week 3- evolution of exaggerated traits Flashcards
what are the diffferent types of selection
artifical selection (breeders equation)
natural selection: viability selection,fecundity selection
sexual selection
when does sexual selection occur
between adults and parents
what are the three forms of selection
disruptive selection, stabilizing selection and directional selection
what are examples of trait exaggeration
rhinoceros beetle horns, long-taied widowbird tail, elk antlers, stag beetle mandibles, fiddler crab chela
what are secondary sexual traits
advantageous when competing for mating or fertilisation opportunities
the primary sexual traits (body structures involved in reproduction)
affect fertilisaition success/ ability to mate irrespective of the presence of rivals,
economic and under natural selection
what is the definition of sexual selection
favours investment in traits that improve fertilisation, given limited access to the opposite sex gametes due to competition with members of the same sex
what are the consequences of anisogamy
Higher variance in reproductive success in males (resulting from male bias in gamete numbers)
what happens if anisogamy variance is non-random
sexual selection occurs
what further fuels competition for mates
sex bias in parental investment (PI), resukting in further shift in operational sex ration (ORS) from the adult sex ratio (ARS)
what is the direction of sexual selection
male-biased ORS means lower, average mating rates for males, but direction of sexual selection unclear
what is Batemans gradient
slope of the regression of offspring production on mating rate. less mates results in less offspring and too many mates results in less offspring
what does a steeper batemans gradient mean
stronger sexual selection on the competing sex
practicing batemans gradient: male parental care in phalaropes
-females are more brightly coloured than males
-the females pursue males, compete for nesting territory, and defend their mates
-after laying, females leave the males to incubate the eggs while she looks for another mate
-means that batemans gradient is steeper for females than for males
what are the benefits (fitness gain) and costs (time out) in case of ORS
ORS captures consequences of differential access to mates between sexes but does not specify fitness gain from mating.
neither BG or ORS specify trade-offs between getting a mate and other fitness-enhancing options
Trade-offs are apparent in mate choice
what is ORS
ratio of sexulaly competing males and females that both are ready to mate
what does batemans gradient not consider
how an individual increases mating success
when does mate choice occur
occurs when traits create mating biases that reduce set of potential mates
when might mating rate not be maximised
in choosy sex, hence trade-off between offspring production and mating rate
when does mate choice evolve
only if mating is costly (e.g. predation risk)
What does Bateman’s gradient suggest/inform about mate choice
suggests which sex can be choosy,
mating costs are not defined in batemans gradient ,
Batemans’ gradient does not inform on the evolution of mate choice
what are the two types of sexual selection
intraxexual selection and intersexual selection
what is intrasexual selection
direct competition for mates between members of the same sex, usually male-male competition
what is intersexual selection
differences in attractiveness to the opposite sex, usually non-random mate choice by females (mate choice)
example of male-male competition (red deer)
red deer (stags), male red deer with the greatest success in combat can retain females for longer periods- directinal selection on antler size and thus have higher reproductive success
stags have higher variance in reproductive success than hinds (batemans gradient)
example of male-male competition (hercules beetles)
engage in titanic jousting matches using their elaborate horns to displace rival males.
this competition has led to an exaggeration of body size and horn size
disruptive selection on horn size.
what can male-male competition explain
the evolution of many morphological and behavioural traits
what does mate choice occur in response to
direct selection on genetic variation in choice
Indirect selection that acts on genetically correlated traits
females and direct selection, reproduction
Females may benefit from increased nutrition or provisioning, or paternal care that increases their reproductive output or the quality of their offspring
explain the good genes hypothesis (indicator selection)
(indirect selection) a chosen mate might have genes that give male and female offspring fitness advantage
explain the handicap principle
(indirect selection) some males may have a heritage;e trait that reduces viability, only males with ‘good genes’ can survive on handicap, females that mate with these males will have offspring with higher fitness
explain fishers runaway process
(indirect selection)
-starts with a genetic correlation between a trait and its preference, and an initial advantage of the trait in attracting mates
-females that prefer males with the trait will produce sons that are more attractive and daughters will inherit the preference
-both choice genes and chosen male trait (genes) have positive selection
-this creates a positive feedback loop that drives the trait and the preference to extreme levels, even if the trait becomes costly or harmful
what is the lek paradox
lekking males face strong directional selection imposed by female choice on display traits
Therefore, we expect that selection erodes heritable variation in those traits, but that is not what is observed
what is the hamilton-zuk hypothesis- assumptions
-female choice on male traits
-male trait expression limited by parasite infection
-females choose males with exaggeratied traits to obtain resistence genes for their offspring
how is heritable variation in paraiste resistance maintained
due to co-evolution between parasites and hots that generates new genetic variation.
what is the Hamilton-zuk hypothesis-prediction
Females should mate with the most ornate males available; these males should carry lower parasite loads than less showy males
what is the immuno-competence handicap hypothesis
suggests that males with robust sexual signals (testosterone) are also more vulnerable to immune challenges due to immunosuppressive effects of T so increases parasite load
what is postcopulatory sexual selection
sexual selection favours investment in traits that improve fertilisation, given limited access to the opposite sex gametes due to competition with members of the same sex. This means that traits favoured by sperm competition are also sexually selected
what is intrasexual selection in postcopulatory sexual selection
sperm competition
competition between the sperm from two or more males for the fertilisation of a given set of ova
what is intersexual selection in postcopulatory sexual selection
cryptic female choice
non-random paternity biases resulting from female morphology, physiology or behaviour that occur after coupling (requires female remating)
what does net magnitude of sexual selection depend on
both precopulatory and postcopulatory processes (interactive effects)
use of weapons
weapons are a precopulatory investment (male-male competition)
but scramblers without weapons have higher reproductive output when their mates are starved (direct selection: nuptial gift)
explain how sexual selection leads to male adaptations (reduce remating oppurtunity of female)
-prolonged copulation following insemination
-post-insemination mating plugs and anti-aphrodisiacs
-prolonged contact after copulation
-guarding of the female by the male after copulation
explain how sexual selection leads to male adaptations (to eliminate/impair sperm of rival males)
-large sperm volume/frequent copulations
-induce females to eject rival sperm
-penile strutures that remove sperm
exaplin mating of black damselfly
-females mate with several males within a few hours
-females store sperm in spermatheca
-males use specially adapted penis to scrub away rival sperm
-very successful males remove 90-100% of competing sperm
how has evolution had consequence on testis size
increased ejaculation size in sperm competition
more sperm, larger testes
sperm in deer mice
sperm discriminate among relatives and cooperate with most closely related sperm
