Evolution and Sex II

Question 1

Define sexual dimorphism.

Answer 1

Any morphological, visual or behavioural difference between the sexes.

Question 2

List the four different types of sexual dimorphisms discussed in class.

Answer 2

Behavioural differences
Behavioural displays
Parental investments
Males and females make different contributions to fitness

Question 3

Describe an example of a sexually dimorphic behavioural difference between males and females.

Answer 3

Ornamentation and Bower birds
Bower = structure built by males during courtship displays
Females like Bowers decorated with blue

Question 4

Give examples of sexually dimorphic behavioural displays (2).

Answer 4

Woodcock winnowing
Ruffed grous drumming

Question 5

Give an example of sexually dimorphic parental investments.

Answer 5

Orangutans
Entire male contribution is 15-20 minute-long copulation
Female pregnant for 8 months and cares for baby for 8 years

Question 6

Describe what is meant by males and females making different contributions to fitness.

Answer 6

o Female reproduction limited by number of eggs and provision for offspring
o Male reproduction limited by number of matings (sperm are cheap)

Question 7

Define a mating system

Answer 7

The pattern of matings between individuals in a population, including number of simultaneous mates, permanence of pair bond, and degree of inbreeding.

Question 8

What is the most common mating system among mammals?

Answer 8

Promiscuity

Question 9

What is promiscuity?

Answer 9

No lasting pair bond or parental care (typically by male)

Question 10

Give an example of a species that practices promiscuity.

Answer 10

Sage Grouse

Question 11

What is polygamy?

Answer 11

Single member of one sex establishes lasting pair bonds with more than one member of the other sex

Question 12

What is polygyny? Polyandry? Which is more common?

Answer 12

Polygyny = single male, many females
Polyandry = single female, many males
Polygyny more common

Question 13

Give an example of a species that follows polygamy.

Answer 13

Elephant seals

Question 14

What is monogamy?

Answer 14

Pair bond between one female and one male, perhaps until death

Question 15

When is monogamy favoured?

Answer 15

When males can contribute parental care

Question 16

Give an example of a species that practices monogamy.

Answer 16

Bay-Breasted Warbler

Question 17

Describe the paradox of self-fertilization.

Answer 17

• 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

Question 18

What are the two benefits of self-fertilization?

Answer 18

Reproductive assurance
Greater relatedness of offspring

Question 19

How is reproductive assurance a benefit of selfing?

Answer 19

If there are no pollinators around, it is better to mate with yourself than no one at all

Question 20

How is greater relatedness of offspring a benefit of selfing?

Answer 20

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

Question 21

What is the symbol for inbreeding depression?

Answer 21

sigma

Question 22

How do you calculate sigma?

Answer 22

1- (fitness of selfed/fitness of outcrossed)

Question 23

What is indicated when sigma is <0.5?

Answer 23

Benefits of selfing outweigh the costs, selfing will spread within populations

Question 24

What is indicated when sigma >0.5?

Answer 24

Outcrossing will spread within populations

Question 25

What types of ecological conditions might favour selfing?

Answer 25

• Unpredictable pollinators
• Weedy species
• Species in extreme environments

Question 26

But when selfing spreads in a population, what about all of those deleterious mutations that are now expressed?

Answer 26

• 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

Question 27

What did Fisher 1930 show?

Answer 27

• 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

Question 28

Describe how negative-frequency dependent selection regulates sex ratios.

Answer 28

• 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