Sexual selection Flashcards
Why do some males have elaborate traits that appear detrimental to survival (Darwin’s view)?
Darwin (1871) proposed two explanations:
-Traits useful in male-male combat.
-Traits preferred by females.
‘Secondary sex traits’
Darwin viewed sexual selection as a separate process from natural selection.
Why are females more picky according to Bateman 1948?
Female fitness is limited by fewer large gametes so females are choosy. However now it has been found that females do benefit from having more than one male
Why Batemans theory may be incorrect
Female solicitation and multiple mating (females keen to mate more)
Higher reproductive output for females that have multiple mates
Male sperm limitation (new info)
Monogamy
Polyandry (females choose to mate with more males)
Male mate choice
Sex role reversal
What is the parental investment theory Trivers (1972)?
The sex which exhibits less parental investment (not necessarily the male) will have to compete to mate with the opposite sex.
In species where both sexes invest heavily, they should be mutually choosy.
Parental investment is any investment made by the parent that benefits their current offspring at the expense of future offspring.
Cost of producing gametes, parental care, etc.
The evolution of mate choice?
Direct phenotypic benefits (attractiveness)
Sensory bias hypothesis
Fisherian runaway
Indicator traits- good genes, Handicap Principle, parasite resistance.
Genetic compatibility
What are the direct phenotypic benefits?
Choosy individuals receive direct benefits from their mates.
Food, breeding territory, parental care, etc.
Male damselfish defend permanent territories.
Females choose mates on the basis of their territory and e.g. size
Female choice influences egg survival
Give an example of phenotypic choice and what the female gains
Males produce spermatophores ~11% of their body weight
which can contain sperm, nutrients, and pyrrolizidine alkaloids
Female gains:
Produce 32 more eggs for each additional mating
Alkaloids incorporated into eggs protect them from predators
But females are choosy even when they gain nothing but gametes: indirect or genetic benefits
Sensory bias (hardwired to prefer a certain trait)
Female mating preferences may be by-products of selection on sensory systems. Males evolve traits that “exploit” these biases.
e.g. Xiphophorus genus: swordtails and platyfish
Swordlessness is the primitive state
Female platyfish prefer conspecific males with artificial swords
A pre-existing bias for swords?
What is Fisherian Runaway?
Fisher (1930): male ornamentation is the result of female preference for males with the most exaggerated ornaments
Initial female preference is random
Strong female choice for the male ornament results in runaway sexual selection, leading to the further exaggeration of the trait.
Preference for the trait (in females) becomes genetically linked to expression of the trait (in males)
Continues until the costs imposed by natural selection balance the benefits of sexual selection.
Runaway selection drives traits past their natural-section optimum
What is the Sexy son hypothesis
Closely related to Fisherian runaway selection
Assumes an indirect benefit to female choice due to the attractiveness of their sons
Females that mate with an attractive male will produce attractive sons
Their fitness will be increased as a result of their sons’ higher mating success
Indicator traits – handicaps (Zahavi 1975)
The handicap principle: reliable signals must be costly to the signaller
Only high quality individuals can afford the cost of the signal.
If signals are cheap, all individuals can display them, so they provide no information to the receiver
Describe the Hamilton-Zuk handicap Hypothesis (1982)
A special case of the handicap principle
Sexual ornaments are indicators specifically of parasite and disease resistance
e.g. intestinal nematodes adversely affect male secondary sexual characters. more parasites more dull
Runaway Selection vs Handicap Principle
Runaway selection:
genes for attracting females only
signal and preference for signal become linked
no positive relationship between signal and genetic quality
signal negatively or uncorrelated with condition
Handicap principle:
genes for survival/reproduction, not just attractiveness
not necessary for signal and preference to be linked
signal positively associated with genetic quality
signal may be positively associated with condition
What can choosiness lead to?
Sexual dimorphism (sexes exhibit different morphological characteristics)
What is a lek
Is lek a bird?
*Leks occur when males unable to defend females or resources
A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking
*A lek is not a formal term, but it generally means a gathering of male birds that strut, dance and/or sing to show off for females.
