Transcript Chapter 10: Life Histories and Evolutionary Fitness

CHAPTER 8: SEX AND
EVOLUTION
Stalk-eyed flies
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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 ->)
 Why does this difference
between the sexes exist?
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Background
 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
individual’s gametes
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Sexual reproduction mixes genetic material
of individuals.
 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.
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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
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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 gametes
 Sexual reproduction:
 self-fertilization through fusion of male and female
gametes (common in plants)
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Sexual reproduction is costly.
 Asexual reproduction is:
 common in plants
 found in all groups of animals, except birds and
mammals
 Sexual reproduction is costly:
 gonads are expensive organs to produce and
maintain
 mating is risky and costly, often involving elaborate
structures and behaviors
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Sexual reproduction
is costly
Sexual reproduction is
costly.
So why does sexual reproduction exist at all?
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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 meiosis
 for females, asexually produced offspring carry
twice as many copies of her genes as sexually
produced offspring:
 thus, mating is undesirable
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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 produced:
 if male parental investment doubles the number of
offspring a female can produce, this offsets the
cost of meiosis
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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?
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Who’s 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
history:
 suggests that long-term evolutionary potential of asexual
reproduction is low:
 because of reduced genetic variability, asexual lines
simply die out over time
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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 proofreading?
 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.
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But…
 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 creatures.
 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 persists.
 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.
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So why have sex?
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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.
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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 virulence.
 Hosts continually change to stay one step ahead of their
pathogens, likened to the Red Queen of Lewis Carroll’s
Through the Looking Glass and What Alice Found There.
 ‘it takes all the running you can do, to keep in the same
place.’
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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.
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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 creature.
 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." 21
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.
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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. antipodarum)
 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 individuals
 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?
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experiment
 ?  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 snails
 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 evolution.
 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.
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Perhaps…
 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 shortlived 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 ladder.
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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)
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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 flowers
 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!
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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
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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°-30°C,
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).
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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.
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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
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Check it out on the web for more explanation…
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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
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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 hermaphrodite
 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
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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 1:1 male:female sex ratios, but there are many
deviations from this in the natural world.
 Despite deviations, 1:1 sex ratios are common. Why?
 Every product of sexual reproduction has one father and one mother
 if the sex ratio is not 1:1, 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 the44other sex
1:1 Sex Ratios: An Explanation
Consider a population with an unequal sex ratio...
 individuals of the rare sex have greater fitness
 mutations that result in production of more offspring
of the rare sex will increase in the population
 when sex ratio approaches 1:1, selective advantage of
producing more offspring of one sex or another
disappears, stabilizing the sex ratio at 1:1
 this process is under the control of frequency-
dependent selection
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Why do sex ratios deviate from 1:1?
 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
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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.
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Female wood rats…
 a laboratory study of female wood rats (Neotoma floridana).
 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.
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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 extinction”
 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.
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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.
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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…
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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.
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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.
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Mating Systems: Rules for Pairing
There is a basic asymmetry in sexually
reproducing organisms:
 a female’s reproductive success depends on her
ability to make eggs:
 large female gametes require considerable resources
 the female’s ability to gather resources determines her
fecundity
 a male’s reproductive success depends on the
number of eggs he can fertilize:
 small male gametes require few resources
 the male’s ability to mate with many females determines his
fecundity
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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
animals
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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
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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
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polyandry
 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?
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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 mate-guarding)
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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
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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 male’s 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 territories
 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
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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 males
 Traits which distinguish sex
above primary sexual organs
are called secondary sexual
characteristics.
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Pathways to Sexual Dimorphism
Sexual dimorphism may arise from:
 (1) life history considerations and ecological
relationships:
 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
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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
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Runaway Sexual Selection
When a secondary sexual
trait confers greater
fitness, the stage is set for
runaway sexual
selection:
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
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The Handicap Principle
Can elaborate male secondary sexual
characteristics actually signal male quality to
females?
 Zahavi’s handicap principle suggests that
secondary characteristics act as handicaps -- only
superior males could survive with such burdens
 Hamilton and Zuk have also proposed that showy
plumage (in good condition) signals genetic
factors conferring resistance to parasites or
diseases
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