Chapter 11: Sex and Evolution

1. Chapter 11: Sex and Evolution Robert E. Ricklefs The Economy of Nature, Fifth Edition

2. 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

3. Sexual reproduction mixes genetic material of individuals. • 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.

4. Asexual Reproduction • 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

5. Other Variants on Reproduction • Asexual reproduction: • production of diploid eggs (genetically identical) without meiosis (common in fishes, lizards and some insects) • production of diploid eggs (genetically different) by meiosis, with suppression of second meiotic division • self-fertilization through fusion of female gametes • Sexual reproduction: • self-fertilization through fusion of male and female gametes (common in plants)

6. 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?

7. Cost of Meiosis 1 • Sex has a hidden cost for organisms in which sexes are separate: • only half of the genetic material in each offspring comes from each parent • each sexually reproduced offspring contributes only 50% as much to the fitness of either parent, compared to asexually produced offspring • this 50% fitness reduction is called the cost of meiosis • for females, asexually produced offspring carry twice as many copies of her genes as sexually produced offspring: • thus, mating is undesirable

8. Cost of Meiosis 2 • The cost of meiosis does not apply: • when individuals have both male and female function (are hermaphroditic) • when males contribute (through parental care) as much as females to the number of offspring produced: • if male parental investment doubles the number of offspring a female can produce, this offsets the cost of meiosis

9. Advantages of Sex • One advantage to sexual reproduction is the production of genetically varied offspring: • this may be advantageous when environments also vary in time and space • Is this advantage sufficient to offset the cost of meiosis?

10. Who’s asexual? • If asexual reproduction is advantageous, then it should be common and widely distributed among many lineages: • most asexual species (e.g., some fish, such as Poeciliopsis) belong to genera that are sexual • asexual species do not have a long evolutionary history: • suggests that long-term evolutionary potential of asexual reproduction is low: • because of reduced genetic variability, asexual lines simply die out over time

11. 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.

12. Sex and Pathogens • The evolution of virulence by parasites that cause disease (pathogens) is rapid: • populations of pathogens are large • their generation times are short • The possibility exists that rapid evolution of virulence by pathogens could drive a host species to extinction.

13. 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.

14. 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)

15. 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!

16. 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

17. 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?

18. 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 sex • such individuals, on average, have greater fitness (contribute to more offspring) than individuals of the other sex

19. 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

20. 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

21. Mating Systems: Rules for Pairing • There is a basic asymmetry in sexually reproducing organisms: • a female’s reproductive success depends on her ability to make eggs: • large female gametes require considerable resources • the female’s ability to gather resources determines her fecundity • a male’s reproductive success depends on the number of eggs he can fertilize: • small male gametes require few resources • the male’s ability to mate with many females determines his fecundity

22. 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

23. 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 displays • less true when sperm and eggs are shed into water or pollen into wind currents

24. 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 harem • polygyny may also arise when one male controls resources (territory) to which multiple females are attracted

25. Monogamy • 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, relatively common among birds (but recent studies provide evidence for extra-pair copulations selecting for mate-guarding)

26. 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

27. Sexual Selection • In promiscuous and polygynous mating systems, females choose among potential mates: • if differences among males that influence female choice are under genetic control, the stage is set for sexual selection: • there is strong competition among males for mates • result is evolution of male attributes evolved for use in combat with other males or in attracting females

28. 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.

29. 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

30. 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

31. 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

32. 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

33. Summary • Sexual reproduction is widespread, yet its benefits are not entirely clear. Genetic diversity among offspring of sexual unions may confer 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.

34. As usual… • Quizzes. Do the quizzes. • 