Instructions for printer-friendly version


x

x

x

x

x

x

x

x

x

x

x

x

x

    Sexual Selection

    Sexual selection is a special level of natural selection.

  • Natural Selection determines who survives.
  • Sexual Selection determines who reproduces.

    When some members of a population have heritable traits
    that are preferred by members of the opposite sex,
    sexual selection can occur.

    Sexual selection can be

    • Intrasexual (competition between members of the same sex)
    • Intersexual (choice exerted by members of opposite sex)

x

x

x

x

x

x

x

x

x

x

    Darwin's Conundrum

    Sexual selection explains observations that perplexed Darwin.
    How could a traits that seemed maladaptive be so prevalent?

    • extravagant male ornamentation
    • conspicuous courtship displays
    • potentially lethal male/male combat
    • sexual dimorphism

    Survival is great.
    But without reproduction,
    an individual is a big evolutionary ZERO.

x

x

x

x

x

x

x

x

x

x

x

x

x

    What Do We Think We Know?

    An anonymous survey:

    Choose from among the statements in the question box to the left.

x

x

x

x

x

x

x

x

x

x

    Bateman's Principle

    In 1948, Angus Bateman published his findings that, in Drosophila,

      • males that had the most mates had higher fitness than other males.
      • females that had the most mates DID NOT have higher fitness than other females.

    He proposed that selection leads to
    "an undiscriminating eagerness in the males
    and a discriminating passivity in the females"
    to obtain mates.

    Bateman's Principle attempted to explain sex-specific differences in

    • mate selection and courtship behaviors
    • secondary sex characteristics
    • parental behaviors towards offspring

x

x

x

x

x

x

x

x

x

x

x

x

x


(click on pic for source)

    Bateman's Reasoning

    Bateman's logic was...
    • The cost an ovum is far greater than that of a sperm.
    • Thus, mother has more impact on offspring survival/success than father.
    • Females become the limiting factor in parental investment.
    • Males compete over this potential female investment.
    • Sexual selection should lead to a pattern of
      • promiscuous, eager males
      • coy, choosy females

    Bateman's ideas, influenced by Darwin and the Victorian
    sexual mores of that time, are still widely held today.

    Even when they're wrong.

x

x

x

x

x

x

x

x

x

x

x

x


(click on pic for source)

    As It Turns Out...

    Empirical investigation of the principle reveals a more complex picture.

    Broad application of the principle to our own species has been challenged.

    Bateman's idea now serves mainly as a predictive model.

    More and more studies reveal that Bateman's predicted patterns,
    when put to the test, often fail to materialize.

x

x

x

x

x

x

x

x

x

x

x

x

    Levels of Sexual Selection

    Sexual selection can occur at several levels during the reproductive process.

    Determination of number of mates

    • male/male competition for territory/mates
    • female choice

    Determination of fertilization success

    • male/male sperm competition
    • cryptic female choice (sperm selection)

    Contribution to offspring survival

    • paternal care
    • maternal care

    Selection has driven a wide variety of adaptations
    in each sex at each of these levels.

x

x

x

x

x

x

x

x

x

x

x

    Sexual Dimorphism

    The sex organs themselves are known as primary sex characteristics.
    They distinguish biological males from biological females on a basic level.

    But in many species, differences go beyond sex organs.

    Both intra-and intersexual selection can result in the evolution
    of phenotypic differences between the sexes.

    These secondary sex characteristics can become
    critical instruments in mate selection for both males and females.

x

x

x

x

x

x

x

x

x

x

x

x

    Mating Systems: Terminology

    monogamy pattern of having one mate at a time

    polygamy pattern of having more than one mate at a time

    pair bond a close relationship through courtship and sexual activity
    with one other partner

    polygyny polygamy in which one male has multiple female mates

    polyandry polygamy in which one female has multiple male mates

x

x

x

x

x

x

x

x

x

x

x


(click for source)

    Intrasexual Selection

    Intrasexual selection involves competition between
    members of the same sex for access to mates.

    Most commonly, competition is among males.

    Adaptations have evolved in response to pressures...

      Pre-mating: physical contest
      • large body size
        xxx(larger male, greater polygyny)
      • anatomical weaponry
      • territory/harem guarding

      Post-mating: sperm competition
      • large ejaculate
      • copulation plugs
      • mate guarding
      • sperm removal

x

x

x

x

x

x

x

x

x

x

    Male/Male Competition: Physical Contest

    Males competing for
    • access to females
    • high-quality territory (to attract females)
    ...must often resort to physical combat.

    Agonistic behavior is any social behaviour related to fighting.

    • aggression

    ...including ways to avoid the potentially lethal risks of fighting.

    • threat displays
    • retreat from fighting
    • placation
    • conciliation

    In a system where males engage in physical combat, there is a correlation
    between body size and reproductive success.

    Larger body size means

    • physical advantage in combat
    • leading to increased frequency of mating opportunities

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x


(click on pic for source)

    Sexual Size Dimorphism

    Polygynous species tend to exhibit sexual size dimorphism (SSD) in which

    • male is larger than the female.
    • the larger dimorphism, the greater the degree of polygyny.
    • Some studies have shown correlation between increased male body size
      x and reduction of paternal parental care.

    So...which member of these pairs is the male?

x

x

x

x

x

x

x

x

x

x

    Male/Male Competition: Territories and Harems

    A male also can exclude other males from mating opportunities by
    • defending a prime territory attractive to females
    • guarding a harem of females from other males

    The Myth of the Leader
    A male guarding a harem is not necessarily its "leader".
    More often, if there is a leader, it is a dominant female.
    The rest of the herd follows her lead, not that of the male.

    The male may protect his harem and offspring from predators or other threats.
    He's there to protect his evolutionary investment, not be the boss.

x

x

x

x

x

x

x

x

x

x

x

x

x


(click on pic for source)

    Infanticide

    When males battle, a resident male will sometimes be replaced.
    The new male will sometimes systematically kill all current offspring.

    Many mammals do not ovulate while lactating. (Not true of humans!)
    Ending suckling can bring females back into sexual receptivity quickly.

    • The new male's paternity is assured
    • He does not spend energy guarding babies not carrying his genes.

    This behavior has been observed in

    • several primates
    • lions
    • some birds
    • jacanas (Females do the killing. See below.)

    And then there's the Cinderella Effect, although this idea is controversial.

x

x

x

x

x

x

x

x

x

x

x

x

x

    Sperm Competition: Ejaculate Volume

    Larger testes manufacture more sperm.
    Testis size is positively correlated with degree of polygamy in many mammals.
    This well known in primates, including humans.

    • Chimpanzee (Pan troglodytes)
      • highly polygamous
      • multiple males mate with estrous females
      • high degree of sperm competition
      • sexually mature male body mass ~ 45kg
      • average ejaculate mass ~ 110g
      • ejaculate ratio: 0.02% of body mass

    • Human (Homo sapiens)
      • intermediately polygamous
      • variable in degree of polyandry
      • intermediate level of potential sperm competition
      • sexually mature male body mass ~ 70kg
      • average ejaculate mass ~ 40g
      • ejaculate ratio: 0.006% of body mass

    • Gorilla (Gorilla gorilla)
      • highly polygynous
      • Usually, only one "silverback" mates with all group females
      • low level of sperm competition
      • sexually mature male body mass ~ 200kg
      • average ejaculate mass ~ 30g
      • ejaculate ratio: 0.002% of body mass

x

x

x

x

x

x

x

x

x

x


(click on pic for source)

    Cryptic Estrus

    Human males can adjust their sperm output to compensate for
    • likelihood of infidelity in a mate
    • mate body size
    • but apparenty cannot adjust for fecundity

    This may be a result of cryptic estrus in our species.
    Cryptic estrus, a female sexual selection strategy, fosters

    • constant female sexual receptivity
    • continued male presence, even in absence of estrus
    • male protection and resources

    Female orgasm causes rhythmic uterine contractions that
    pull sperm through the cervix.

    This increases chance of fertilization, and may help explain
    the adaptive significance of female orgasm.

x

x

x

x

x

x

x

x

x

x

    Sperm Competition: Copulation Plugs

    In species in which females mate sequentially with multiple males,
    another male strategy is to physically block mating by another male.

    This can be accomplished with a mating plug:

    • aka
      • copulation plug
      • sperm plug
      • vaginal plug
      • sement
      • sphragis (from Greek for "seal")
    • thick, gelatinous component of ejaculate OR
    • instilled after mating by male
    • thickens to block vulva or cloaca
    • can also be achieved via autotomy of mating structure (e.g., spider pedipalp)

    Female can eventually remove the plug, but it buys the sperm some time.

x

x

x

x

x

x

x

x

x

x

    It Could Never Happen Here, Right?

    In case you didn't think our own species would ever resort to such
    mediaeval tactics, think again.

    Never mind. Evolution already beat you to it.

x

x

x

x

x

x

x

x

x

x

x

x

    Sperm Competition: Mate Guarding

    Some species engage in mate-guarding.
    This involves the male staying close to the female after copulation
    to ensure that no other males interferes with his investment.

    Documented in diverse taxa, from arthropods to primates.

x

x

x

x

x

x

x

x

x

x

x

x

    Sperm Competition: Sperm Removal

    In some species in which females mate sequentially with more than one male, mechanisms have evolved in males that enable them to remove sperm from a female's previous matings.

    In some bird species, the male will peck at the female's cloaca
    to induce her to eject sperm from a previous mating.

    Some arthropods will physically remove previously deposited sperm from a female's cloaca.

x

x

x

x

x

x

x

x

x

x

x

x

x

x

    Intersexual Selection

    Intersexual selection involves selection of preferred mates by one sex from among a group of the other sex.

    Most commonly, choice is exerted by females.

    Adaptations have evolved in response to pressures...

      Pre-mating: overt female choice
      • male display and ornamentation
      • elaborate courtship behaviors

      Post-mating: cryptic female choice
      • copulation interference
      • sperm removal

x

x

x

x

x

x

x

x

x

x

Before we continue, a quick (anonymous) survey for the females among us.

x

x

x

x

x

x

x

x

x

x

x

x

x


Some bower birds will go that extra mile.


(click on pic for more about the Lion's Mane.)

    Overt Female Choice:
    Male Display and Ornamentation

    Selection sometimes results in males evolving costly physical and/or behavioral traits that potentially send an honest signal about his health and genetic vigor.

    • peacock tail
    • Bower Bird bower
    • human risky behavior
    • nuptial gifts (male to female)

    Carotenoid production in birds is correlated with parasitic infection:
    bright plumage can thus be an honest signal of immune system genetic quality.

    This is the basis of the Handicap Principle. <-- required link!

    For ornamentation like this to function on a populational level,
    ornaments must be costly enough to make cheating difficult.
    (THINK: Frequency-dependent selection)

x

x

x

x

x

x

x

x

x

x

    Runaway Sexual Selection

    Persistent, directional selection by females exerting mate preference
    can result in traits (like the above) that are a liability to the male.
    (Ronald Fisher, 1930)

    Only the most vigorous males can support such expense.
    Hence, the ornaments or behaviors become an honest signal.

    • complexity of male bird song
    • bright plumage
    • cumbersome structures

x

x

x

x

x

x

x

x

x

x

x

x

x


(click on pic for source)

    Balancing Selection

    In many cases, an evolutionary compromise must be struck
    • showy enough to attract mates
    • sufficiently able to avoid predation
    • (at least until you share your genes)

    On a related note...the Sexy Son Hypothesis (Ronald Fisher, 1930):

      A female's best choice among potential males is one whose genes
      will produce male offspring with the best chance of reproductive success.

x

x

x

x

x

x

x

x

x

x

x

x

x


A lek is an aggregation of male animals gathered to engage in competitive courtship displays (lekking). Females visit the lek to survey prospective mating partners. Each male hopes to entice as many visiting females as possible, maximizing his fitness.

    Overt Female Choice: Courtship Behaviors

    In many species, female preference has driven the evolution
    of elaborate courtship rituals.
    • insects (e.g., firefly patterns)
    • fish (e.g., nest building; showy displays)
    • amphibians (frog calls)
    • birds and other reptiles (numerous!)
    • mammals (e.g., deer rutting leks)

    Once again, these are costly for the male.
    But the reproductive differential gained by chosen males
    continues to drive evolution of traits until it meets
    opposing selective forces.

x

x

x

x

x

x

x

x

x

x

x

x

Annual "Dance your Ph.D." Competition Winner.
I had no idea this was a Thing.

    Cryptic Female Choice: Sperm Ejection

    Cryptic female choice (CFC) comprises female-driven mechanisms
    at or after mating that

    • result in differential sperm use from multiple male partners
    • impact the share of each males' share of paternity

    In some species, females can eject sperm of "substandard" males after copulation.

    For example, females of several bird species (e.g., chickens)
    can eject sperm of socially subordinate males.

x

x

x

x

x

x

x

x

x

x

x

x


(click on pic for source and more amazing photos!)

    Intersexual Selection: Exceptions to the Rule

    In some species, competition for mating opportunities is more intense
    among females than among males.

    In such cases, females tend to evolve secondary sex characteristics
    usually associated with males.

    But these cases are the exceptions, not the rule.

x

x

x

x

x

x

x

x

x

x

x

x

x

    The Sexual Arms Race

    Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction.

    The is particularly evident in conflict over

    • mode of mating
    • frequency of mating

    This can lead to an "evolutionary arms race" between the sexes.
    The result: some remarkable strategies to thwart

    • (1) male sexual aggression or
    • (2) female choice

x

x

x

x

x

x

x

x

x

x

    Altruism and Kin Selection

    An animal enages in altruism when it performs an act that benefits another individual its own expense.

    Altruism stumped Darwin and many others for more than 100 years.

    But in 1964, William D. Hamilton published his theory of kin selection.

x

x

x

x

x

x

x

x

x

x

    A Quick Survey

    If you agree with a statement, check its box.

x

x

x

x

x

x

x

x

x

x

x

x

required video


Richard Dawkins popularized Hamilton's idea in The Selfish Gene.

(Meet Professor Richard Dawkins.)

    Acts of Fitness

    Hamilton proposed that altruism could be evolutionarily adaptive IF

    R x B > C

    Where

    • R = genetic relatedness of individual receiving altruism
    • B = benefit of the action to the individual performing an altruistic act
    • C = cost to the individual performing the altruistic act

    Hamilton coined the terms

    • personal/individual fitness - # of offspring an indivual begets

    • inclusive fitness - combination of
      • # of offspring an individual begets
      • # of shared genes passed on by the individual's relatives

    Individuals whose behavior fosters reproductive success
    in their relatives, even at the expense of their individual fitness,
    are engaging in kin selection

x

x

x

x

x

x

x

x

x

x

x

x

    Whither, Natural Selection?

    With all these mechanisms in play, one might think that species could quickly adapt to whatever new challenge their world throws at them.

    Polar bears will likely be extinct before the end of the century,
    largely because climate change is drastically changing their habitat.
    Why Can't Polar Bears Just Evolve Gills?

    Because of four evolutionary constraints.

x

x

x

x

x

x

x

x

x

x

    #1. Natural Selection works only on existing traits.

    • Apple seeds grow into apple trees, and manatees give birth to manatees.

    • Body plan changes are based on what is already present in the genome.

    • Mutations do not generate complex structures de novo.

    • Natural selection conscripts existing structures for new functions.

    • So you're not going to get that Pegasus you wanted for your birthday.

x

x

x

x

x

x

x

x

x

x

    #2. Natural Selection acts only on existing polymorphism.

    • Mutations do not generate ideal traits.

    • Selection acts only upon alleles already present in a population.

    • Mutations are random.

    • Traits don't evolve simply because organisms need them.

x

x

x

x

x

x

x

x

x

x

    #3. Natural Selection is compromise.

    • Certain traits may be well adapted to serve in a particular habitat.

    • But life involves multi-tasking.

    • Phenotypic constraints result from evolutionary compromise.

x

x

x

x

x

x

x

x

x

x

    #. Random events can have
    xx major evolutionary consequences.

    • Super Frog's call was the world's best.

    • His colors were perfect.

    • Every female wanted him.

    • He shouldn't have crossed the road that rainy night.

x

x

x

x

x

x

x

x

x

x

    Evolution does not necessarily build ideal organisms.

    We're just as good as we can be, given the circumstances.

    And that's...okay.

x

x

x

x

x

x

x

x

x

x

    A Musical Finale

    ...starring Sir David Attenborough, Prof. Richard Dawkins, Jane Goodall
    and other friends.

x

x

x

x

x

x

x

x

x

x