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Chapter 11: Sex and Evolution


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Title: Chapter 11: Sex and Evolution

Chapter 11 Sex and Evolution
  • Robert E. Ricklefs
  • The Economy of Nature, Fifth Edition

  • Among the most fascinating attributes of
    organisms are those related to sexual function,
    such as
  • gender differences
  • sex ratios
  • physical characteristics and behaviors that
    ensure the success of an individuals gametes

Sexual reproduction mixes genetic material of
  • In most plants and animals reproduction is
    accomplished by production of male and female
    haploid gametes (sperm and eggs)
  • gametes are formed in the gonads by meiosis
  • Gametes join in the act of fertilization to
    produce a diploid zygote, which develops into a
    new individual.

Asexual Reproduction
  • Progeny produced by asexual reproduction are
    usually identical to one another and to their
    single parent
  • asexual reproduction is common in plants
    (individuals so produced are clones)
  • many simple animals (hydras, corals, etc.) can
    produce asexual buds, which
  • may remain attached to form a colony
  • may separate to form new individuals

Other Variants on Reproduction
  • Asexual reproduction
  • production of diploid eggs (genetically
    identical) without meiosis (common in fishes,
    lizards and some insects)
  • production of diploid eggs (genetically
    different) by meiosis, with suppression of second
    meiotic division
  • self-fertilization through fusion of female
  • Sexual reproduction
  • self-fertilization through fusion of male and
    female gametes (common in plants)

Sexual reproduction is costly.
  • Asexual reproduction is
  • common in plants
  • found in all groups of animals, except birds and
  • Sexual reproduction is costly
  • gonads are expensive organs to produce and
  • mating is risky and costly, often involving
    elaborate structures and behaviors
  • So why does sexual reproduction exist at all?

Cost of Meiosis 1
  • Sex has a hidden cost for organisms in which
    sexes are separate
  • only half of the genetic material in each
    offspring comes from each parent
  • each sexually reproduced offspring contributes
    only 50 as much to the fitness of either parent,
    compared to asexually produced offspring
  • this 50 fitness reduction is called the cost of
  • for females, asexually produced offspring carry
    twice as many copies of her genes as sexually
    produced offspring
  • thus, mating is undesirable

Cost of Meiosis 2
  • The cost of meiosis does not apply
  • when individuals have both male and female
    function (are hermaphroditic)
  • when males contribute (through parental care) as
    much as females to the number of offspring
  • if male parental investment doubles the number of
    offspring a female can produce, this offsets the
    cost of meiosis

Advantages of Sex
  • One advantage to sexual reproduction is the
    production of genetically varied offspring
  • this may be advantageous when environments also
    vary in time and space
  • Is this advantage sufficient to offset the cost
    of meiosis?

Whos asexual?
  • If asexual reproduction is advantageous, then it
    should be common and widely distributed among
    many lineages
  • most asexual species (e.g., some fish, such as
    Poeciliopsis) belong to genera that are sexual
  • asexual species do not have a long evolutionary
  • suggests that long-term evolutionary potential of
    asexual reproduction is low
  • because of reduced genetic variability, asexual
    lines simply die out over time

Sex A Short-Term Advantage?
  • Theoretical models based on environmental
    variability fail to find an advantage to sexual
  • A promising alternative is that genetic
    variability is necessary to respond to biological
    changes in the environment.

Sex and Pathogens
  • The evolution of virulence by parasites that
    cause disease (pathogens) is rapid
  • populations of pathogens are large
  • their generation times are short
  • The possibility exists that rapid evolution of
    virulence by pathogens could drive a host species
    to extinction.

The Red Queen Hypothesis
  • Genetic variation represents an opportunity for
    hosts to produce offspring to which pathogens are
    not adapted.
  • Sex and genetic recombination provide a moving
    target for the evolution by pathogens of
  • Hosts continually change to stay one step ahead
    of their pathogens, likened to the Red Queen of
    Lewis Carrolls Through the Looking Glass and
    What Alice Found There.

Individuals may have female function, male
function, or both.
  • The common model of two sexes, male and female,
    in separate individuals, has many exceptions
  • hermaphrodites have both sexual functions in the
    same individual
  • these functions may be simultaneous (plants, many
    snails and most worms) or
  • sequential (mollusks, echinoderms, plants, fishes)

Sexual Functions in Plants
  • Plants with separate sexual functions in separate
    individuals are dioecious
  • this condition is relatively uncommon in plants
  • Most plants have both sexual functions in the
    same individual (hermaphroditism)
  • monoecious plants have separate male and female
  • plants with both sexual functions in the same
    flower are perfect (72 of plant species)
  • most populations of hermaphrodites are fully
  • Many other possibilities exist in the plant world!

Separate Sexes versus Hermaphroditism
  • When does adding a second sexual function
    (becoming hermaphroditic) make sense?
  • gains from adding a second sexual function must
    not bring about even greater losses in the
    original sexual function
  • this seems to be the case in plants, where basic
    floral structures are in place
  • for many animals, adding a second sexual function
    entails a net loss in overall sexual function

Sex ratio of offspring is modified by evolution.
  • When sexes are separate, sex ratio may be defined
    for progeny of an individual or for the
    population as a whole.
  • Humans have 11 malefemale sex ratios, but there
    are many deviations from this in the natural
  • Despite deviations, 11 sex ratios are common.

11 Sex Ratios Background
  • Every product of sexual reproduction has one
    father and one mother
  • if the sex ratio is not 11, individuals
    belonging to the rarer sex will experience
    greater reproductive success
  • such individuals compete for matings with fewer
    individuals of the same sex
  • such individuals, on average, have greater
    fitness (contribute to more offspring) than
    individuals of the other sex

11 Sex Ratios An Explanation
  • Consider a population with an unequal sex
  • individuals of the rare sex have greater fitness
  • mutations that result in production of more
    offspring of the rare sex will increase in the
  • when sex ratio approaches 11, selective
    advantage of producing more offspring of one sex
    or another disappears, stabilizing the sex ratio
    at 11
  • this process is under the control of
    frequency-dependent selection

Why do sex ratios deviate from 11?
  • One scenario involves inbreeding
  • inbreeding may occur when individuals do not
    disperse far from their place of birth
  • a high proportion of sib matings leads to local
    mate competition among males
  • from the parents standpoint, one male offspring
    serves just as well as many to fertilize his
    female siblings, while production of more female
    offspring will lead to production of more progeny
  • the result is a shift of the sex ratio to
    predominance of females, the case in certain
    parasitic wasps

Mating Systems Rules for Pairing
  • There is a basic asymmetry in sexually
    reproducing organisms
  • a females reproductive success depends on her
    ability to make eggs
  • large female gametes require considerable
  • the females ability to gather resources
    determines her fecundity
  • a males reproductive success depends on the
    number of eggs he can fertilize
  • small male gametes require few resources
  • the males ability to mate with many females
    determines his fecundity

Promiscuity 1
  • Promiscuity is a mating system for which the
    following are true
  • males mate with as many females as they can
    locate and induce to mate
  • males provide their offspring with no more than a
    set of genes
  • no lasting pair bond is formed
  • it is by far the most common mating system in

Promiscuity 2
  • Promiscuity is a mating system for which the
    following are true
  • it is universal among outcrossing plants
  • there is a high degree of variation in mating
    success among males as compared to females
  • especially true where mating success depends on
    body size and quality of courtship displays
  • less true when sperm and eggs are shed into water
    or pollen into wind currents

  • Polygamy occurs when a single individual of one
    sex forms long-term bonds with more than one
    individual of opposite sex
  • a common situation involves one male that mates
    with multiple females, called polygyny
  • polygyny may arise when one male controls mating
    access to many females in a harem
  • polygyny may also arise when one male controls
    resources (territory) to which multiple females
    are attracted

  • Monogamy involves the formation of a lasting pair
    bond between one male and one female
  • the pair bond persists through period required to
    rear offspring
  • the pair bond may last until one of the pair dies
  • monogamy is favored when males can contribute
    substantially to care of young
  • monogamy is uncommon in mammals, relatively
    common among birds (but recent studies provide
    evidence for extra-pair copulations selecting for

The Polygyny Threshold
  • When should polygyny replace monogamy?
  • For territorial animals
  • a female increases her fecundity by choosing a
    territory with abundant resources
  • polygyny arises when a female has greater
    reproductive success on a males territory shared
    with other females than on a territory in which
    she is the sole female
  • the polygyny threshold occurs when females are
    equally successful in monogamous and polygynous
  • polygyny should only arise when the quality of
    male territories varies considerably

Sexual Selection
  • In promiscuous and polygynous mating systems,
    females choose among potential mates
  • if differences among males that influence female
    choice are under genetic control, the stage is
    set for sexual selection
  • there is strong competition among males for mates
  • result is evolution of male attributes evolved
    for use in combat with other males or in
    attracting females

Consequences of Sexual Selection
  • The typical result is sexual dimorphism, a
    difference in the outward appearances of males
    and females of the same species.
  • Charles Darwin first proposed in 1871 that sexual
    dimorphism could be explained by sexual selection
  • Traits which distinguish sex above primary sexual
    organs are called secondary sexual

Pathways to Sexual Dimorphism
  • Sexual dimorphism may arise from
  • life history considerations and ecological
  • females of certain species (e.g., spiders) are
    larger than males because the number of offspring
    produced varies with size
  • combats among males
  • weapons of combat (horns or antlers) and larger
    size may confer advantages to males in
    competition for mates
  • direct effects of female choice
  • elaborate male plumage and/or courtship displays
    may result

Female Choice
  • Evolution of secondary sexual characteristics in
    males may be under selection by female choice
  • in the sparrow-sized male widowbird, the tail is
    a half-meter long
  • males with artificially elongated tails
    experienced more breeding success than males with
    normal or shortened tails

Runaway Sexual Selection
  • When a secondary sexual trait confers greater
    fitness, the stage is set for runaway sexual
  • regardless of the original reason for female
    preference, female choice exaggerates fitness
    differences among males
  • leads to evolution of spectacular plumage (e.g.,
    peacock) and other seemingly outlandish plumage
    and/or displays

The Handicap Principle
  • Can elaborate male secondary sexual
    characteristics actually signal male quality to
  • Zahavis handicap principle suggests that
    secondary characteristics act as handicaps --
    only superior males could survive with such
  • Hamilton and Zuk have also proposed that showy
    plumage (in good condition) signals genetic
    factors conferring resistance to parasites or

  • Sexual reproduction is widespread, yet its
    benefits are not entirely clear. Genetic
    diversity among offspring of sexual unions may
    confer fitness in the face of environmental
    variation and rapidly-evolving diseases.
  • Sex ratios, mating systems, and secondary sexual
    characteristics arise in sexually reproducing
    organisms in response to selective pressures
    affecting both males and females.

As usual
  • Quizzes. Do the quizzes.
  • ?