W9L2 sex selection Flashcards
Female tendency in reproduction
-female invest more in reproduction than male
-Female more likely to exhibit parental care ( as parental care increase offspring survival but lower ability to generate further offspring)
Two forms of sexual selection
- Intrasexual selection: direct competition for mates between members of the same sex, usually male-male competition
- Intersexual selection: differences in attractiveness to the opposite sex, usually non-random mate choice by females
Cause of sexual dimorphism
The asymmetric nature of sexual selection often leads to dramatic sexual dimorphism in characters directly related to male-male competition and/or female choice
What result from sexual dimorphism
- Weapons and ornaments
- Body size (fecundity and fighting)
- Disruptive selection between sex
- Can affect development time
- Differing cost of rearing sons and daughters
- Affect sex ratio
trait that help with male male competition
Weaponry
Large body size
Sensory/locomotory apparatus
Dominance hierarchies
Trait that help with inter-sexual seletion
Direct benefits: nutrient and care, high viability offspring, attractive sons
Indirect benefits
Cost of sexual seletion
- Sexual selection can produce individuals with elaborate ornaments that are costly
- Typically, the cost is in the maintenance of the ornament
- The ornament may lead to a direct survival cost
- Sexual selection has the capacity to evolve maladaptive traits
A pre-existing sensory bias
Guppies eat Clusia fruits which are orange due to carotenoids. These carotenoids also accumulate in males and lead to colourful patterns
Observation: females prefer colourful males
Why? Preexisting sensory bias for detecting orange fruits
-male use courtship signals to exploit pre-existing sensory bias in female
Chase away selection theory
-male exploit sensory bias which cause female fitness declines, leading to maiting threshold to increase
-male attraction would then decrease
-causing male to exaggerate their display and the cycle repeat
What is sperm competition
-male male competition can continue after insemination
-when female remates and ejaculation overlap. This lead to sperm competition
The lottery principle
In many instances sperm competition is analogous to a raffle or lottery. Sperm equal the tickets, fertilizations the prize. As in a raffle, the more tickets (sperm), the greater the probability of securing the prize (fertilization). Hence sperm are produced in large numbers
Evolution of two sexes zygotes
Fixed amount of resources
* Zygote fitness size dependent
* Large gametes = high zygote fitness
* Small gametes = numerous
* Size dimorphic gametes
* Conflict over size of fusion partners
Sperm competition: risk and intensity
- As risk increases, males invest in more sperm
- Within species variation in the NUMBER of competitors = intensity
- Reduce ejaculate expenditure ≥2 competitors due to diminishing returns
how costly are sperm
- Energetically costly investment in 6 sperm= 1 eggs
- Can reduce male lifespan
- Food shortage reduce sperm number
- Accumulate nutrients
Sperm release strategy in different condition
- fruit flies beetle and brown rat release more sperm in the present of rival
>50 species - tend to provide more sperm to mated female (>40 species)
-provide more sperm to bigger female >35 species
The reason for limit sperm release
- Sperm limited commodity
- Optimize fertilization returns
- Several models by Parker & co-workers:
1. Increase ejaculate expenditure under high risk of sperm competition (0-1)
2. Decrease ejaculates under high intensity of sperm competition (≥2 competitors)
What can sperm competition select for
- Increased testis size
- Strategic ejaculation
- Mate guarding
- Mating plugs
- Sperm displacement (removal/repositioning)
- Repeated/longer copulation
-bigger sperm size
Mechanism for female post reproductive selction
- Selective prevention of full copulation
- Selective sperm ejection
- Selective sperm transportation/storage
- Selective sperm use
- Selective ovulation/egg laying
- Selective allocation to eggs
- Selective abortion
- Selective mating plug removal
- Selective remating
Reproductive Fitness of Females and Males
- Female Reproductive Fitness is determined by the Numbers of Eggs and Numbers of Clutches of Eggs that a female lays
- Male Reproductive Fitness is determined by the Numbers of Mates a male has
- Average Female Fitness must be equal toAverage Male Fitness Wfemale = Wmale
Why is the variance in fitness of males LARGER than the variance in fitness of females
-same average fitness but different variance of fitness as male have a much larger variation in reproductive fitness (stronger selection)
-due to variation in mate number between male lead to greater selection on males generate different between sex
