Sexual Selection Flashcards
What is the fundamental difference between males and females?
◦ Males and females are defined by the size of their gametes.
◦ Female – large, non motile
◦ Male – small, motile
Isogamy to anisogamy:
Survival of a zygote (two gametes fused) depends on its size -> Selects for larger gametes
Preponderance of larger gametes in population selects for production of more smaller gametes (although larger gametes would be selected to fuse with larger gametes):
◦ Large-small mating predominate – (small gametes are in excess)
◦ Little-little mating has higher cost than large-small
◦ Little selected to evolve traits that ensure mating with large gametes
Competition over mates:
- Sexual selection proposed by Darwin to explain male traits that didn’t fit natural selection
- There were two main types of traits:
◦ Male weaponry (large body size, antlers, horns)
◦ Exaggerated traits/ornaments (e.g. bright colours/long tails)
• Both types of male trait reduce survival:
◦ Weapons are expensive to produce and carry around
◦ Ornaments can attract predators
Two types of sexual selection:
Intrasexual selection:
• Competition between members of the same sex for access to mates
• Usually male-male competition
• This has resulted in evolution of large body size & weaponry
Intersexual selection:
• Choice of mate of opposite sex
• Usually females that choose
• This often explains evolution of exaggerated male traits
How did exaggerated male traits evolve?
Several mechanisms proposed to explain evolution of male ornaments and exaggerated traits
◦ Selection for ecological sex differences
◦ Unprofitable prey hypothesis
◦ Male-male competition
◦ Female mate choice
females don’t generally evolve ornaments or weapons…
E.g.
• Deer females do not have antlers for fighting
• Mandrill female doesn’t have colourful face
• Hercules beetle female doesn’t have horn for fighting
Why don’t females generally evolve ornaments or weapons?
Bateman’s Experiment measured reproductive success of Male and female Drosophila
◦ Male fitness increases with number of matings
◦ Female fitness does not.
Conclusions of baseman’s experiment:
- Males - usually higher potential reproductive success than females - egg production (large gametes) limits female reproductive rate
- Males often show greater variation in reproductive success than females (but contentious…)
- Traits that influence male reproductive success experience stronger selection
Sexual selection favours:
- Males that mate with many females
* Females that choose good quality males
Male-male competition:
The Intensity of male-male conflict varies between species - male-male conflict increases with…
- ratio of female:male parental investment
- More F investment more M-M conflict
• operational sex ratio (OSR)
◦ OSR is the ratio of males ready to mate in relation to females ready to mate – more male-biased OSR – more male conflict
In many species males assess the quality of rivals without fighting
e.g. male stalk-eyed flies compare eyespan
Courtship displays may serve to resolve male-male conflicts
e.g. Pacific Tree Frog, Hyla regilla - male calls attract females and repel other males (avoid conflict)
Age can influence outcome of competition
Older males more experienced and have less to lose
E.g. giraffes fight using their necks
In some species Females gain Direct benefits from mate choice:
- Avoidance of interspecific hybridisation
- Nuptial gifts
- Parental care
- Access to a territory
- Access to food
- Lower risk of predation or harassment
- Avoidance of inbreeding
But…in some species females only get genes from Male (no direct benefit)
Avoidance of interspecific hybridisation:
◦ E.g. In Central America several species of frogs breed in same ponds - Males make calls preferred by conspecific females
