Sexual Selection Flashcards
Natural Selection
• 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
Sexual selection
Selection arising from differences in mating success.
A component of natural selection that explains
differences in mating success.
Differential Reproduction
• Mate choice
– Does mating happen?
– Fitness of offspring
– # of offspring
• Asexual reproduction
2 basic kinds of reproductive modes
- Copulation
* Spawning
Copulation requires
• 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.
Spawning requires…
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.
Selective “opportunities” depend on life history
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)
polyandry
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
polygyny
Oldest and largest individual is a male; he maintains a harem of females all of whom he fertilizes
-males maximize mating opportunities
Sexual selection
Selection arising from competition between or within sexes in a single population.
Demonstration of mate preference violates the
“random mating” tenet of Hardy-Weinberg.
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.
Random mating
• 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
Assortative Mating
• 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
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)
Components of (reproductive) fitness
• 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.
Fertilization Success
Alleles impact the probability that fertilization will take place.
Gamete viability
Alleles impact the longevity or quality of the egg or sperm
Fecundity
Number of gametes per individual.
Mating Success
Number and quality of mates
Viability
Probability that individual bearing genotype will survive
Figuring out what is being chosen can be hard
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.
Why is it females typically do the
picking?
• 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.
What kinds of things typically act as cues in female choice?
• 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?
Sneaking and cheating
Some males “compete” by pretending to be females, or by sneaking. – If success is ever > 0, this is worth it.
Indirect male-male competition
Raise your chances by
mating more, or having
more gametes.
But male competition can continue,
too.
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
Extravagant male morphologies
• Males are elaborate, brightly colored, or otherwise more conspicuous than females.
• A special case of
sexual dimorphism.
Zahavi’s handicap hypothesis
“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)
Zahavi’s hypothesis requires…
• 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
Costs associated with extreme
morphologies
• 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
Who’s more likely to be noticed
(and then eaten)?
normally the male because of his coat.
Zahavi’s hypothesis is a form of
“honest advertisement”
• 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.
Female choice in widowbirds
• 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.
Choosiness
“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.
More reinforcing consequences of sexual selection
- Offspring that resemble their fathers have greater reproductive success (thus higher fitness)
- Female choice is primary differentiator for fitness
Runaway sexual selection
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.
Zahavi’s handicap hypothesis is a
form of “honest advertisement”
• 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.
