10 Sexual Selection

Question 1

What is sexual selection?

Answer 1

The choice and number of mates

Question 2

What is sexual dimorphism?

Answer 2

Differences in the appearance between males and females of the same species

Question 3

What is intrasexual selection?

Answer 3

Competition among members of the same species for access to mates - few winners
e.g. males fighting, mate guarding, sperm competition
can lead to evolution of traits beneficial for this

Question 4

What is intersexual selection

Answer 4

Individuals of one sex choose mates from the opposite sex
e.g. by attractiveness, displays of fitness

Question 5

What is the diversity of mating systems?

Answer 5

Caused by competition for mates
Monogamy - 1M & 1F
Polygamy - male and female have multiple partners
Polyandry - F has multiple partners
Polygyny - M has multiple partners

Question 6

What is monogamy?

Answer 6

One partner
Little sexual selection
No sexual dimorphism

Question 7

What is extra pair paternity?

Answer 7

e.g. in Dunnocks
Frequently polyandrous despite appearing monogamous
Females frequently mate with unpaired males

Question 8

What comes with polygamy, polyandry and polygyny?

Answer 8

Multiple partners
Sexual selection
Sexual dimorphism

Question 9

What is Lek competition?

Answer 9

Gathering of males that compete with each other
Females observe
Females choose best males
Most costly for males - fighting

Question 10

What do females gain by choosing males?

Answer 10

1) Access to resources
- Courtship feeding - If given high quality food at high rate, more likely to mate with the male

2) Access to good genes
- Choose genes that confer high survival to offspring e.g. resistance to parasites
- Choose attractiveness genes that would make male offspring more attractive to females

Question 11

What did Anderson (1982) do?

Answer 11

Experimentally altered tail length of males
Found that:
Reduced tail = lower mating success
Increased tail = more mating success

Question 12

What is runaway selection?

Answer 12

The evolution of exaggerated male ornamentation by persistent, directional female choice
e.g. peacock’s plumage
However selection on survival and reproduction operates in different directions

Question 13

Why does an optimum tail length evolve?

Answer 13

Female preference evolves to increase tail length
There is a survival advantage to having a shorter tail

Optimum tail length - survival advantage + female choice
- Shorter lifespan but fitness still high

Question 14

What is the Lek Paradox?

Answer 14

Would expect that in a strongly selected population, variation between individuals are random and not due to genetics
Actually observed:
Sexually selected traits have high variability and additive variance

Question 15

What is an example of the relationship between sexual and natural selection?

Answer 15

e.g. in spotted fish
There are variations of colourations of fish
- Natural selection favours spot pattern matching background
- Sexual selection favours male patterns that contrast background

Question 16

How are sexual and natural selection related?

Answer 16

Prediction that in species that experience stronger natural selection, sexual selection should be weaker

The strength of sexual selection is correlated with sexual dimorphism - males are more different from females when sexual selection is stronger

So species in habitats that are more prone to predation should have less dimorphism e.g. ground nesting birds - stronger natural selection for being camoflagued