L10 - Sex-based PI

Question 1

Sex Allocation Theory

Answer 1

• concerns the allocation of parental investment to male vs. female offspring
• prediction: NS favors sex allocation which maximizes parental fitness

Question 2

Primary sex ratio

Answer 2

at conception

Question 3

Secondary sex ratio

Answer 3

at birth

Question 4

Tertiary sex ratio

Answer 4

at any point in maturity

Question 5

Operational sex ratio

Answer 5

sexually active population

Question 6

Fisher’s Principle

Answer 6

• Assuming a 1:1 operational sex ratio, mean male and female reproductive success much always be equal (since every child has 1 mother and 1 father)
• If we shift away from a 1:1 sex ratio, the RS of the less common sex will increase –> generating selection for more of that sex
• thus, we should expect stabilizing selection on a 1:1 operational sex ratio
• ratio will always even out

Question 7

Evidence for Fisher’s Principle

Answer 7

• confirmed by experimental studies –> evolution of balanced sex ratios in mutant Drosophilia
• Evidence for stabilization of equal operational sex ratios, not sex ratios at birth –> humans and animal males more vulnerable to early life disease/mortality so we should expect section to favor slightly more male offspring to be born

Question 8

Hamilton on local resource competition/enhancement

Answer 8

• clarified that within-group interaction between family members alters the costs and benefits of sons vs. daughters
• when one sex is particularly competitive or cooperative with parents or siblings = predicted a departure from Fisherian sex ratios

Question 9

The dispersing sex will be favored if…

Answer 9

…one sex is more likely to compete for local resources

Question 10

The helping sex will be favored if…

Answer 10

…one sex is more likely to enhance the fitness of other family members (e.g. helpers at the nest)

• ex: Seychelles Warbler. Daughters help parents raise offspring, sons disperse.
• in high-quality territory, having helper is advantageous = 90% daughters!
• the bigger the benefit provided by helpers, the larger departure from 50/50 sex ratio
• in low-quality, increased competition for food means helpers are disadvantageous = 80% sons!

Question 11

Trivers-Willard Hypothesis

Answer 11

• biased sex ratios will be favored when reproductive success is differentially influenced by parental condition/invested resources
• when conditions are good, both male and female likely to benefit reproductively, but males benefit more because of greater reproductive potential
• when conditions are bad, both sexes suffer but males will suffer more
• parents in good conditions will favor sons
• parents in bad conditions will favor daughters to have more grandchildren

Question 12

Hypergamy

Answer 12

the action of marrying a person of a superior caste or class.

Question 13

Male-biased investment in India

Answer 13

-preference for sons because of difficulty paying dowry if poor

Question 14

Daughter-biased investment

Answer 14

• Hutterites nurse daughters for longer

- Hungarian gypsies have higher RS if have first born daughters and invest more in girls