Sex allocation

Question 1

What is sex allocation

Answer 1

The allocation of resources to male vs female reproduction in sexual organisms.

Sex allocation determines the sex ratio and operation sex ratio.

Operational sex ratio: the ratio of sexually mature competing male compared to sexually mature competing females.

Question 2

The advancements in understanding sex raiots

Answer 2

• Foundational sex ratio theory (R.A. Fisher 1930)
• Fisher set the foundations and the other scientists build upon his theory altering te assumptions.
• Extraordinary Sex Ratios (W.D. Hamilton 1967)
• Eusocial hymenopterans (R. L. Trivers and H. Hare 1976)
• Local Resource Competition (A. Clark 1978)

Question 3

Fisher’s foundational sex ratio theory

Answer 3

Fisher’s principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females using the idea of reprodutive value and frequency dependent selection.

Males biased in the population: Females have a higher reproductive value (contribute more to the next generation) leading to increased sex allocation and a female biased sex ratio.

Females biased in the population: Males have a higher reproductive value (contribute more to the next generation) leading to increased sex allocation and a male biased sex ratio.

Equal population: Males and females have equal reproductive value leading equal sex allocation and a 1:1 ratio

This theory assumes that males and females are equally costly to produce.
- If investment in males and females is equal then sex ratios are equal.

Question 4

What is reproductive value

Answer 4

Reproductive value is a measure of the genetic influence that males/females have on future generations.

F= f’/2 + m’/2

M= f’/2 + m’/2

f= reproductive value of females in this generations
f’= reproductive value of females in the next generation

Therefore male reproductive value is equal to female reprodutive value when there is an equal sex ratio. (f=m)

Question 5

Fisher’s assumptions

Answer 5

1) Sexual diploidy
2) Non-overlapping generations
3) Panmixia (random mating)
4) No individual variation
5) Agents / parents the same
6) Fixed cost per offspring (no difference between males and females)
7) Returns don’t change as no. of sex increases

Question 6

Hamilton’s Extraordinary sex ratio’s

Answer 6

Assumption violated: panmixia

Hamilton investigated biased sex ratios and came up with local mate competition theory:
- If competition for mates is higher in one species than another, then natural selection may favour the sex with less competition.

Example: Female biased wasp ratios

• n foundresses lay their eggs together in a patch
• The offspring mate at random.
• Females disperse to own breeding patch and males die
• Lots of competition between males (often related) for females
• This leads to female biased sex ratio to reduce the competition for the males and provide more mates for each son. -> trading sons for daughters reduces waste energy on sons that compete

Equilibrium sex ratio= (n-1)/2n
- The more foundresses, the more equal the sex ratio as less male competition between related sons.

Over 40 insect species have female biased sex ratios.

But recent study Greef 2022-> data shows substantial mismatch with theory
- They observed an increase in the proportion of male offspring as a function of foundress number in 5 fig wasps
- foundresses can not quantitatively adjust their sex ratio according to increased environmental selection pressure
- More complex interacting factors than just LMC

Question 7

Trivers and Hare Eusocial hymenopterans

Answer 7

Assumption violated: Diploidy, All agents the same

They considered sex ratio in haploidiplontic species

Reproductive value:

f= f’/2 + m’
m= f’/2

female reproductive value is 2x male reproductive value.

Relatedness:

Queen to son= 0.5 and queen to daugher=0.5
Worker to sister=0.75 and to brother =0.25

Result:

Queen in control: equal investment in males and females as she is equally related

Worker in control: 3x investment in males than females

Excpetion:
- slaving ants bring brood from other nests to reduce effort
- These offspring are not related to the queen and no involved in the conflict
- Queen will control the sex ratio.

Question 8

Clark’s local resource competition

Answer 8

Assumption violated: Panmixia

They investigated how dispersal and competition effects sex ratios.

In mammals males tend to disperse and females stay put (opposite in birds)

Stay at home sex has increased resource competition w/ parents

So sex ratio biased to dispersing sex

Example: Passerines vs anseriforms
- Passerines have predominantly daughter-biased natal dispersal -> female biased
- anseriforms have predominantly son-biased natal dispers. -> male biased

Unless stay at home sex helps the mother eg. wild dogs – sons help mother

Question 9

OVERVIEW

Answer 9

Fisher’s theory creates a good null model for sex ratios.
If there is variation from this model, then an assumption has been violated and there is something interesting going on.

Hamilton, Trive/ Hare and Clark each used Fisher’s model as a framework to understand why some sex ratios are 1:1 and they came up with different theories to explain this.