(c) 2001 w.h. freeman and company chapter 11: sex and evolution robert e. ricklefs the economy of...

(c) 2001 W.H. Freeman and Company

Chapter 11: Sex and Evolution

Robert E. RicklefsThe Economy of Nature, Fifth Edition


Stalk-eyed flies, 有柄眼果蝇


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


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

meiosisGametes join in the act of fertilization

to produce a diploid zygote, which develops into a new individual.


Asexual ReproductionProgeny 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水螅 , corals珊瑚虫 , etc.) can produce asexual buds, which:may remain attached to form a colonymay separate to form new individuals


Other Variants on Reproduction

Asexual reproduction: production of diploid eggs (genetically identical)

without meiosis (common in fishes, lizards and some insects) parthenogenesis孤雌生殖

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.

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 1Sex 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

Figure 11.5


Cost of Meiosis 2

The cost of meiosis does not apply: when individuals have both male and

female function (are hermaphroditic雌雄同体 )

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

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弥补 the cost of meiosis?


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若花鳉鱼 ) 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


Sex: A Short-Term Advantage?

Theoretical models based on environmental variability fail to find an advantage to sexual reproduction!

A promising alternative is that genetic variability is necessary to respond to biological changes in the environment.


藤黄科书带木属


Sex and Pathogens

The evolution of virulence致病力 by parasites that cause disease (pathogens) is rapid: populations of pathogens are large their generation times are short

The possibility exists that rapid evolution of virulence by pathogens could drive a host species to extinction.


The Red Queen Hypothesis

Genetic variation represents an opportunity for hosts to produce offspring to which pathogens are not adapted.

Sex and genetic recombination provide a moving target for the evolution by pathogens of 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.


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) orsequential (mollusks软体动物 ,

echinoderms棘皮动物 , plants, fishes)


Sexual Functions in Plants

Plants with separate sexual functions in separate individuals are dioecious: this condition is relatively uncommon in plants

Most plants have both sexual functions in the same individual (hermaphroditism): monoecious plants have separate male and female

flowers plants with both sexual functions in the same flower are

perfect (72% of plant species) most populations of hermaphrodites are fully outcrossing

Many other possibilities exist in the plant world!


Separate Sexes versus Hermaphroditism

When does adding a second sexual function (becoming hermaphroditic) make sense? gains from adding a second sexual function must

not bring about even greater losses in the original sexual function

this seems to be the case in plants, where basic floral structures are in place

for many animals, adding a second sexual function entails承受 a net loss in overall sexual function


Sex ratio of offspring is modified by evolution.

When sexes are separate, sex ratio may be defined for progeny of an individual or for the population as a whole.

Humans have 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?


1:1 Sex Ratios: Background

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 sexsuch individuals, on average, have greater

fitness (contribute to more offspring) than individuals of the other 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


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 from the parent’s standpoint, one male offspring

serves just as well as many to fertilize his female siblings, while production of more female offspring will lead to production of more progeny

the result is a shift of the sex ratio to predominance of females, the case in certain parasitic wasps


Mating Systems: Rules for Pairing

There is a basic asymmetry in sexually reproducing organisms: a female’s reproductive success depends on

her ability to make eggs:large female gametes require considerable resourcesthe 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 resourcesthe male’s ability to mate with many females

determines his fecundity


Promiscuity 1

Promiscuity is a mating system for which the following are true: males mate with as many females as they

can locate and induce to mate males provide their offspring with no more

than a set of genes no lasting pair bond is formed it is by far the most common mating system

in animals


Promiscuity 2

Promiscuity is a mating system for which the following are true: it is universal among outcrossing plants there is a high degree of variation in mating

success among males as compared to females:especially true where mating success depends on

body size and quality of courtship displaysless true when sperm and eggs are shed into water

or pollen into wind currents


Polygamy

Polygamy occurs when a single individual of one sex forms long-term bonds with more than one individual of opposite sex: a common situation involves one male that

mates with multiple females, called polygyny:polygyny may arise when one male controls

mating access to many females in a harempolygyny may also arise when one male controls

resources (territory) to which multiple females are attracted


MonogamyMonogamy involves the formation of a lasting

pair bond between one male and one female: the pair bond persists through period required to

rear offspring the pair bond may last until one of the pair dies monogamy is favored when males can contribute

substantially to care of young monogamy is uncommon in mammals, relatively

common among birds (but recent studies provide evidence for extra-pair copulations结合 selecting for mate-guarding)


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


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

matesresult is evolution of male attributes evolved

for use in combat with other males or in attracting females


Consequences of Sexual Selection

The typical result is sexual dimorphism, a difference in the outward appearances of males and females of the same species. Charles Darwin first proposed in 1871 that

sexual dimorphism could be explained by sexual selection

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


Pathways to Sexual Dimorphism

Sexual dimorphism may arise from: 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

combats among males:weapons of combat (horns or antlers) and larger size may

confer advantages to males in competition for mates

direct effects of female choice:elaborate male plumage and/or courtship displays may

result


Female Choice

Evolution of secondary sexual characteristics in males may be under selection by female choice: in the sparrow-sized male widowbird,

the tail is a half-meter long:males with artificially elongated tails

experienced more breeding success than males with normal or shortened tails


Runaway Sexual Selection

When a secondary sexual trait confers greater fitness, the stage is set for runaway sexual 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

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


Summary

Sexual reproduction is widespread, yet its benefits are not entirely clear. Genetic diversity among offspring of sexual unions may confer 授予 fitness in the face of environmental variation and rapidly-evolving diseases.

Sex ratios, mating systems, and secondary sexual characteristics arise in sexually reproducing organisms in response to selective pressures affecting both males and females.

(c) 2001 w.h. freeman and company chapter 11: sex and evolution robert e. ricklefs the economy of...

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