Evolution and Sex II Flashcards
Define sexual dimorphism.
Any morphological, visual or behavioural difference between the sexes.
List the four different types of sexual dimorphisms discussed in class.
Behavioural differences
Behavioural displays
Parental investments
Males and females make different contributions to fitness
Describe an example of a sexually dimorphic behavioural difference between males and females.
Ornamentation and Bower birds
Bower = structure built by males during courtship displays
Females like Bowers decorated with blue
Give examples of sexually dimorphic behavioural displays (2).
Woodcock winnowing
Ruffed grous drumming
Give an example of sexually dimorphic parental investments.
Orangutans
Entire male contribution is 15-20 minute-long copulation
Female pregnant for 8 months and cares for baby for 8 years
Describe what is meant by males and females making different contributions to fitness.
o Female reproduction limited by number of eggs and provision for offspring
o Male reproduction limited by number of matings (sperm are cheap)
Define a mating system
The pattern of matings between individuals in a population, including number of simultaneous mates, permanence of pair bond, and degree of inbreeding.
What is the most common mating system among mammals?
Promiscuity
What is promiscuity?
No lasting pair bond or parental care (typically by male)
Give an example of a species that practices promiscuity.
Sage Grouse
What is polygamy?
Single member of one sex establishes lasting pair bonds with more than one member of the other sex
What is polygyny? Polyandry? Which is more common?
Polygyny = single male, many females
Polyandry = single female, many males
Polygyny more common
Give an example of a species that follows polygamy.
Elephant seals
What is monogamy?
Pair bond between one female and one male, perhaps until death
When is monogamy favoured?
When males can contribute parental care
Give an example of a species that practices monogamy.
Bay-Breasted Warbler
Describe the paradox of self-fertilization.
- Self-fertilization increases homozygosity
- Homozygosity often leads to inbreeding depression
- Despite this, many flowering plants (~20%) have evolved to predominately self-fertilize, and most hermaphroditic plant species exhibit some selfing
What are the two benefits of self-fertilization?
Reproductive assurance
Greater relatedness of offspring
How is reproductive assurance a benefit of selfing?
If there are no pollinators around, it is better to mate with yourself than no one at all
How is greater relatedness of offspring a benefit of selfing?
o A gene that confers complete selfing has a 50% transmission advantage over a gene that leads to outcrossing
o Alleles that increase selfing will quickly spread, even if bad in the long term, can have short term benefits
What is the symbol for inbreeding depression?
sigma
How do you calculate sigma?
1- (fitness of selfed/fitness of outcrossed)
What is indicated when sigma is <0.5?
Benefits of selfing outweigh the costs, selfing will spread within populations
What is indicated when sigma >0.5?
Outcrossing will spread within populations
What types of ecological conditions might favour selfing?
- Unpredictable pollinators
- Weedy species
- Species in extreme environments
But when selfing spreads in a population, what about all of those deleterious mutations that are now expressed?
- Selfing should not be as dire as complete asexuality
- Recombination and segregation are still operating, allowing deleterious alleles to be purged
- It is the case that complete selfing will make it difficult to bring multiple beneficial alleles together, thus selfing may have reduced adaptive potential
What did Fisher 1930 show?
- Every product of sexual reproduction has one father and one mother
- If the sex ratio is not 1:1, individuals belonging to the rarer sex will experiences greater reproductive success
o Such individuals compete for matings with fewer individuals of the same sex
o Therefore, on average, have greater fitness (i.e., contribute more offspring) than individuals of the other sex
Describe how negative-frequency dependent selection regulates sex ratios.
- Individuals of the rare sex have greater fitness
- Mutations that result in production of more offspring of the rare sex will increase in the population
- When ratio approaches 1:1, selective advantage of producing more offspring of one sex or another disappears, stabilizing the sex ratio at 1:1
- This process is an example of negative frequency-dependent selection
