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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

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Your oral presentations 5 min max
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

Life histories
Life Histories

  • 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

What is life history
What is life history?

  • 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
What influences life histories?

  • 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
A Classic Study

  • 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
Lack’s Proposal

  • Lack made 3 key points, suggesting that life histories are shaped by natural selection:

    • 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
An Experimental Test

  • 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
Life Histories: A Case of Trade-Offs

  • 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
Components of Fitness

  • 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)

The slow fast continuum 1
The Slow-Fast Continuum 1 and reproduction

  • 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
The Slow-Fast Continuum 2 and reproduction

  • 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
Grime’s Scheme for Plants and reproduction

  • 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 plants1
Grime’s Scheme for Plants and reproduction

Stress tolerators
Stress Tolerators and reproduction

  • Stress tolerators:

    • grow under extreme environmental conditions

    • grow slowly

    • conserve resources

    • emphasize vegetative spread, rather than allocating resources to seeds

Ruderals and reproduction

  • Ruderals:

    • are weedy species that colonize disturbed habitats

    • typically exhibit

      • rapid growth

      • early maturation

      • high reproductive rates

      • easily dispersed seeds

Competitors and reproduction

  • Competitors:

    • grow rapidly to large stature

    • emphasize vegetative spread, rather than allocating resources to seeds

    • have long life spans

Life histories resolve conflicting demands
Life histories resolve conflicting demands. and reproduction

  • Life histories represent trade-offs 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

Life histories balance tradeoffs
Life histories balance tradeoffs. and reproduction

  • 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
Age at First Reproduction and reproduction

  • 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

Fecundity versus survival 1
Fecundity versus Survival 1 and reproduction

  • 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
Fecundity versus Survival 2 and reproduction

  • 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
In other words… and reproduction

  • 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… ?

Your oral presentations 5 min max1
Your oral presentations: 5 min max together

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
*Growth versus Fecundity together

  • Some species grow throughout their lives, exhibiting indeterminategrowth:

    • 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
Semelparity and Iteroparity together

  • 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
Semelparity: Agaves and Bamboos together

  • 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

Why semelparity versus iteroparity
Why semelparity versus iteroparity? together

  • 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
More on Semelparity in Plants together

  • 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
Senescence is a decline in function with age together

  • 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

Why does senescence occur
Why does senescence occur? together

  • 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
Why does aging vary? together

  • 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

Life histories respond to variation in the environment
Life histories respond to variation in the environment together

  • 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
What are the stimuli for change together

  • 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

Food supply and timing of metamorphosis
Food Supply and Timing of Metamorphosis when growth rates differ

  • 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
Metamorphosis Under Varied Environments when growth rates differ

  • 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
An Experiment with Tadpoles when growth rates differ

  • 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… when growth rates differ

  • 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
Animals and feeding when growth rates differ

  • 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

CHAPTER 8: SEX AND EVOLUTION when growth rates differ

Stalk eyed flies
Stalk-eyed flies when growth rates differ

Background when growth rates differ

  • 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
Sexual reproduction mixes genetic material of individuals. when growth rates differ

  • In most plants and animals reproduction is accomplished by production of male and female haploidgametes (sperm and eggs):

    • gametes are formed in the gonads by meiosis

  • Gametes join in the act of fertilization to produce a diploidzygote, which develops into a new individual.

Asexual reproduction
Asexual Reproduction when growth rates differ

  • 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
Other Variants on Reproduction when growth rates differ

  • 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
Sexual reproduction is costly. when growth rates differ

  • 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

  • So why does sexual reproduction exist at all?

Cost of meiosis 1
Cost of Meiosis 1 when growth rates differ

  • 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
Cost of Meiosis 2 when growth rates differ

  • 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
Advantages of Sex when growth rates differ

  • 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
Who’s asexual? when growth rates differ

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
Why have sex? when growth rates differ

  • 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… when growth rates differ

  • 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
So why have sex? when growth rates differ

Sex and pathogens
Sex and Pathogens when growth rates differ

  • 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
The when growth rates differRed 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.’

Sex vs asex
Sex vs Asex when growth rates differ

  • 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
Now add parasites when growth rates differ

  • 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
Real-world evidence when growth rates differ

  • 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.

More on sex and evolution
More on sex and evolution when growth rates differ

  • 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… when growth rates differ

  • 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
**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
Sexual Functions in Plants both.

  • 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!

Separate sexes versus hermaphroditism
Separate Sexes versus Hermaphroditism both.

  • 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
Sex ratio of offspring is modified by evolution. both.

  • 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
1:1 Sex Ratios: An Explanation both.

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

Why do sex ratios deviate from 1 1
Why do sex ratios deviate from 1:1? both.

  • 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
Sex ratio and pollution both.

  • 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.

Human sex ratio and pollution pcbs
Human sex ratio and pollution: PCBs… both.

  • 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
Is it PCBs? both.

  • 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
So? What do we know? both.

  • 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
Human sex ratio and pollution both.

  • 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
Mating Systems: Rules for Pairing both.

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

Promiscuity is a mating system for which the following are true
Promiscuity: is a both.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

Promiscuity 2
Promiscuity 2 … both.

  • 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 both.

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 both.

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
The Polygyny Threshold both.

  • 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

Sexual selection
Sexual Selection both.

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
Consequences of Sexual Selection both.

  • The typical result is sexual dimorphism, a difference in the outward appearances of males and females of the same species.

    • Charles Darwin first proposed in 1871 that sexual dimorphism could be explained by sexual selection

  • Traits which distinguish sex above primary sexual organs are called secondary sexual characteristics.

Pathways to sexual dimorphism
Pathways to Sexual Dimorphism both.

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

Female choice
Female Choice both.

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
Runaway Sexual Selection both.

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

The handicap principle
The Handicap Principle both.

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