Chapter 11 Sex and Evolution Robert E Ricklefs

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Background z. Among the most fascinating attributes of organisms are those related to sexual function, such as: ygender differences ysex ratios yphysical characteristics and behaviors that ensure the success of an individual’s gametes (c) 2001 W. H. Freeman and Company

Sexual reproduction mixes genetic material of individuals. z. In most plants and animals reproduction is accomplished by production of male and female haploid gametes (sperm and eggs): ygametes are formed in the gonads by meiosis z. Gametes join in the act of fertilization to produce a diploid zygote, which develops into a new individual. (c) 2001 W. H. Freeman and Company

Asexual Reproduction z. Progeny produced by asexual reproduction are usually identical to one another and to their single parent: yasexual reproduction is common in plants (individuals so produced are clones) ymany simple animals (hydras水螅, corals珊瑚虫, etc. ) can produce asexual buds, which: xmay remain attached to form a colony xmay separate to form new individuals (c) 2001 W. H. Freeman and Company

Other Variants on Reproduction z Asexual reproduction: yproduction of diploid eggs (genetically identical) without meiosis (common in fishes, lizards and some insects) parthenogenesis孤雌生殖 yproduction of diploid eggs (genetically different) by meiosis, with suppression of second meiotic division yself-fertilization through fusion of female gametes z Sexual reproduction: yself-fertilization through fusion of male and female gametes (common in plants) (c) 2001 W. H. Freeman and Company

Sexual reproduction is costly. z Asexual reproduction is: ycommon in plants yfound in all groups of animals, except birds and mammals z Sexual reproduction is costly: ygonads are expensive organs to produce and maintain ymating is risky and costly, often involving elaborate structures and behaviors z So why does sexual reproduction exist at all? (c) 2001 W. H. Freeman and Company

Cost of Meiosis 1 z Sex has a hidden cost for organisms in which sexes are separate: yonly half of the genetic material in each offspring comes from each parent yeach sexually reproduced offspring contributes only 50% as much to the fitness of either parent, compared to asexually produced offspring xthis 50% fitness reduction is called the cost of meiosis z for females, asexually produced offspring carry twice as many copies of her genes as sexually produced offspring: ythus, mating is undesirable (c) 2001 W. H. Freeman and Company

Figure 11. 5

Cost of Meiosis 2 z. The cost of meiosis does not apply: ywhen individuals have both male and female function (are hermaphroditic雌雄同体) ywhen males contribute (through parental care) as much as females to the number of offspring produced: xif male parental investment doubles the number of offspring a female can produce, this offsets the cost of meiosis (c) 2001 W. H. Freeman and Company

Advantages of Sex z. One advantage to sexual reproduction is the production of genetically varied offspring: ythis may be advantageous when environments also vary in time and space z. Is this advantage sufficient to offset弥补 the cost of meiosis? (c) 2001 W. H. Freeman and Company

Who’s asexual? z If asexual reproduction is advantageous, then it should be common and widely distributed among many lineages: ymost asexual species (e. g. , some fish, such as Poeciliopsis若花鳉鱼) belong to genera that are sexual yasexual species do not have a long evolutionary history: xsuggests that long-term evolutionary potential of asexual reproduction is low: • because of reduced genetic variability, asexual lines simply die out over time (c) 2001 W. H. Freeman and Company

Sex: A Short-Term Advantage? z. Theoretical models based on environmental variability fail to find an advantage to sexual reproduction! z. A promising alternative is that genetic variability is necessary to respond to biological changes in the environment. (c) 2001 W. H. Freeman and Company

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Sex and Pathogens z. The evolution of virulence致病力 by parasites that cause disease (pathogens) is rapid: ypopulations of pathogens are large ytheir generation times are short z. The possibility exists that rapid evolution of virulence by pathogens could drive a host species to extinction. (c) 2001 W. H. Freeman and Company

The Red Queen Hypothesis z Genetic variation represents an opportunity for hosts to produce offspring to which pathogens are not adapted. z Sex and genetic recombination provide a moving target for the evolution by pathogens of virulence. z 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. (c) 2001 W. H. Freeman and Company

Individuals may have female function, or both. z. The common model of two sexes, male and female, in separate individuals, has many exceptions: yhermaphrodites have both sexual functions in the same individual: xthese functions may be simultaneous (plants, many snails and most worms) or xsequential (mollusks软体动物, echinoderms棘皮动物, plants, fishes) (c) 2001 W. H. Freeman and Company

Sexual Functions in Plants z Plants with separate sexual functions in separate individuals are dioecious: ythis condition is relatively uncommon in plants z Most plants have both sexual functions in the same individual (hermaphroditism): ymonoecious plants have separate male and female flowers yplants with both sexual functions in the same flower are perfect (72% of plant species) ymost populations of hermaphrodites are fully outcrossing z Many other possibilities exist in the plant world! (c) 2001 W. H. Freeman and Company

Separate Sexes versus Hermaphroditism z When does adding a second sexual function (becoming hermaphroditic) make sense? ygains from adding a second sexual function must not bring about even greater losses in the original sexual function ythis seems to be the case in plants, where basic floral structures are in place yfor many animals, adding a second sexual function entails承受 a net loss in overall sexual function (c) 2001 W. H. Freeman and Company

Sex ratio of offspring is modified by evolution. z. When sexes are separate, sex ratio may be defined for progeny of an individual or for the population as a whole. z. Humans have 1: 1 male: female sex ratios, but there are many deviations from this in the natural world. z. Despite deviations, 1: 1 sex ratios are common. Why? (c) 2001 W. H. Freeman and Company

1: 1 Sex Ratios: Background z. Every product of sexual reproduction has one father and one mother yif the sex ratio is not 1: 1, individuals belonging to the rarer sex will experience greater reproductive success: xsuch individuals compete for matings with fewer individuals of the same sex xsuch individuals, on average, have greater fitness (contribute to more offspring) than individuals of the other sex (c) 2001 W. H. Freeman and Company

1: 1 Sex Ratios: An Explanation z Consider a population with an unequal sex ratio. . . yindividuals of the rare sex have greater fitness ymutations that result in production of more offspring of the rare sex will increase in the population ywhen sex ratio approaches 1: 1, selective advantage of producing more offspring of one sex or another disappears, stabilizing the sex ratio at 1: 1 ythis process is under the control of frequencydependent selection (c) 2001 W. H. Freeman and Company

Why do sex ratios deviate from 1: 1? z One scenario involves inbreeding: yinbreeding may occur when individuals do not disperse far from their place of birth ya high proportion of sib matings leads to local mate competition among males yfrom 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 ythe result is a shift of the sex ratio to predominance of females, the case in certain parasitic wasps (c) 2001 W. H. Freeman and Company

Mating Systems: Rules for Pairing z There is a basic asymmetry in sexually reproducing organisms: ya female’s reproductive success depends on her ability to make eggs: xlarge female gametes require considerable resources xthe female’s ability to gather resources determines her fecundity ya male’s reproductive success depends on the number of eggs he can fertilize: xsmall male gametes require few resources xthe male’s ability to mate with many females determines his fecundity (c) 2001 W. H. Freeman and Company

Promiscuity 1 z Promiscuity is a mating system for which the following are true: ymales mate with as many females as they can locate and induce to mate ymales provide their offspring with no more than a set of genes yno lasting pair bond is formed yit is by far the most common mating system in animals (c) 2001 W. H. Freeman and Company

Promiscuity 2 z Promiscuity is a mating system for which the following are true: yit is universal among outcrossing plants ythere is a high degree of variation in mating success among males as compared to females: xespecially true where mating success depends on body size and quality of courtship displays xless true when sperm and eggs are shed into water or pollen into wind currents (c) 2001 W. H. Freeman and Company

Polygamy z Polygamy occurs when a single individual of one sex forms long-term bonds with more than one individual of opposite sex: ya common situation involves one male that mates with multiple females, called polygyny: xpolygyny may arise when one male controls mating access to many females in a harem xpolygyny may also arise when one male controls resources (territory) to which multiple females are(c)attracted 2001 W. H. Freeman and Company

Monogamy z Monogamy involves the formation of a lasting pair bond between one male and one female: ythe pair bond persists through period required to rear offspring ythe pair bond may last until one of the pair dies ymonogamy is favored when males can contribute substantially to care of young ymonogamy is uncommon in mammals, relatively common among birds (but recent studies provide evidence for extra-pair copulations结合 selecting for mate-guarding) (c) 2001 W. H. Freeman and Company

The Polygyny Threshold z When should polygyny replace monogamy? z For territorial animals: ya female increases her fecundity by choosing a territory with abundant resources ypolygyny 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 ythe polygyny threshold occurs when females are equally successful in monogamous and polygynous territories xpolygyny should only arise when the quality of male (c) 2001 W. H. Freeman and territories varies considerably Company

Sexual Selection z. In promiscuous and polygynous mating systems, females choose among potential mates: yif differences among males that influence female choice are under genetic control, the stage is set for sexual selection: xthere is strong competition among males for mates xresult is evolution of male attributes evolved for use in combat with other males or in attracting females (c) 2001 W. H. Freeman and Company

Consequences of Sexual Selection z The typical result is sexual dimorphism, a difference in the outward appearances of males and females of the same species. y. Charles Darwin first proposed in 1871 that sexual dimorphism could be explained by sexual selection z Traits which distinguish sex above primary sexual organs are called secondary sexual characteristics. (c) 2001 W. H. Freeman and Company

Pathways to Sexual Dimorphism z Sexual dimorphism may arise from: ylife history considerations and ecological relationships: xfemales of certain species (e. g. , spiders) are larger than males because the number of offspring produced varies with size ycombats among males: xweapons of combat (horns or antlers) and larger size may confer advantages to males in competition for mates ydirect effects of female choice: xelaborate male plumage and/or courtship displays may result (c) 2001 W. H. Freeman and Company

Female Choice z. Evolution of secondary sexual characteristics in males may be under selection by female choice: yin the sparrow-sized male widowbird, the tail is a half-meter long: xmales with artificially elongated tails experienced more breeding success than males with normal or shortened tails (c) 2001 W. H. Freeman and Company

Runaway Sexual Selection z. When a secondary sexual trait confers greater fitness, the stage is set for runaway sexual selection: yregardless of the original reason for female preference, female choice exaggerates fitness differences among males: xleads to evolution of spectacular plumage (e. g. , peacock) and other seemingly outlandish plumage and/or displays (c) 2001 W. H. Freeman and Company

The Handicap Principle z Can elaborate male secondary sexual characteristics actually signal male quality to females? y. Zahavi’s handicap principle suggests that secondary characteristics act as handicaps -- only superior males could survive with such burdens y. Hamilton and Zuk have also proposed that showy plumage (in good condition) signals genetic factors conferring resistance to parasites or diseases (c) 2001 W. H. Freeman and Company

Summary z 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. z 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