L8

Question 1

Darwin’s Natural selection

Answer 1

• The fittest individuals will survive and reproduce
  • Succeeding generations will become adapted to their environment

Question 2

Darwin’s problem

Answer 2

• Natural selection can’t explain costly traits that lower an individual’s survival
  • Peacocks tail increases predation chance, is costly to produce, carry and fly with
  • Must lower peacocks chances of survival
  • Argued if males were willing to suffer increase cost of traits they must be getting something important in return
  • Eg increased mating with females

Question 3

Sexual selection

Answer 3

• Males compete with each other by fighting or displaying
  
  • Intrasexual selection
  • Female choice
  • Intersexual selection
  • Choice, by one sex, of mates of the other sex
  • In Victorian times female choice was looked upon with suspicion as it suggested the course of evolution could be determined by females
  • Mechanisms of choice were therefore poorly understood

Question 4

Secondary sexual characters

Answer 4

• Not characters for reproduction eg genitalia, these evolved by natural selection
  • These involve size, behaviour, colours etc
  • Only males have these traits, sexual dimorphism

Question 5

What is the basis for sexual selection

Answer 5

• Differences in gamete size
  • Anisogamy is a difference in gamete size across genders, sexual selection acts differently eg male vs female sperm production

Question 6

Experimental study

Answer 6

• 5 males and females into pots and identified who produced what offspring

How did reproductive success vary with amount of copulation?

Females

- Offspring produced didn’t alter significantly after first copulation

- One copulation fertilised all offspring

Males

- Reproductive success increased almost linearly with increasing copulations

- More copulation = more offspring

- Male have higher reproductive potential than females

Question 7

Females are limited by?

Answer 7

The number of eggs they can produce

Question 8

How does sexual dimorphism occur?

Answer 8

Males compete and females choose

Males aim for quantity and females aim for quality

Question 9

Sexual Dimorphism

Answer 9

• No selection pressure on females to evolve ornaments
  • May be selection to keep them plain to protect offspring when rearing young

Question 10

How does sexual dimorphism arise?

Answer 10

Intrasexual competition

Question 11

Intrasexual competition in seals

Answer 11

• Males compete for harems of females by roaring matches proceeding to fighting
  • Winners become beachmasters
  • Other males get few or no mating’s

Question 12

Seals study

Answer 12

• Higher the dominance rank the more copulations
  • DNA analysis shows more mating’s = more offspring produced
  • Selection pressure to grow large as biggest males copulate more
  • If males cannot monopolise many females dimorphism will not occur
  • Doesn’t occur in water as it is harder to monopolise females
  • Some males stay small and sneak copulations from females In the sea

Question 13

Most important criterion to male success

Answer 13

Weight/ size

- Large males often displace smaller males when they are attempting to mate with a female

- Small males sometimes manage to hold on 

- In cases where the initial male is large they can hold on when smaller males try and copulate

- Small males never displaced large males

- Large size exerts a huge effect on reproductive success

- The relationship between success and size is more positive more males than females

- This leads to size sexual dimorphism

- We would expect in species with more competition there would be more dimorphism

Question 14

Body size and Harem size in Pinnipeds

Answer 14

• As sexual dimorphism increases so does harem size
  • Sexes are of similar size in species where males are monogamous
  • This is because there is little male-male competition
  • Where there is more competition there is more size dimorphism

Question 15

Body size does not always determine success

Answer 15

• Association between polygyny and degree of sexual dimorphism in structures eg size etc
  • Antlers longer in polygynous species than monogamous ones
  • Monogamous species have smaller bodies, but proportionally have smaller antlers even for body size

Question 16

Sexual dimorphism facts

Answer 16

1. Sexual selection doesn’t have to lead to sexual dimorphism
   2. Sexual dimorphism can arise from processes other than sexual selection
   • Eg bill shape for same foods in different niches
   • Reduced feeding competition between partners

Question 17

Female choice

Answer 17

• Often difficult to distinguish male- male competition and female choice
  • Requires experiments controlling for differences in competitive ability between males
  • Females must be given choice to reject males
  • Acceptance of a partner is not proof of choice

Question 18

Do female widowbirds prefer long-tailed males?

Answer 18

• Male lekking
  • Females choose a mate
  • Were long tailed males better at competing or choosing longer tailed males
  • Long tailed males received more mates than short and control
  • Males were same level of competition , just a change in physical characteristics
  • Both competition occurrence and choice can occur together

Question 19

Competition and choice depends on relative investment in offspring production

Answer 19

• Sex investing least competes
  • Sex investing most chooses
  • In monogamous birds both sexes invest

Question 20

Crested auklets

Answer 20

• Both sexes invest in offspring
  
  • Both sexes have crests which is only used in breeding season
  • We’d predict mutual sexual selection
  • Compared crest length using models of auklets from dead birds
  • Compared attractiveness between long and short crests

Question 21

Choice in crested auklets

Answer 21

• Both males and females chose longer crests birds to mate with than shorter crest birds

Male-male interactions

- 86% won by long crested birds

Female-female interactions

- 83% won by long crested birds

Question 22

Sex role reversal

Answer 22

Observation: in some species males invest more in reproduction than females

Prediction: there will be sex role reversal in species in which males invest a lot in offspring

- Females will compete and males will be choosy

Question 23

Pipefish

Answer 23

• Female provides male with a clutch of eggs that he stores and fertilises.
  • He then carries them and provides nutrients till they are birthed
  • Females produce more eggs to then produce more egg pouches for males
  • Males are the limited sex as they have a set pouch space
  • This means operational sex ratio is female biased, more females looking for males
  • Males choose large females that can produce more eggs

Question 24

Nutritious spermatophores

Answer 24

• Male transfers nutrients along with his sperm
  • These are spermatophores
  • Other species males provide nuptial gift while male prepares to mate
  • Providing gifts and spermatophores is costly, so male crickets don’t to give away large investment to any female
  • Choosiness of male is linked to resource availability
  • When resources are scarce, males should be more choosy

Question 25

Australian Katydid

Answer 25

- Food supply varies greatly through the breeding season - When food is limited spermatophores are costly to produce so are very valuable - Males then become choosy and females compete - When food is abundant males are rarely choosy and females don’t compete - When food is abundant males compete more - Environmental effect on sexual selection and behaviours of males and females

Question 26

Sex role reversal: Mormon crickets

Answer 26

- Large flightless - Males transfer a large nuptial gift as an edible spermatophore when they mate - Males can only mate once in their lifetime - Females can produce several clutches - Females can mate more often, so operational sex ratio is female biased - More females looking for males so females have to compete - High density groups form - Males call to attract females and females compete - Male then chooses - Females accepted by male are heavier and produce more eggs 48 vs 30 for rejected females - Being choosy is therefore beneficial for the male

Question 27

Sexual selection summary

Answer 27

- Any pattern of choice and competition can occur eg male female mutual etc - Each sex can play both roles of competing and choosing - The environment can influence the pattern of sexual selection