer swordtails?
Yes. Only because they make the male look bigger. This
is just a special case of size preference.

Do females prefer males with symmetrical or asymmetrical bars?
Symmetrical bars in general, but it depends on the species.

Question 23

Why is it females typically do the

picking?

Answer 23

• Females produce the expensive gametes (by
definition)
– May also have post-mating responsibilities.

• Female strategy is typically focused on obtaining the best mate so that HER investment
in eggs pays the greatest genetic dividend.
– Higher proportion of offspring survive and reproduce.

Question 24

What kinds of things typically act as cues in female choice?

Answer 24

• Features that suggest health and capability in feeding
– “Honest” demonstration of greater fitness

• Features that emphasize distinction between males and
females
– Minimized wasted time/effort

• “Novelties” that suggest that males are from another
population
– Decrease possibility of inbreeding?

Question 25

Sneaking and cheating

Answer 25

Some males “compete” by
pretending to be females, or
by sneaking.
– If success is ever > 0, this is
worth it.

Question 26

Indirect male-male competition

Answer 26

Raise your chances by
mating more, or having
more gametes.

Question 27

But male competition can continue,

too.

Answer 27

Crickets have “time release”
packets of sperm
(spermatophores)

Male crickets “guard” females after mating
– Prevent rejection of spermatophore?
– Prevent additional matings?

This kind of behavior is most common in males when mating is costly

Question 28

Extravagant male morphologies

Answer 28

• Males are elaborate, brightly colored, or otherwise more conspicuous than females.

• A special case of
sexual dimorphism.

Question 29

Zahavi’s handicap hypothesis

Answer 29

“An individual with a well developed sexually selected
character [such as a peacock’s flashy tail] is an individual
which has survived a test. A female which could
discriminate between a male possessing a sexually
selected character, from one without it, can discriminate
between a male which has passed a test and one which has not been tested. Females which selected males with the
most developed characters can be sure that they have
selected from among the best genotypes of the male
population. “ (Zahavi 1975)

Question 30

Zahavi’s hypothesis requires…

Answer 30

• Females exert choice

• Males have extreme morphologies compared to
females
– Having the extreme morphology entails cost

• Males differ in terms of the expression of the extreme morphology

Question 31

Costs associated with extreme

morphologies

Answer 31

• Plumage (or antlers, or complex songs, etc) require energy to make

• Males bearing extreme trait are more conspicuous to predators, so they face a greater threat of being eaten.
– Those males that survive DESPITE this handicap have demonstrated their superior fitness

Question 32

Who’s more likely to be noticed

(and then eaten)?

Answer 32

normally the male because of his coat.

Question 33

Zahavi’s hypothesis is a form of

“honest advertisement”

Answer 33

• Males bearing the traits DO face greater threat and entail higher cost as a consequence.
• Survival despite handicap demonstrates greater fitness.
• Female preference for extreme morphology is
therefore a preference for higher-fitness males.

Question 34

Female choice in widowbirds

Answer 34

• Male widowbirds differ in tail
length and coloration
• Males differ also in territory
size and quality, rate of
display, and other attributes
that may affect female
preference

Tail length
• Experimental manipulation
of tail length demonstrated
that female red-collared
widowbirds prefer longer
tailed males
• Red-collared widowbird
males with experimentally shortened tails lose more male-male contests over
territory than longertailed
birds.
So is tail length under sexual selection by females, or does it confer advantage in male-male
competition?
Are females cued into the “meaning” of long tails in terms of male-male competitive success?

What about coloration?
• Carotenoid pigments in collar
derived from food
• Brightness of collar varies
among males in a species
Does color send a conflicting signal, amplify the signal sent by tails, or no signal at all?

Significant POSITIVE covariance ONLY between tail length and nest activity—a longer tail predicts MORE nest activity
Significant NEGATIVE covariance between both collar area and body size and nest activity—a larger collar area and body size predict LESS nest activity.

Question 35

Choosiness

Answer 35

“Choosiness” and elaborate features can nonetheless reinforce one another

• Choosy females select elaborate males
• Unchoosy females select males randomly

If male quality correlates with fitness, the offspring of choosy
females will have higher average fitness than unchoosy females because choosy females have an increased frequency of mating with higher fitness males.

Choosiness and elaborate traits will both be favored and will spread through the population.

Question 36

More reinforcing consequences of sexual selection

Answer 36

• Offspring that resemble their fathers have greater reproductive success (thus higher fitness)
• Female choice is primary differentiator for fitness

Question 37

Runaway sexual selection

Answer 37

Co-evolution of choosiness and elaboration and the self-fulfilling prophecy of female-discrimination benefiting fitness means that sexual selection is potentially a positive feedback loop.

Female choosiness and male elaboration form a positive feedback loop.

Male appearance and fitness become de-coupled; extreme
morphology NOT correlated with higher fitness.

Female’s choice NOT a guarantee of better genes for her offspring.

Trait in males is maintained SOLEY because females select it
—the males have no inherent advantage because of it.

Question 38

Zahavi’s handicap hypothesis is a

form of “honest advertisement”

Answer 38

• Males bearing the traits DO face greater threat and entail higher cost as a consequence.

• Survival despite this handicap demonstrates greater
fitness.

• Female preference for extreme morphology is
therefore a preference for higher-fitness males.

• This is NOT the case in runaway sexual selection.