CHAPTER 8: SEX AND EVOLUTION

1. CHAPTER 8: SEX AND EVOLUTION

2. Stalk-eyed flies

3. Stalk-eyed flies • Both M &F have these stalks • In some species – they are up to twice as long in males as they are in females (see pic ->) • Why does this difference between the sexes exist?

4. 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

5. 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.

6. Asexual Reproduction Progeny produced by asexual reproduction are usually identical to one another and to their single parent: This fern sprouts a fully formed plant from the tip of a leaf • 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

7. 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)

8. 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

9. Sexual reproduction is costly. So why does sexual reproduction exist at all?

10. 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

11. 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

12. So why have sex?

13. 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.

14. 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. • ‘it takes all the running you can do, to keep in the same place.’

15. Sex vs Asex • 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.

16. Now add parasites • 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."

17. Real-world evidence • 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.

18. 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)

19. Check it out: more on the web Sequential hermaphroditism Some organisms are male first and then become female later in their lives Some organisms are female first and then become male later in their lives

20. Whether a sequential hermaphrodite is first male or first female depends on how reproductive success through male or female function potentially increases with increasing body size. Larger females have larger reproductive organs and can lay more eggs. Small males can have high reproductive success where fertilization is internal and males do not contest social status, as in the case of the slipper shell. Where males compete for territories, as in the wrasse, large size is prerequisite to successful reproduction.

21. Sequential hermaphroditism reflects changes in the costs and benefits of male and female sexual function as an organism grows. In some marine gastropods having internal fertilization, such as the slipper shell Crepidula, insemination requires the production of only small amounts of sperm. Hence male function consumes few resources and has little effect on growth. Consequently, individuals of many such species are male when they are small and become female when they are large and thus able to produce correspondingly large clutches of eggs For others…

22. Check it out on the web for more explanation…

23. Mechanisms of sex determination • Determined by the physical environment • Several species of turtles, lizards, alligators: sex determined by the temperature at which it develops in the egg • Embryos that develop at lower temp produce males; higher temp: females; for turtles. Opposite for alligators and lizards. • Hmm… ? • Determined by the social environment • The wrasse (discussed earlier) is a sequential hermaphrodite • Raised in isolation  females; raised in small groups, at least one develops initially into a male w/o passing through a female phase • Females may become males later when they grow large enough to compete for territories ; primary males never change their sex

24. Sex ratio and pollution • 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.

25. So? What do we know? • in-utero exposure to pollutants could 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.

26. Human sex ratio and pollution • 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.

27. 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

28. Promiscuity 2 … • 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

29. 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

30. polyandry • Rare cases of a single female having more than one male mate • Some human communities • 1% of birds.. • A common example of this can be found in the Field Cricket Gryllus bimaculatus of the invertebrate order Orthoptera • Widely shown in frogs (Agile frogs, Rana dalmatina), polyandry was also documented in polecat (Mustela putorius) and other mustelids • Why?

31. Monogamy: the formation of a lasting pair bond betw one male &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)

32. Real monogamy? • Extra-pair copulations (EPC) • 1/3 or more of the broods produced by some monogamous species contain 1 or more offspring sired by a different male • Mate guarding behavior on the part of males during their mates’ periods of fertility

33. 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

35. 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

36. 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 • Females of many spider species are larger than males • Traits which distinguish sex above primary sexual organs are called secondary sexual characteristics.

37. Pathways to Sexual Dimorphism 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

38. 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