Mating effort and sexual selection, Lecture 10 & 11 Flashcards
Natural selection
Causes the evolution of traits which enhance chances of survival.
Why have some traits which seem to reduce survival evolved?
Darwin’s theory about this is sexual selection
Sexual selection
Causes the evolution of traits which gives the owners a mating advantage over other of the same sex
Secondary sexual characteristics
Traits which differ between the sexes
Examples of secondary sexual characteristics
3
size dimorphism,
elaborate displays and colours
weapons - antlers, horns, etc.
What aren’t secondary sexual characteristics connected with?
gamete production or transfer
Why is it usually the males that compete?
2 reasons
- gamete size
2. potential rate of reproduction
Gamete size males
sperm,
very small,
large numbers ‘cheap’
Gamete size females
eggs,
large,
small numbers ‘expensive’
Investment per gamete in females is …
relatively high
Potential rate of reproduction
higher in males than females
Bateman’s experiment
drosophila, mating and success
What limits mating in males
according to Bateman’s experiments
access to females
What limits mating in females
according to Bateman’s experiments
egg production
Ardent males and …
choosy females
… and choosy females
ardent males
The sex with the greater potential rate of reproduction competes whilst…
the other sex is choosy
The advantages of being choosy
- investment in eggs is relatively high
- breeding opportunities relatively rare
- needs to get:
- right species
- best quality male available
Two types of sexual selection
intra and inter
Intra-sexual selection
Competition for mates between individuals of same sex
Inter-sexual selection
Mate choice
Examples of use of secondary sexual characteristics in intra-sexual selection
weapons
size
signals
Examples of secondary sexual characteristics in male ungulates
antlers and horns in males
none in females
Types of ungulates
gazelle, moose, deer (red), ram, antelope,
Were the evolution of antlers and horns in ungulates due to sexual selection or natural selection
sexual selection,
- not always used for defense
- many shed when predation risk is highest: in winter
Example of intra-sexual selection: size
competitions often won by the largest male,
selects for larger than average size male,
which can lead to extremes of size dimorphism
Example of intra-sexual selection: size dimorphisms in…
elephant seals
Intra-sexual selection, rituals and displays
neither benefit from fighting,
‘sizing up’ rituals beneficial to both parties
Example of Intra-sexual selection
rituals and displays
Red deer, display increases in intensity, 1/roaring 2/ parallel walking 3/ fighting
Rituals can lead to extravagant body parts
2 examples
- peacock tails
2. stalk-eyed flies
Three aspects of inter-sexual selection
- getting the right species
- direct benefits
- indirect benefits
Getting the right species
inter-sexual selection
hybrids often less fertile or less fit
Direct benefits (inter-sexual selection)
males may come with resources:
- food
- nests or laying sites
- parental care
- protection from harassment
Indirect benefits (inter-sexual selection)
males provide genetic benefits:
- parasite resistance
- longevity
- ‘good genes’ - inherited by the offspring and increase their viability
When is female preference greatest?
inter-sexual selection
when cost of error is greatest
Overlap or hybrid zone
Areas where two species overlap and may produce hybrid offspring
Are females from the hybrid zone more or less likely to go to the correct male?
more
Are females basing their choice on secondary sexual characteristics?
example
Long-tailed widow bird,
males have very long tails but females don’t - suggests a role in mate choice
experiment: trapped males, changed tail length,
females preferred longer tailed so yes to question
Sedge warbler song
females don’t sing,
males have a complex song which is made up of elements, more elements = larger repertoire,
males with larger repertoires pair early,
this could be related to other factors such as age,
song complexity may signal useful traits such as longevity
Evolution of elaborate traits
2 main theories
- Fisher’s runaway theory
2. good genes or handicap principle - Zahavi
Fisher’s runaway theory about elaborate traits
females prefer for a trait becomes genetically linked with a trait
Fisher’s runaway theory about elaborate traits
how can a preference arrive?
arbitrary - no cost or benefit
Fisher’s runaway theory about elaborate traits
how does a preference become genetically linked to a trait?
natural selection to start with
Fisher’s runaway theory about elaborate traits
how does genetic linking of a trait lead to runaway selection?
sensory bias,
eg: loud calls or obvious mating signals perceived first
The handicap principle about elaborate traits
eg: with tails
long tails are expensive handicaps,
females prefer them because they signal an ability to survive despite the handicap,
may be a signal male viability (longevity, parasite load, etc.)
