the evolution of genitalia

Question 1

what are genitalia?

Answer 1

all structures directly associated with the male
gonopore, or specialised for transmission or
holding the female near her copulatory organs are
genitalia (Eberhard, 1985)

Question 2

what kind of allometric relationship and variation do sexually selected traits usually display?

Answer 2

Typically, sexually selected traits show positive allometry and increased phenotypic variation as a result of directional selection (Green, 1992).

Question 3

Dave 2005 said CV is indicatve of sexual selection, it need not always be the case - why?

Answer 3

(Hosken et al, 2005). As an alternative to sexual selection, similar patterns could arise from pleiotropy.

Question 4

what is the evolutionary paradigm with the complexity of genitalia?

Answer 4

why male genitalia need to be so complex when all they need to do is deliver sperm

Question 5

why could pleiotropy be responsible for positive allometry and an increase in genital variation?

Answer 5

genitalia are affected by many genes and therefore any change at any of these loci will affect genitalia.

Question 6

what are the problems with pleitropy as an explanation for genital variation?

Answer 6

1) assumes genitals are more or less neutral as long as sperm transfer is not impaired.
2) why would genitals be especially polygenic?
3) if pleiotropic gene effects are responsible, why do we not see the same variation in external fertilisers?
4) why would there be a correlation between the sexes if there are other genes associated with genital morphology?

Question 7

is there any evidence of pleiotropic influence on genital morphology?

Answer 7

yes- but limited. general applicability is questionable.

however, (Arnqvist & Thornhill, 1998) - pleiotropic gene effects can influence genital organs in water striders

Question 8

what is the lock and key hypothesis for genital variation?

Answer 8

genital divergence occurs as a result of hybridisation avoidance

Question 9

if lock and key is true, you’d expect differences in genital morphology in sister species across space. Does this happen?

Answer 9

larger divergence in genital morphology between sister species in sympatry is expected than sister species in allopatry if reinforcement of anti-hybridisation occurs. This was found in marine Littorid species, genital morphology has a role in reproductive isolation in this clade (hollander et al, 2012)

Question 10

what are the problems with lock and key?

Answer 10

island species do not seem to have less complex/divergent
genitals, ditto parasites.and there can be cross species
intromission. SO could there be natural selection on genitals unrelated to avoiding hybridization?

Question 11

what is the sexual selection hypothesis for genital evolution?

Answer 11

male genitalia are highly variable and
divrgent due to the actions of sexual
selection. (reproductive competition)

Question 12

why do we expect sexual selection to be post-copulatory?

Answer 12

we expect selection to be post copulatory
because genitals are typically hidden except
during copulation, so sperm competition and
cryptic female choice

Question 13

what did Eberhard, 1996 argue about the role of post copulatory sexual selection in shaping genital diversity?

Answer 13

specifically, he argued that genital divergence is driven by cryptic female choice

Question 14

what is cryptic female choice?

Answer 14

Broadly, it is any post-coupling process or structure controlled by females that biases paternity towards males with a certain phenotype when females are polyandrous

Question 15

what are the four main sexual selection mechanisms that have been implicated in genital evolution?

Answer 15

sexy sons (also known as runaway or fisherian selection)
good genes
sperm competition
sexual conflict

Question 16

what is genital divergence driven by for both the sexy sons and the good genes hypothesis?

Answer 16

cryptic female choice for males with genitalia which best stimulate them during copulation

Question 17

what are the fitness benefits associated with the sexy sons mechanism?

Answer 17

the inherited mate-attracting potential, or the female stimulation potential of the sons of the female

Question 18

what is the additional benefit of the good genes mechanism compared with sexy sons?

Answer 18

also increased offspring viability

Question 19

when could sperm competition facilitate the divergence of genitalia?

Answer 19

it might select for sperm removal devices (so the selection pressure is more sperm competition avoidance). BUT… females are also benefitting from the sperm removal ability of their sons

Question 20

which animals can remove sperm?

Answer 20

Odonates (damselflies) can remove competition by removing sperm from the female reproductive tract (Waage, 1979)

Question 21

when will evolutionary conflict between the sexes occur?

Answer 21

when there is not strict genetic monogamy and reproduction is costly - leading to arms race dynamics

Question 22

Eberhard (2004) argues that sexual conflict doesn’t promote divergence… why?

Answer 22

a widespread study by Eberhard, 2004 predicted that new arms races are less likely to occur when females are protected from unwanted sexual attention. The pattern didn’t hold across 361 genera of insects and spiders.

Question 23

what are the weak points for sexual selection in explaining genital variation?

Answer 23

1) good genes requires genes to increase both viability and
attractiveness. genes with this double advantage should rapidly go to fixation (natural + sexual selection) , leaving females with little to choose from. if choice is costly then there is no benefit, means no choice.

Question 24

what is R=Sh^2?

Answer 24

the univariate breeders equation. h^2 is heritability and S is the selection differential. it reflects how strong selection is

Question 25

what is sexual selection

Answer 25

it depends on the advantage that certain individuals ave over each other (same sex) in exclusive relation to competition. evolution through sexual selection occurs as a result of non random associations between a trait and a component of mating success

Question 26

what does the slope of the line of a regression of offspring trait values and parent trait values tell you?

Answer 26

how heritable the trait is

Question 27

what is runaway/fisherian selection? (sexy sons)

Answer 27

females prefer males with exaggerated traits because females prefer them. the only benefits to females is via the mating success of their sons. females prefer certain, naturally selected traits. Because females mate with males with that trait, preference for the trait and trait become genetically correlated. this can generate accelerated evolution beyond a fitness optima

Question 28

what are the characteristics of landes model, 1981?

Answer 28

based on ra fisher's model, he said that sexual selection could become self reinforcing and that male trait and female preference could evolve together at ever increasing speeds.

Question 29

what type of trait is lande's model referring to?

Answer 29

a sex limited trait, assumes characters are polygenic

Question 30

in landes model, what happens to average male trait distribution when genetic covariance is weak?

Answer 30

it depends on the starting point, whether it is already beyond the naturally selected optima or not. Btu t leads to a line of stable equilibrium - an eventual balance between sexual and natural selection (although generally, male trait size does not correspond to an NS optima)

Question 31

what happens in lands model when genetic covariance between female preference and male trait value are highly correlated?

Answer 31

when preference is strong / stereotyped, fisherman runaway selection occurs. strong covariance leads to a line of unstable equilibria