Sexual selection

Question 1

Define isogamy.

Answer 1

Sexual reproduction via fusion of gametes that are almost identical.

Question 2

In isogamy are the gametes motile or non-motile?

Answer 2

Both are motile.

Question 3

What is said about isogamous organisms?

Answer 3

They are single sex as they are virtually identical.

Question 4

Instead of sexes what do isogamous organisms have?

Answer 4

Mating types - these differ at a single locus only.

Question 5

How is inbreeding prevented in isogamous organisms?

Answer 5

Mating between organisms of the same mating type is incompatible.

Question 6

Define anisogamy.

Answer 6

Sexual reproduction via the fusion of dissimilar gametes.

Question 7

In anisogamy, are gametes motile or non-motile?

Answer 7

They are both motile. Differences between gametes are not profound.

Question 8

Define oogamy.

Answer 8

A form of anisogamy whereby the differences between gametes are profound.

Question 9

What are the gametes like in oogamy?

Answer 9

The sperm is small, motile and only contributes genetic material.

The egg is large, non-motile and contributes genetic material and yolk proteins.

Question 10

Almost all multicellular organisms are oogamous. What kind of ancestor did they all evolve from?

Answer 10

An isogamous, marine ancestor with external fertilisation.

Question 11

Define disruptive selection.

Answer 11

When there are opposing selection pressures on males and females.

Question 12

Disruptive selection is responsible for sexual dimorphism. True or false?

Answer 12

True.

Question 13

What could be described as the fundamental sexual dimorphism?

Answer 13

Anisogamy

Question 14

What does n = R/m mean?

Answer 14

n = the number of gametes that can be produced

R = the amount of resource allocated by each parent

m = the size of each gamete

Question 15

The size of the zygote is the sum of the sizes of the parental gametes. True or false?

Answer 15

True.

Question 16

What effect does zygote size have on fitness, f?

Answer 16

Fitness increases with zygote size.

Question 17

Is it always true that fitness increases with zygote size?

Answer 17

No, that is only true for anisogamous organisms: in isogamous organisms zygote size has a decreasing effect on fitness.

Question 18

Why does disruptive selection select for larger eggs in oogamy?

Answer 18

Zygote size it affected by egg volume: larger eggs lead to fitter zygotes

Question 19

Why does disruptive selection select for smaller sperm in oogamy?

Answer 19

They are faster and better at swimming so will be successful in competition for fertilisation.

Question 20

Why is oogamy the only solution to disruptive selection?

Answer 20

Because there are opposing selection pressures on each gamete, the gametes must be different.

Question 21

The transition from uni to multicellularity is usually accompanied by a transition from external to internal fertilisation. Why?

Answer 21

To reduce sperm competition.

Question 22

Anisogamy is stable if A > N/(N-1). Explain this equation.

Answer 22

A = anisogamy ratio, calculated as ovum size/sperm size

N = the number of competing ejaculates

The equation means sperm will not increase in size to contribute to zygote survival unless N is extremely low.

Question 23

The equation p > 4/A refers to the probability that a given ejaculate will face sperm competition. Sperm will only get bigger to contribute to zygote survival if p is very low. Why will this never happen?

Answer 23

There is always sperm competition in nature.

Question 24

Anisogamy is the fundamental difference. What does sexual dimorphism create?

Answer 24

Sex-specific fitness optima: males and females strive for different physical attributes that compliment their reproductive roles. For example in fish large females are favoured because they produce more eggs.

Question 25

Do males and females have the same sex hormones?

Answer 25

Yes but they are differentially expressed.

Question 26

What do sex hormones control?

Answer 26

The expression of down-stream genes that result in phenotypic differences between then sexes.

Question 27

Testosterone is often associated with males. Give an example of a species in which it is highly expressed in females.

Answer 27

Fish: females need to be larger and testosterone makes them grow bigger.

Question 28

Are different sexes of each species always dimorphic?

Answer 28

Not always: there is a scale of dimorphism. In some species males and females look identical, in others they don’t.

Question 29

Give 3 reasons for sexual dimorphism.

Answer 29

1. Sexual selection
2. Intersexual niche partitioning
3. Reproductive role division

Question 30

Why does sexual selection create a need for sexual dimorphism?

Answer 30

Females choose males based on attributes that display his genetic prowess, i.e. what he can contribute to their offspring.

Question 31

Why does intersexual niche partitioning create a need for sexual dimorphism?

Answer 31

In some species males and females behave differently so as not to compete with each other. In green wood-hoopoes, males and females feed on different things thus their beak morphs are different.

Question 32

Why does reproductive role partitioning create a need for sexual dimorphism? Give an example.

Answer 32

Each sex needs to be adapted to different tasks. In female humans the pelvis has been altered to accommodate childbirth. This impairs locomotion, thus males are often better at running. Males need to be better at running as ancestrally they would hunt for food to feed their family.

Question 33

There is always a trade-off of function in reproductive role partitioning. True or false?

Answer 33

True.

Question 34

What is developmental decoupling?

Answer 34

Males and females share the same autosomal genes, thus varying gene expression is what creates phenotypic disparity.

Question 35

What is created in developmental decoupling?

Answer 35

Sex-specific gene expression.

Question 36

The extent of dimorphism depends on the strength of conflict between the fitness optima. Use turkeys as an example to explain this.

Answer 36

Dominant male turkeys and female turkeys look hugely different. Subordinate male turkeys look very much like females. Subordinate males never mate, so never have to compete for mates and thus express far less male-biased and more female-biased genes.

Question 37

What effect does sexual dimorphism have on growth trajectory?

Answer 37

Species that are highly dimorphic have longer, more rapid growth trajectories.

Question 38

Apoptosis has been observed as a way to create sexual dimorphism.

a) Give an example of this
b) Why is it very rare?

Answer 38

a) In horn beetles the horn forms in both sexes but in females it dies away
b) Because this is an extremely energy expensive process