Lecture 10 - sexual selection Flashcards
What is a rule for evolution?
- selection acts on replicators
- these are individual units that replicate themselves
- those that leave most copies are the most successful
(species & individuals aren’t replicators)
What are the ingredients for natural selection & competition?
- ecological competition is the consequence of limited amounts of resources
- resources can include food, water, shelter, space or mates
- there is a direct relationship between amount of resource captured & fitness
What is sexual selection?
- intrasexual selection
- intersexual selection
What is fecundity?
the amount of offspring
What is fertility?
the amount of gametes
What does sexual selection lead to?
different adaption
What is sexual dimorphism?
- male & female
- they have different adaptions - e.g. male has claws to fight off predators
- sexual selection produces differences between the sexes
What are mating systems?
competitions for mates had led to diversity of outcomes in terms of mating systems
What is monogamy?
1 male & 1 female - one partner, little sexual selection - no sexual dimorphism
What is polygamy?
male & females have multiple partners
What is polyandry?
females has multiple partners
What is polygyny?
male has multiple partners
What is the extra-pair paternity?
- discovered in 1990s with advent of genetic fingerprinting
- surprising high rates in supposedly monogamous species
How is Dunnock an example of extra-pair paternity?
- very variable mating systems
- frequently polyandrous, despite appearing monogamous
- however females frequently mate with unpaired males (sneak)
- mates 100 times a day
- important consequences
Describe extra pair paternity statistics
- chaffinch 17%
- blue tit 10-15%
- dunnock 0-36%
- tree swallow 38-76%
- superb fairy wren 75-85%
What are the characteristics of polygamy, polyandry & polygyny?
- multiple partners optimal
- sexual selection
- sexual dimorphism
What is intrasexual competition?
- in polygamous mating systems, there are a few winners and lot of losers –> intense competition for mates
- competition between members of the same sex is called intrasexual selection
- thus had led to the evolution of several behaviours as a consequence
What are examples of behaviours carried out due to intrasexual competiton?
- male-male competition - fighting
- mate guarding - pre-copulatory & post-copulatory
- sperm competition - direct competition between sperm (led to adaptions in sperm - e.g. being motile)
What is ‘lek competition’?
- a gathering of males that compete with each other
- they display with others and females observe the lek
- the females choose the ‘best males’
- lemming is costly for males
- may be less costly for females
- example (red grouse)
What is intersexual selection?
- females choose mates
- in lek species, females choose one male
- other species, males display to females
- typically in polygynous mating systems (females mate with one male)
- but reverse can happen - polyandrous mating system
Females choose males based on their characteristics
What do females gain from intersexual selection?
- Access to resources
- Access to good genes - utilitarian
- Access to good genes - attractiveness
What access to resources do females gain?
e.g. courtship feeding
What access to good genes (utilitarian) do females gain?
- females choose genes that confer high survival to offspring - resistance to parasites
What access to good genes (attractiveness) do females gain?
- females choose attractiveness genes that make male offspring more attractive to females
What is runaway sexual selection?
Anderson (1982):
- experimentally altered tail length of males
- reduced tail = lower mating success
- increased tail = more mating success
How are peacocks an example of runaway sexual selection?
female preference for long tail leads to selection for long tailed males - this loops
Are natural selection & sexual selection operating in the same direction?
not always
How is Endlers an example of sexual vs natural selection?
- Endler: course vs fine gravel selection
- if predator present - natural selection (favours fish that fit into background)
- if predator isn’t present - sexual selection (favours fish that look different from their background)
What is comparative evidence?
look at patterns across multiple species to see whether traits vary as we would predict
What is the prediction regarding sexual vs natural selection?
- in species that experience stronger natural, sexual selection should be weaker.
- strength of sexual selection is correlate with sexual dimorphism, i.e. males are more different from females when sexual selection is stronger
- e.g. predation pressure - species in habitats that are prone to predation should have less dimorphism
What predation risk is associated with off-ground nesting?
less predation risk
What predation risk is associated with ground-nesting?
more predation risk
What is runaway sexual selection?
- exaggerated traits can evolve through sexual selection
- however, as selection proceeds, genetic variation becomes used up
- in the extreme, in a strongly selected population, variations between individuals are not due to genetics but random (additive genetic variation is low)
- although this is expected, the reverse is actually observed
- sexually selected traits have high variability & additive variance
- The Lek Paradox
