Reproduction: The evolution of sex and gender differences

1. Reproduction: The evolution of sex and gender differences ZOL 313 June 5, 2008

2. Reproduction: The evolution of sex and gender differences • Objectives: • Understand and be able to explain potential costs and benefits of sexual reproduction. • Understand why males and females use different mating strategies. Be able to explain why males usually compete and females usually choose. • Be able to predict whether we would expect sex role reversal in a given situation. ZOL 313 June 5, 2008

4. Why reproduce sexually? ‘Evolutionary biologists are much teased for their obsession with why sex exists.  People like to ask, in an amused way, “isn’t it obvious?” Joking apart, it is far from obvious....  Sex is a puzzle that has not yet been solved; no one knows why it exists’ ~Mark Ridley 2001 VS

5. Costs of sexual reproduction 1. Making males Organisms that reproduce sexually face a “two-fold cost” of making males. Example: Imagine a population with both sexual (pink) and asexual (green) females. Each individual produces 2 offspring each generation.

6. Costs of sexual reproduction 2. Sex can break up good genetic combinations If an organism’s genes are ideally suited to its environment: Asexual reproduction Sexual reproduction

7. Costs of sexual reproduction 3. Time/energy/survival Finding a mate and the process of mating itself It can also greatly increase Hypothesis: Mating is energetically costly to male Himalayan tahrs. Prediction:

8. Costs of sexual reproduction 4. Sexually transmitted diseases Example: Example:

9. Benefits of sexual reproduction 1. DNA repair through recombination In an asexual organism, (Like photocopying a blurry photocopy.) In a sexual organism , (A copy for your offspring can be made from your mate’s correct copy.)

10. Benefits of sexual reproduction 2. Lottery Hypothesis Sexual reproduction provides Asexual reproduction is like Sexual reproduction is like

11. Benefits of sexual reproduction Proportion males 2. Red Queen Hypothesis Sexual reproduction enables organisms Example: New Zealand snails have some populations that are all female (asexual) and some with males (sexual). Prediction:

12. Why do males and females use different mating strategies? Example: Bowerbird courtship In general: Females: Males:

13. What is the definition of a female vs a male?

14. Why do males and females use different mating strategies? 1. Differences in gametes MaleFemale Size: Number: Function:

15. Why do males and females use different mating strategies? 2. Differences in parental investment Parental Investment: Costly parental activities that Prezygotic investment: Postzygotic investment:

16. Why do males and females use different mating strategies? 3. Differences in reproductive rate Example: What is the maximum number of offspring a male vs a female human can have in his/her lifetime? Females reproductive rate is limited by Males reproductive rate is limited by Example:

17. Why do males and females use different mating strategies? 4. Differences in levels of sexual activity __________ tend to have higher levels of sexual activity than _________ . This leads to a male biased operational sex ratio Example: Number of matings in male and female katydids. The Coolidge Effect:

18. Why do males and females use different mating strategies? 4. Differences in levels of sexual activity Origin of the term “The Coolidge Effect”: During a tour of a poultry farm, Mrs. Coolidge asked the farmer how his farm could produce so many fertile eggs with such a small number of roosters. The farmer proudly explained that his roosters performed their duty dozens of times each day. "Perhaps you could point that out to Mr. Coolidge," replied the First Lady in a pointedly loud voice. The President, overhearing the remark, asked the farmer, "Does each rooster service the same hen each time?" "No," replied the farmer, "there are many hens for each rooster." "Perhaps you could point that out to Mrs. Coolidge," replied the President. Example: Prediction:

19. Why do males and females use different mating strategies? Figure 10.8

20. Sex role reversals: The exception that proves the rule When males make a larger parental investment and/or have a lower potential reproductive rate than females, we would expect the sex roles to switch. Example: Females mate with multiple males and lay eggs in their nests. Males incubate and care for young. Males are limited by: Females are limited by: Which gender to you predict will compete and which will chose?

21. Sex role reversals: The exception that proves the rule When males make a larger parental investment and/or have a lower potential reproductive rate than females, we would expect the sex roles to switch. Example: Males have brood pouch where females place eggs. Females can make enough eggs to fill 2 pouches. Males are limited by: Females are limited by: Which gender to you predict will compete and which will chose?

22. Sex role reversals: The exception that proves the rule When males make a larger parental investment and/or have a lower potential reproductive rate than females, we would expect the sex roles to switch. Example: When food is scarce, males are only able to produce one spermatophore (nutrient rich nuptial gift). When food is abundant, males can produce multiple spermatophores. Under which condition should male choice/female competition be greater? Why?

23. Sex role reversals: The exception that proves the rule Figure 10.8