CHAPTER 8: SEX AND EVOLUTION - PowerPoint PPT Presentation


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Title: Chapter 10: Life Histories and Evolutionary Fitness Author: Thomas R. Wentworth Last modified by: Rania Masri Created Date: 11/25/2009 8:09:20 AM – PowerPoint PPT presentation

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Stalk-eyed flies
Stalk-eyed flies
  • Both M F have these stalks
  • In some species they are up to twice as long in
    males as they are in females (see pic -gt)
  • Why does this difference between the sexes exist?

  • 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
  • This fern sprouts a fully formed plant from the
    tip of a leaf
  • 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

  • Sexual reproduction is costly

Sexual reproduction is costly.
  • 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

Why have sex?
  • By the late 1980s, in the contest to explain sex,
    only two hypotheses remained in contention.
  • One the deleterious mutation hypothesis
  • sex exists to purge a species of damaging genetic
    mutations Alexey Kondrashov (at the National
    Center for Biotechnology Information) argues that
    in an asexual population, every time a creature
    dies because of a mutation, that mutation dies
    with it. In a sexual population, some of the
    creatures born have lots of mutations and some
    have few. If the ones with lots of mutations die,
    then sex purges the species of mutations. Since
    most mutations are harmful, this gives sex a
    great advantage.
  • But But why eliminate mutations in this way,
    rather than correcting more of them by better
  • Kondrashov It may be cheaper to allow some
    mistakes through and remove them later. The cost
    of perfecting proofreading mechanisms escalates
    as you near perfection.

  • According to Kondrashov's calculations, the rate
    of deleterious mutations must exceed one per
    individual per generation if sex is to earn its
    keep eliminating them if less than one, then his
    idea is in trouble.
  • The evidence so far is that the deleterious
    mutation rate teeters on the edge it is about
    one per individual per generation in most
  • But even if the rate is high enough, all that
    proves is that sex can perhaps play a role in
    purging mutations. It does not explain why sex
  • The main defect in Kondrashov's hypothesis is
    that it works too slowly. Pitted against a clone
    of asexual individuals, a sexual population must
    inevitably be driven extinct by the clone's
    greater productivity, unless the clone's genetic
    drawbacks can appear in time. Currently, a great
    deal of effort is going into the testing of this
    model by measuring the deleterious mutation rate,
    in a range of organisms from yeast to mouse. But
    the answer is still not entirely clear.

So why have sex?
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.
  • it takes all the running you can do, to keep in
    the same place.

Sex vs Asex
  • One of the main proponents of the Red Queen
    hypothesis was the late W. D. Hamilton.
  • In the late 1970s, with the help of two
    colleagues from the University of Michigan,
    Hamilton built a computer model of sex and
    disease, a slice of artificial life. It began
    with an imaginary population of 200 creatures,
    some sexual and some asexual. Death was random.
    Who won?
  • As expected, the sexual race quickly died out. In
    a game between sex and "asex," asex always wins
    -- other things being equal. That's because
    asexual reproduction is easier, and it's
    guaranteed to pass genes on to one's offspring.

Now add parasites
  • Next they introduced 200 species of parasites,
    whose power depended on "virulence genes" matched
    by "resistance genes" in the hosts.
  • The least resistant hosts and the least virulent
    parasites were killed in each generation.
  • Now the asexual population no longer had an
    automatic advantage -- sex often won the game. It
    won most often if there were lots of genes that
    determined resistance and virulence in each
  • In the model, as resistance genes that worked
    would become more common, then so too would the
    virulence genes. Then those resistance genes
    would grow rare again, followed by the virulence
    genes. As Hamilton put it, "antiparasite
    adaptations are in constant obsolescence." But in
    contrast to asexual species, the sexual species
    retain unfavored genes for future use. "The
    essence of sex in our theory," wrote Hamilton,
    "is that it stores genes that are currently bad
    but have promise for reuse. It continually tries
    them in combination, waiting for the time when
    the focus of disadvantage has moved elsewhere."

Real-world evidence
  • asexuality is more common in species that are
    little troubled by disease boom-and-bust
    microscopic creatures, arctic or high-altitude
    plants and insects.
  • The best test of the Red Queen hypothesis,
    though, was a study of a little fish in Mexico
    called the topminnow. The topminnow, which
    sometimes crossbreeds with another similar fish
    to produce an asexual hybrid, is under constant
    attack by a worm that causes "black-spot
    disease." The asexually reproducing topminnows
    harbored many more black-spot worms than did
    those producing sexually.
  • That fit the Red Queen hypothesis The sexual
    topminnows could devise new defenses faster by
    recombination than the asexually producing ones.

Parasites and sex in freshwater snails
  • One of the most compelling tests in the Red Queen
    Hypothesis has been conducted by Curt Lively and
    his coworkers at Indiana University
  • Test focuses on the freshwater snail (P.
  • Most are asexual, all-female clones
  • Populations in some localities have 13 males
    enough to maintain some genetic diversity
  • Trematode worms of the genus Microphallus infect
    the snails and sterilize them
  • Hosts in the life cycle of the worm are ducks

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Snails and parasites
  • Asexual snails reproduce faster than sexual
  • Where the prevalence of Micorphallus infection is
    high ? sexual individuals are common
  • Why? ? asexual clones cannot persist in the face
    of high rates of parasitism
  • Why?

  • ? ? if the parasites had evolved to specialize on
    local (depth-specific) populations of snails,
    then they should have the greatest success in
    infecting the populations they evolved with
  • Took snails from 3 different depths exposed
    them to parasites obtained from each group of
  • Remember the definitive hosts (ducks) feed
    mostly in shallow water, and so only the
    shallow-water parasite populations cycled
    regularly through snail host populations

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More on sex and evolution
  • a 2005 study shows that sex leads to faster
  • To demonstrate this, a team of scientists created
    a mutant strain of yeast that, unlike normal
    yeast, was unable to divide into the sexual
    spores that allow yeast to engage in sexual
    reproduction. Yeast can reproduce either sexually
    or asexually.
  • When testing this mutant strain in stress-free
    conditions, the scientists found that it
    performed as well as normal yeast. In more
    extreme conditions, however, the normal yeast
    grew faster than the asexual mutants.
  • This shows "unequivocally that sex allows for
    more rapid evolution," said Matthew Goddard of
    the School of Biological Sciences at the
    University of Auckland in New Zealand.

  • It could well be that the deleterious mutation
    hypothesis and the Red Queen hypothesis are both
    true, and that sex serves both functions.
  • Or that the deleterious mutation hypothesis may
    be true for long-lived things like mammals and
    trees, but not for short-lived things like
    insects, in which case there might well be need
    for both models to explain the whole pattern.
  • Perpetually transient, life is a treadmill, not a

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
    outcrossing (fertilization takes place between
    gametes of different individuals)
  • Many other possibilities exist in the plant world!

Dioecious plants have 2 separate sexes
  • Perfect flowers contain both male and female
    sexual organs

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

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Sequential hermaphroditism
  • Check it out more on the web
  • Some organisms are male first and then become
    female later in their lives
  • Some organisms are female first and then become
    male later in their lives

From more on the web
In most organisms, including humans, sex is
determined by genetic factors. Nature is
endlessly inventive, however, and in some species
of reptiles and fish the temperature at which the
egg develops determines sex. In the case of
reptiles, females lay their eggs in sand or dirt
and the temperature of the soil at the depth at
which the clutch is laid determines the
temperature of the eggs. The mechanism of
temperature-dependent sex determination (TSD) has
not been worked out fully, but it involves an
effect of temperature on gene expression in the
developing gonads of the embryo. Each species
with TSD has a critical temperature, usually
around 28-30C, below which offspring become one
sex and above which offspring become the other
sex. Whether high temperatures produce males or
females depends on the species, although in most
cases higher temperatures produce females. Why do
you think that some groups of animals would adopt
TSD? How is temperature likely to affect the
length of the embryo growth period and the
relative size of the hatchling, and how would
these factors differentially affect the fitness
of male and female offspring? Would it be
possible for an egg-laying female to control the
sex of her offspring? What factors might
constrain the temperature regime that a female
can provide for her clutch of eggs? How would a
string of unusually hot or cold seasons affect
sex ratio in a population, and how should females
respond to this in determining where to lay their
eggs? A recent discussion of these issues can be
found in R. Shine (1999).
Whether a sequential hermaphrodite is first male
or first female depends on how reproductive
success through male or female function
potentially increases with increasing body size.
Larger females have larger reproductive organs
and can lay more eggs. Small males can have high
reproductive success where fertilization is
internal and males do not contest social status,
as in the case of the slipper shell. Where males
compete for territories, as in the wrasse, large
size is prerequisite to successful reproduction.
For others
  • Sequential hermaphroditism reflects changes in
    the costs and benefits of male and female sexual
    function as an organism grows. In some marine
    gastropods having internal fertilization, such as
    the slipper shell Crepidula, insemination
    requires the production of only small amounts of
    sperm. Hence male function consumes few resources
    and has little effect on growth. Consequently,
    individuals of many such species are male when
    they are small and become female when they are
    large and thus able to produce correspondingly
    large clutches of eggs

Check it out on the web for more explanation
Mechanisms of sex determination
  • Inheritance of sex-specific chromosomes (eg
    humans, birds)
  • Other factors
  • Competition among X- and Y-bearing sperm to
    fertilize eggs or selection abortion of male or
    female embryos
  • Birds control is so precise that the chance of
    an offspring being male changes predictably from
    the first to the last laid egg in the clutch as
    a way of controlling (maybe) competitive
    interactions between male and female siblings

Mechanisms of sex determination
  • Determined by the physical environment
  • Several species of turtles, lizards, alligators
    sex determined by the temperature at which it
    develops in the egg
  • Embryos that develop at lower temp produce males
    higher temp females for turtles. Opposite for
    alligators and lizards.
  • Hmm ?
  • Determined by the social environment
  • The wrasse (discussed earlier) is a sequential
  • Raised in isolation ? females raised in small
    groups, at least one develops initially into a
    male w/o passing through a female phase
  • Females may become males later when they grow
    large enough to compete for territories primary
    males never change their sex

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.
  • Sex ratio number of males relative to the number
    of females
  • Humans have 11 malefemale sex ratios, but there
    are many deviations from this in the natural
  • Despite deviations, 11 sex ratios are common.
  • 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

Female condition and offspring
  • In some situations, a parent may benefit from a
    skewed sex ratio among its progeny, meaning that
    it should produce a preponderance of either male
    or female offspring.
  • competition for matings among individuals of one
    sex (usually males) can create variation in
    reproductive success when competition is keen,
    some males may achieve many matings, others none.
  • it is often the largest males that win the lion's
    share of contests over access to females.
  • In mammals, a mother cares directly for her
    offspring, and her condition is likely to
    influence the fitness of her offspring.
    Therefore, females in poor condition should
    invest more in female offspring, which are likely
    to mate successfully regardless of the parental
    care they receive. Females in good condition
    ideally should produce male offspring, which will
    grow large and fare well in male-male competition
    for mates.

Female wood rats
  • a laboratory study of female wood rats (Neotoma
  • when investigators restricted food intake during
    the first 3 weeks of lactation to below the
    maintenance level of a nonreproductive female,
    mothers actively rejected the attempts of male
    offspring to nurse.
  • As a result, males starved, and the sex ratio of
    the offspring at 3 weeks shifted to about one
    male for every two females.
  • Faced with the likelihood that their young would
    be poorly nourished and that some of them would
    probably die before they were weaned, the mothers
    favored their female offspring.

Sex ratio and pollution
  • Recent study Lower oxygen levels in polluted
    waters could lead to a higher ratio of male fish
    that may threaten certain species with
  • hypoxia (O2 depletion) can affect sex
    development, sex differentiation and the sex
    ratio in fish species. hypoxia can inhibit the
    activities of certain genes that control the
    production of sex hormones and sexual
    differentiation in embryonic zebra fish.
  • In his study, Wu found that 61 of zebra fish -
    a universal freshwater fish widely used in
    scientific and pollution research - spawned into
    males under regular oxygen conditions. Under
    hypoxia conditions, the ratio of males increased
    to 75 .
  • Hypoxia can be a naturally occurring phenomenon,
    particularly in areas where salt and fresh waters
    meet in estuaries such as the Pearl River Delta.
    It can also be caused by pollution.

Human sex ratio and pollution PCBs
  • PCBs were banned in the 1970s, they are linked
    to problems with the brain, nervous and hormone
    systems, and although average levels in the human
    body have dropped, human exposure continues. Why?
    PCBs are persistent contaminants, which means
    they build up in the environment and in us.
  • Evidence continues to build that PCBs also affect
    birth sex. A recent study of blood serum from
    women who were pregnant in San Francisco in the
    '60s found that those with higher PCB levels were
    more likely to give birth to boys than those with
    low PBC levels.

Is it PCBs?
  • Dr. Pete Myers brings up an important point in
    his summary of the report The exposure levels
    observed in the study are high compared to today.
    Thus if these results are indicative of a causal
    relationship (never possible to confirm with
    epidemiological studies) then the simplest
    prediction would be that the chances of having a
    boy baby should be increasing because PCBs have
    been decreasing. That is not the case, at least
    as of the most recent analysis from Canada and
    the US.
  • Evidence from a large-scale study of four
    industrialized nations indicates that the sex
    ratio is skewed, and fewer boys are being born
    But PCB levels have dropped

So? What do we know?
  • in-utero exposure to pollutants can affect a
    child's sex.
  • There are more than 80,000 chemicals in
    production today, many of which are known to be
    persistent or to disrupt hormone systems, and
    most of which haven't really tested for their
    impact on human health.
  • A 2007 study from the University of Pittsburgh
    found that during the past thirty years, the
    number of male births has steadily decreased in
    the U.S. and Japan. The study found a decline of
    17 males per 10,000 births in the U.S. and a
    decline of 37 males per 10,000 births in Japan.

Human sex ratio and pollution
  • The steepest sex ratio declines observed in the
    world have occurred on the 3,000-acre Aamjiwnaang
    (pronounced AH-jih-nahng) First Nation
    reservation in Canada.
  • The ratio of boys to girls there began dropping
    in the early 1990s. Between 1999 and 2003,
    researchers found, only 46 boys were born out of
    132 recorded births. (35)
  • Dozens of petrochemical, polymer and chemical
    plants border the reservation on three sides.
    Mercury and PCBs contaminate the creek that runs
    through the land, and air-quality studies show
    the highest toxic releases in Canada, said Jim
    Brophy, executive director of Occupational Health
    Clinics for Ontario Workers, based in Sarnia, the
    nearest city.
  • Boys made up only 42 of the 171 babies born
    from 2001 to 2005 to Aamjiwnaang living on the
    reserve or nearby.

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 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
  • 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 (eg
    elephant seals)
  • 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

  • Rare cases of a single female having more than
    one male mate
  • Some human communities
  • 1 of birds..
  • A common example of this can be found in the
    Field Cricket Gryllus bimaculatus of the
    invertebrate order Orthoptera
  • Widely shown in frogs (Agile frogs, Rana
    dalmatina), polyandry was also documented in
    polecat (Mustela putorius) and other mustelids
  • Why?

Monogamy the formation of a lasting pair bond
betw one male 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 (why?),
    relatively common among birds (but recent studies
    provide evidence for extra-pair copulations in as
    many as a 1/3 of the broods leading to

Real monogamy?
  • Extra-pair copulations (EPC)
  • 1/3 or more of the broods produced by some
    monogamous species contain 1 or more offspring
    sired by a different male
  • Mate guarding behavior on the part of males
    during their mates periods of fertility

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

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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
  • Females of many spider species are larger than
  • Traits which distinguish sex above primary sexual
    organs are called secondary sexual

Pathways to Sexual Dimorphism
  • Sexual dimorphism may arise from
  • (1) 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
  • (2) combats among males
  • weapons of combat (horns or antlers) and larger
    size may confer advantages to males in
    competition for mates
  • (3) 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

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