Sexual Selection #2 Flashcards
(40 cards)
Why do some sexes (usually males) have elaborate traits that appear detrimental to survival
Darwin (1871) proposed two explanations:
1. Traits are useful in male-male combat
2. Traits preferred by females ‘secondary sexually selected traits’
What does this mean: “female fitness is limited by fewer large gametes so females are choosy”
Females produce fewer larger gametes and males produce many small gametes. Therefore each egg is valuable and females must maximize the success of each one that means being choosy about who fertilizes it.
Typically preferring males with:
- good genes
- strong displays or traits
- ability to provide resources or protection
What is the parental investment theory
Any time, energy or resources a parent give to an offspring that increases the offsprings survival but reduces the parents ability to invest in other offspring or future offspring
Who chooses the mate
- The sex which exhibits less parental investment (not necessarily the male) will have to compete to make the opposite sex.
- In species where both sexes invest heavily they should be mutually choosy
How does choosiness by one mate lead to sexual dimorphism
By sexual selection
How does sexual selection determine sexual dimorphism
When one sex (usually females) is choosy, it creates pressure on the other sex (usually males) to evolve traits that increase their chances of being selected. Overtime this leads to sexual dimorphism- a clear physical or behavioural difference between males and females
Intrasexual selection
Members of the same sex (usually males) compete with each other for access to mates
Some males fight for exclusive mating access to a female group
Intersexual selection
Type of sexual selection where one sex (usually females) chooses mates based on specific (attractive/advantageous) traits in the opposite sex
Consequences of intrasexual selection
- Evolution of weapons and body sizes= males evolve larger bodies, antlers, horns and tusks to fight
- Increased aggressive or dominant behaviours= more Territorial and leads to hierarchies where dominant males get more access to females
- Sperm competition= females mate with multiple males:
- more sperm production
- faster or more competitive sperm
- mating plugs or mate guarding - Sexual dimorphism- males and females evolve different appearances because males are under pressure to outcompete eachother
How does intrasexual selection lead to alternative male strategies like sneaker males
- smaller or weaker males cant win fights they use other strategies like sneaking in to mate while stronger males are distracted.
- “Sneaker males” avoid fights but still pass on their genes.
- Over time different successful tactics evolve like pretending to be a female or waiting to steal chances
What is sperm competition
Form of post copulatory competition where the males reproductive success depends on how well his sperm outcompetes the sperm rivals inside the females body
What is intersexual selection based on
- mate choice
- selection of mate is dependent on attractiveness
- in most cases females choose among competing males
What is the direct phenotypic benefit in intersexual selection and provide an example
Choosy individuals receive direct benefits from their mates= food, breeding territory and parental care.
Damselfish:
- males build and guard nests while females lay their eggs
- choosy females select males with the best nests and they benefit as eggs more likely to survive and the male guards and fans the eggs
How many spermatophores do tiger moths produce in comparison to their body weight
Males produced spermatophores equivalent to 11% of their body weight
What does tiger moths spermatophores contain
Sperm, nutrients and pyrrolizidine
What do female tiger moths gain from their male companions (2)
- produce 32 more eggs for each additional mating due to a nutritional gift
- alkaloids incorporated into eggs protect from predators
What are indirect or genetic benefits
Females choose mates based on traits that signal genetic quality even when those traits are costly for males to maintain
What traits can signal genetic quality and how can it be shown
- disease resistance
- high survival ability
- good overall genetic health
Examples include;
Bright plumage in birds
Elaborate displays or calls in frogs or insects
What are costly traits in males
Bright feathers, long tails or loud calls
What do costly traits show
That they are healthy strong males as only they can afford them
What is the fisherian runaway selection
- Some females happen to prefer a certain trait (longer tails)
- Males with that trait get more mates
- Trait spreads
- At the same time females with that preference also spread
Why do females benefit indirectly from choosing attractive males
- their offspring inherit genes that make them more likely to survive or attract mates
- this increases the females genetic contribution to future generations
What is sensory bias
Is the idea that females preferences for certain male traits evolve because of pre existing biases in their sensory systems. Males evolve traits that exploit these biases
Ie, some fish species, females prefer red coloured food. Over time males with red colouration (like fins) are more likely to be notices and so they evolve red fins exploiting sensory bias
What is an example of a sensory bias in fish using swordtails and platyfish
Female platyfish prefer males with long tails, even though their species never had them. When given males with fake swords they showed more interest. This shows a pre existing preference (sensory bias) that likely helped the swordtail trait evolve related species
