Transcript Chapter 10: Life Histories and Evolutionary Fitness

Your oral presentations: 5 min max

November 2: Reina
November 11: Wael; Elie G

November 18: Elie D; Chris






November 23: Tony; Bianca
November 25: Omar; Kareem
November 30: Stephanie; Yuri
December 2: Melissa; Reem
December 7: Sabine; Elizabeth

December 9: Fouad; Olivia

December 16: Tarek;

December 21: Anthony;

January 4: Ziad; Zena

January 11: Gerard; Nawal

January 13: Gaby; Riyad
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January 18: Georgio

January 20: Iman
2
CHAPTER 7: LIFE HISTORIES
AND EVOLUTIONARY FITNESS
Life Histories
3

Consider the following remarkable differences in life
history between two birds of similar size:

thrushes




reproduce when 1 year old
produce several broods of 3-4 young per year
rarely live beyond 3 or 4 years
storm petrels



do not reproduce until they are 4 to 5 years old
produce at most a single young per year
may live to be 30 to 40 years old
Parental
investment
What is life history?
5

The life history is the schedule of an organism’s life,
including:
 age
at maturity
 number of reproductive events
 allocation of energy to reproduction
 number and size of offspring
 life span
What influences life histories?
6

Life histories are influenced by:
 body
plan and life style of the organism
 evolutionary responses to many factors, including:
 physical
conditions
 food supply
 predators
 other biotic factors, such as competition
A Classic Study
7

David Lack of Oxford University first placed life
histories in an evolutionary context:
 tropical
songbirds lay fewer eggs per clutch than their
temperate counterparts
 Lack speculated that this difference was based on
different abilities to find food for the chicks:
 birds
nesting in temperate regions have longer days to find
food during the breeding season
Lack’s Proposal
8

Lack made 3 key points, suggesting that life histories
are shaped by natural selection:
1.
2.
3.
because life history traits (such as number of eggs per
clutch) contribute to reproductive success they also influence
evolutionary fitness
life histories vary in a consistent way with respect to factors
in the environment
hypotheses about life histories are subject to experimental
tests
An Experimental Test
9


Lack suggested that one could artificially increase
the number of eggs per clutch to show that the
number of offspring is limited by food supply.
This proposal has been tested repeatedly:
 Gören
Hogstedt manipulated clutch size of European
magpies:
 maximum
number of chicks fledged corresponded to
normal clutch size of seven
Life Histories: A Case of Trade-Offs
10

Organisms face a problem of allocation of scarce
resources (time, energy, materials):



the trade-off: resources used for one function cannot be
used for another function
Altering resource allocation affects fitness.
Consider the possibility that an oak tree might
somehow produce more seed:
how does this change affect survival of seedlings?
 how does this change affect survival of the adult?
 how does this change affect future reproduction?

Components of Fitness
11

Fitness is ultimately dependent on producing
successful offspring, so many life history attributes
relate to reproduction:
 maturity
(age at first reproduction)
 parity (number of reproductive episodes)
 fecundity (number of offspring per reproductive
episode)
 aging (total length of life)
Life history: set of rules and choices
influencing survival and reproduction
The Slow-Fast Continuum 1
14


Life histories vary widely among different species
and among populations of the same species.
Several generalizations emerge:
 life
history traits often vary consistently with respect to
habitat or environmental conditions
 variation in one life history trait is often correlated with
variation in another
The Slow-Fast Continuum 2
15

Life history traits are generally organized along
a continuum of values:
 at
the “slow” end of the continuum are organisms
(such as elephants, giant tortoises, and oak trees)
with:
 long life
 slow development
 delayed maturity
 high parental investment
 low reproductive rates
 at the “fast” end of the continuum are organisms with
the opposite traits (mice, fruit flies, weedy plants)
Grime’s Scheme for Plants
16

English ecologist J.P. Grime envisioned life history
traits of plants as lying between three extremes:
 stress
tolerators (tend to grow under most stressful
conditions)
 ruderals (occupy habitats that are disturbed)
 competitors (favored by increasing resources and
stability)
Grime’s Scheme for Plants
17
Stress Tolerators
18

Stress tolerators:
 grow
under extreme environmental conditions
 grow slowly
 conserve resources
 emphasize vegetative spread, rather than allocating
resources to seeds
Ruderals
19

Ruderals:
 are
weedy species that colonize disturbed habitats
 typically exhibit
 rapid
growth
 early maturation
 high reproductive rates
 easily dispersed seeds
Competitors
20

Competitors:
 grow
rapidly to large stature
 emphasize vegetative spread, rather than allocating
resources to seeds
 have long life spans
Life histories resolve
conflicting demands.
21

Life histories represent tradeoffs among competing
functions:
a
typical trade-off involves
the competing demands of
adult survival and allocation
of resources to reproduction:
 kestrels
with artificially reduced
or enlarged broods exhibited
enhanced or diminished adult
survival, respectively
22
Parental
investment
affects
parental
survival
Life histories balance tradeoffs.
23

Issues concerning life histories may be phrased in
terms of three questions:
 when
should an individual begin to produce offspring?
 how often should an individual breed?
 how many offspring should an individual produce in
each breeding episode?
Age at First Reproduction
24


At each age, the organism chooses between
breeding and not breeding.
The choice to breed carries benefits:
 increase

in fecundity at that age
The choice to breed carries costs:
 reduced
survival
 reduced fecundity at later ages
Long-lived organisms mature later than short-lived ones
Fecundity versus Survival 1
26


How do organisms optimize the trade-off between current
fecundity and future growth?
Critical relationship is:
 = S0B + SSR
where:  is the change in population growth
S0 is the survival of offspring to one year
B is the change in fecundity
S is annual adult survival independent of reproduction
SR is the change in adult survival related to reproduction
Fecundity versus Survival 2
27

When the previous relationship is rearranged, the
following points emerge:
changes in fecundity (positive) and adult survival (negative)
are favored when net effects on population growth are
positive
 effects of enhanced fecundity and reduced survival depend
on the relationship between S and S0
 one thus expects to find high parental involvement
associated with low adult survival and vice versa

In other words…
28



The number of offspring produced today can
reduce the number produced tomorrow
Natural selection should optimize the trade-off
between present and future reproduction
What factors influence the resolution of this conflict?
 High
mortality rates for adults… ?
 Long adult life span… ?
Fecundity and mortality rates for 33 species of birds:
vary together
Your oral presentations: 5 min max

November 2: Reina

November 11: Wael; Elie G

November 18: Elie D; Chris





November 23: Tony; Bianca
November 25: Omar; Kareem
November 30: Stephanie; Yuri
December 2: Melissa; Reem
December 7: Sabine; Elizabeth

December 9: Fouad; Olivia

December 16: Tarek;

December 21: Anthony;

January 4: Ziad; Zena

January 11: Gerard; Nawal

January 13: Gaby; Riyad

January 18: Georgio

January 20: Iman
*Growth versus Fecundity
33

Some species grow throughout their lives, exhibiting
indeterminate growth:
fecundity is related to body size
 increased fecundity in one year reduces growth, thus
reducing fecundity in a later year
 for shorter-lived organisms, optimal strategy emphasizes
fecundity over growth
 for longer-lived organisms, optimal strategy emphasizes
growth over fecundity

Semelparity and Iteroparity
34

Semelparous organisms breed only once during
their lifetimes, allocating their stored resources to
reproduction, then dying in a pattern of
programmed death:
 sometimes

called “big-bang” reproduction
Iteroparous organisms breed multiple times during
the life span.
Semelparity: Agaves and Bamboos
35

Agaves are the century plants
of deserts:



grow vegetatively for several
years
produce a gigantic flowering
stalk, draining plant’s stored
reserves
Bamboos are woody tropical
to warm-temperate grasses:


grow vegetatively for many years
until the habitat is saturated
exhibit synchronous seed
production followed by death of
adults
Agaves: semelparous
38
Why semelparity versus
iteroparity?



iteroparity might offer the advantage of bet
hedging in variable environments
but semelparous organisms often exist in highly
variable environments
this paradox may be resolved by considering the
advantages of timing reproduction to match
occasionally good years
More on Semelparity in Plants
39


Semelparity seems favored when adult survival is
good and interval between favorable years is long.
Advantages of semelparity:
timing reproductive effort to match favorable years
 attraction of pollinators to massive floral display
 saturation of seed predators

Senescence is a decline in function with age
40


Senescence is an inevitable decline in physiological
function with age.
Many functions deteriorate:
 most
physiological indicators (e.g., nerve conduction,
kidney function)
 immune system and other repair mechanisms

Other processes lead to greater mortality:
 incidence
of tumors and cardiovascular disease
Senescence… (males in English population in 1980s)
Why does senescence occur?
42

Senescence may be the inevitable wearing out of the
organism, the accumulation of molecular defects:
ionizing radiation and reactive forms of oxygen break
chemical bonds
 macromolecules become cross-linked
 DNA accumulates mutations


In this sense the body is like an automobile, which
eventually wears out and has to be junked.
Why does aging vary?
44

Not all organisms senescence at the same rate,
suggesting that aging may be subject to natural
selection:
 organisms
with inherently shorter life spans may
experience weaker selection for mechanisms that
prolong life
 repair and maintenance are costly; investment in these
processes reduces investment in current fecundity
45
Life histories respond to variation in
the environment




Storage of food and buildup of reserves
Dormancy  physiologically inactive states
Hibernation  spending winter in a dormant state
Diapause  (insects) – water is chemically bound or
reduced in quantity to prevent freezing and
metabolism drops so low to become barely
detectable
What are the stimuli for change
46



Proximate factors (day length, for example) – an
organism can assess the state of the environment but
these factors do not directly affect its fitness
Ultimate factors (food supplies, for example) –
environmental features that have direct
consequences on the fitness of the organism
Photoperiod: the length of daylight: proximate
factor to virtually all organisms
Relationships between age and size at maturation
may differ when growth rates differ
Food Supply and Timing of Metamorphosis
49



Many organisms undergo metamorphosis from
larval to adult forms.
A typical growth curve relates mass to age for a
well-nourished individual, with metamorphosis
occurring at a certain point on the mass-age curve.
How does the same genotype respond when
nutrition varies?
Metamorphosis Under Varied Environments
50


Poorly-nourished organisms grow more slowly and
cannot reach the same mass at a given age.
When does metamorphosis occur?
fixed mass, different age?
 fixed age, different mass?
 different mass and different age?


Solution is typically a compromise between mass and
age, depending on risks and rewards associated with
each possible combination.
An Experiment with Tadpoles
51

Tadpoles fed different diets illustrate the
complex relationship between size and age at
metamorphosis:
 individuals
with limited food tend to metamorphose
at a smaller size and later age than those with
adequate food (compromise solution)
 the relationship between age and size at
metamorphosis is the reaction norm of
metamorphosis with respect to age and size
Size…
53


Risks of all sorts depend on size and those risks
influence the allocation of resources between
functions that support growth and those that support
maintenance and survival
In the Kalahari sand vegetation of Zimbabwe…
Animals and feeding
55



Optimal feeding: what do you think that means?
Central place foraging – offspring in one location
and parents search for food at some distance
Risk sensitive foraging: every activity carries a risk
of mortality
CHAPTER 8: SEX AND
EVOLUTION
Stalk-eyed flies
60
Background
61

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

Sexual reproduction mixes genetic material of
individuals.
62

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
63

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
64

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)
Sexual reproduction is costly.
65

Asexual reproduction is:



Sexual reproduction is costly:



common in plants
found in all groups of animals, except birds and mammals
gonads are expensive organs to produce and maintain
mating is risky and costly, often involving elaborate
structures and behaviors
So why does sexual reproduction exist at all?
Cost of Meiosis 1
66

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
Cost of Meiosis 2
67

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
Advantages of Sex
68

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?
Who’s asexual?
69
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
Why have sex?

70

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.
But…
71




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.
So why have sex?
72
Sex and Pathogens
73

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
74



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.’
Sex vs Asex
75



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
76




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."
Real-world evidence
77



asexuality is more common in species that are little troubled
by disease: boom-and-bust microscopic creatures, arctic or highaltitude 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.
More on sex and evolution
78




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.
Perhaps…
79



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
ladder.
**Individuals may have female function, male
function, or both.
80

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
81

Plants with separate sexual functions in separate individuals
are dioecious:


Most plants have both sexual functions in the same individual
(hermaphroditism):




this condition is relatively uncommon in plants
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!
82
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.
83




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 the other sex
1:1 Sex Ratios: An Explanation
84
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 frequencydependent selection
Why do sex ratios deviate from
1:1?
85

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

Sex ratio and pollution
86




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.
87
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?
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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?
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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
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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
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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
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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
Promiscuity 2 …
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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
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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
Monogamy
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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 (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)
The Polygyny Threshold
96
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When should polygyny replace monogamy?
For territorial animals:
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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
Sexual Selection
97
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
98

The typical result is sexual dimorphism, a difference
in the outward appearances of males and females of
the same species.

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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 characteristics.
Pathways to Sexual Dimorphism
99
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
result
male plumage and/or courtship displays may
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
102
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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