L13 - Sexual Selection and Coevolution

Question 1

Why would a seemingly maladapted trait persist in a population (e.g., peacock tail)?

Answer 1

• Sexual selection! While the maladaptive traits are not beneficial to an individual’s survival, they are attractive to potential mates, essential for the reproduction of a species

Question 2

Define primary sexual characteristics

Answer 2

Differences in the biological sexes directly related to reproduction (e.g. genitalia)

Question 3

Define secondary sexual characteristics

Answer 3

• Traits which are used in sexual selection, but are
  not required for sexual reproduction
  
  • Horns, pretty feathers, birdsongs…
  • Direct competition or attractiveness

Question 4

Define sexual dimorphism

Answer 4

Difference in appearance between males and females of the same species

Question 5

What are the two main types of sexual selection?

Answer 5

• Intrasexual: within the same sex
• Epigamic (intersexual): between sexes

Question 6

What is intrasexual selection?

Answer 6

• Direct competition between two members of the same sex for the opposite sex
  
  • Most often seen as male-male competition for a female
  • Thus, female has limited to no choice in mate selection
• This type of selection is most common among species where the males generally do not help care for offspring

Question 7

What are common sexual dimorphism patterns among species that undergo intrasexual selection?

Answer 7

• Males are larger than females
• Males have sexual ornaments (antlers) to fight competing males
• Males tend to be similar colours to females

Question 8

What is epigamic (intersexual) selection?

Answer 8

• Selective pressure from the member of one sex having COMPLETE CHOICE over a mate of the opposite sex
  
  • Courtship rituals used to attract members of the opposite sex
• This type of selection is most common in species where individuals help raise the offspring together, also among individuals that mate for life

Question 9

What are common sexual dimorphism patterns among species that undergo epigamic selection?

Answer 9

• Where males do not stay to help raise the offspring:
  
  • Males are often much more colourful than females (extreme sexual ornamentation)
  • Females are often larger than males, or of a similar size
• Where males help raise offspring:
  
  • Both sexes are often the same colour and size

Question 10

Who typically chooses in epigamic selection?

Answer 10

• Females !
• Male choice does exists but it is much rarer

Question 11

How do males invest into propagating their genes?

Answer 11

• Sperm: low input & high output
• Reproductive success comes from mating with as many females as possible
• Many mates: male can afford to select a suboptimal mate

Question 12

How do females invest into propagating their genes?

Answer 12

• Eggs: high energy investment, fixed number
• Pregnancy: energy intensive
• Reproductive success comes from breeding with the BEST male - to ensure that offspring are as fit as possible
  
  • Can be very costly to choose a poor mate
    
    • THUS: females get to choose

Question 13

What is Bateman’s principle?

Answer 13

• The sex which invests the most in producing offspring will become a limiting resource over
  which the other sex competes
• **Males can be the sex that invest the most

Question 14

Do we ever see male competition in epigamic selection?

Answer 14

Yes ! However, any system where mate choice exists is considered primarily epigamic

Question 15

What is the Fischerian Runaway Selection Hypothesis?

Answer 15

1. Female prefer a secondary sexual characteristic
2. Female selects that mate, then passes that secondary character to her male offspring
3. She also passes on a genetic trait in her daughters to desire that secondary sexual characteristic
4. Creates a positive feedback, increases the desirability and extreme nature of the character over generations!

Question 16

What is the Sexy Son Hypothesis?

Answer 16

Females choose attractive mates because it means that their own male offspring will be better able to attract mates

Question 17

What is the Handicap Hypothesis? (think peacock)

Answer 17

• Intense secondary sexual characteristics indicate to the opposite sex that the individual can afford to waste resources on developing such an elaborate secondary characteristic
• The individual can “handicap” itself and still survive = very fit

Question 18

What is coevolution?

Answer 18

Where two (or more) species impact each other’s evolution

Question 19

What’s the difference between natural selection and coevolution?

Answer 19

• Natural selection: broad changes in phenotypes in response to environmental conditions
• Coevolution: Small number of traits/genes in one species are directly impacting the evolution of a second suite of traits/genes in another species

Question 20

What is specific (pairwise) evolution?

Answer 20

Two species reciprocally impacting the other’s traits or genes

Question 21

What is an evolutionary arms race?

Answer 21

• Symmetrical arms race
  
  • Selection pressures are identical on two species
  • Two species whose interactions result in them co-evolving the same trait
• Asymmetrical arms race
  
  • Contrasting selection pressures
  • Both species are co-evolving, however the selection pressure acting on the two species is different

Question 22

Example of a symmetrical arms race?

Answer 22

• Only the tallest trees receive full sunlight in a mature forest
• Different trees evolve to have the same trait: height
• A tree is only tall because the next tree threatens to overshadow it

Question 23

Example of an asymmetrical arms race?

Answer 23

• African cheetah can only run for a short distance at its top speed (120km/hr)
• Thomson’s gazelle’s (top speed = 97km/hr) is the preferred prey of African cheetahs
  
  • Cheetah evolved traits for hunting: speed & stealth
  • Gazelle evolved different traits for evading capture: endurance & evasion

Question 24

Define symbiosis

Answer 24

• A close and prolonged interaction between organisms of different species
• Can be beneficial to both partners, neutral to one partner, harmful to one partner

Question 25

Define Mutualism

Answer 25

- Interactions between two different species mutually benefit each other

Question 26

Define obligate mutualism

Answer 26

- Two species must co-operate in order for both to survive - The survival of one species depends on the survival of the other - EX: inside out flowers are ONLY pollinated by fig wasps

Question 27

Define facultative mutualism

Answer 27

- Both species derive benefit from each other, but one would survive if the other went extinct - EX: Tarantula protects frog from other predators and the frog protects tarantula eggs (as the frog eats ants, the primary predator of tarantula eggs) - But they can survive alone

Question 28

Define parasitism?

Answer 28

- Interaction between two different species where one benefits at the expense of the other - Parasitism relationships are generally very finely tuned to preserve the life of the host - Parasite needs the host to survive

Question 29

What are brood parasites?

Answer 29

- Cuckoo finch = obligate brood parasite - Does not raise its own offspring - Relies on birds of other species to raise its offspring by laying its own eggs in other birds' nests - When the cuckoo eggs hatch, the babies will push the other eggs out of the nest - Ensures that only the cuckoo offspring is raised - The host can sometimes identify the cuckoo egg before it hatches - Will push the cuckoo egg out of the nest - The host evolves a diversity of egg patterns to try to distinguish its eggs from the cuckoo