Sexual Selection Flashcards
w6
compare and contrast natural and sexual selection
- natural selection = traits evolve that enhance survival & reproduction
- sexual selection = costly traits evolve that enhance mating success
- **sexually selected traits can be detrimental to survival**, so they cannot be explained by natural selection
describe 2 main pathways to sexual selection
(intrasexual competition vs. intersexual selection - mate choice)
- intrasexual competition = male competition
-> mechanisms: physical combat, displays of strength / dominance, or contests for territory / resources
=> evolution of traits that enhance competitive abilities
- intersexual selection = female choice
-> mechanisms: physical appearance, courtship displays, behavioral traits, or possession of resources
=> evolution of traits that are attractive to potential mates
contrast variety of Pre-copulation & post-copulatory traits selected for male-male competition
sexual selection
evolutionary process where selection acts on heritable traits that enhance reproductive success either through sex competition or mate choice
costs of sexual selection
- predation
- energetically expensive
- cumbersome
- time spent displaying
Why do males tend to be under stronger sexual selection for competition than females?
Bateman’s principle
- females = few, large, expensive gametes -> high parental investment
- males = many, small, cheap, gametes -> often little / no parental investment
- females are limiting factor to male production due to lower reproduction rates
- males compete, females choose to maximise their investment
Bateman’s principle
the sex investing most in offspring production drives sexual selection for competitiveness in the opposite sex
Resource holding potential (RHP)
- refers to individs ability to successfully acquire & defend valuable resources, eg. territories, food, or mates, against competitors
- in context of pre-copulation fighting, RHP plays crucial role in determining outcome of aggressive interactions between individuals competing for access to mates
- good RHP characteristics:
-> large body size
->effective weapons
->aggression/effort
-> coalitions
RHP characteristics and examples in species
-
successfully defending territories
-> body size determines who holds highest traffic territories (marine iguana)
-> aggression predicts past success holding territories (song sparrow) -
guard females/harems
-> lifetime reproductive success & antler size in red deer
-> oldest/heaviest group members mate with highest fecundity females in group -
defend resources females are attracted to
-> larger males win exclusive access to shrubs where female Ligurotettix lay eggs
some females fight!
why?
- often in cooperative groups
- over carers for their offspring
pre-copulation: scramble
aquatic mating in Weddell seals…
- females are dispersed
- RHP has no benefit, experience in past seasons looking for females does!
- limited sexual dimorphism
if access to females is not defendable, RHP is not sexually selected
pre-copulation: alternative, sneaky strategies
- in systems where access to females is defendable, low RHP males sometimes employ sneaky strategies
- eg. small sneaker males in Round headed goby
- eg. femae mimicry in cuttlefish
Pre-copulatory sexual selection
overview
- RHP is important for protecting access to females if that access is defendable
- to have a chance, low RHP males may be selected to use sneaky strategies
Why do males produce so many sperm cells?
(post-copulation)
intrasexual selection:
-> RHP: males compete for access to mates & those with higher RHP (eg. larger body size / ↑ physical strength), have adv in pre-copulation fighting
-> producing large no. of sperm allows males to ↑ their chances of fertilizing eggs during mating encounters, maximizing reproductive success even if they don’t win every competition for mates
in summary, production of numerous sperm cells by males is adaptive strategy shaped by intrasexual & intersexual selection pressures.
it allows males to compete more effectively for mates, maximize reproductive success in competitive mating environments, & increase likelihood of fertilizing eggs during mating encounters
Post-copulatory interference: barriers
sperm plugs as insurance policy against rival copulations by…
- obstructing access to female’s reproductive tract & reducing likelihood of fertilization by rival males’ sperm
- largely absent in monogamous mating systems
- male ejaculates coagulation plug correlates with promiscuity e.g in primates
how does male sperm plug correlate with promiscuity?
- in promiscuous mating systems, females mate with multiple males => intense sperm competition where sperm from diff males compete to fertilize female’s eggs
- males face challenge of outcompeting rival males’ sperm to ensure their genes are passed on to next gen…
- so males in some species ejaculate coagulation, which forms plug in female’s reproductive tract after mating
- coagulation serves as physical barrier, obstructing subsequent mating by other males and reducing chances of their sperm reaching female’s eggs
Post-copulatory interference: flushing
- Grevy’s zebra have disproportionate large testes compared to body size to produce large quantities of ejaculate
- while guarding female, copulate hourly to flush previous sperm out of reproductive tract
flushing: females engage in urination / defecation immediately after mating
Post copulatory sexual selection
overview
- sperm have traits selected to win race to egg
- other male traits are selected to interfere with race, & tilt outcome in their favour
- sexual selection can drive rapid evolution & diversity in traits, often even more so than natural selection
cost of competition
- pre-copulation: injuries / death / energy
-> eg. Fidler crabs can lose claws - post-copulatory: germline DNA damage
sexually selected traits can be extremely costly,
BUT as long as fitness benefits outweigh the costs…
they will be selected
Explain the selection pressures that drive male sexual signalling
Describe main benefits of female choosiness and how they can identify informative signals of male quality
Explain how trait variation is maintained despite “Lek paradox”
female cryptic choice
represents female-driven mechanisms at / after mating that bias sperm use & impact male paternity share
- selective ejection
- spermicide
- selective transport/storage
eg. Red jungle fowl cloacal contractions eject sperm of less desirable partners
eg. separate storage receptacles in flower beetle
who (females) stores sperm longest?
snake
bat
shark
chicken
snake = 7 years
bat = 6 months
shark = 2 months
chicken = 2 weeks
If females have so much control, how do males prove themselves?
SIGNALLING
what is signalling?
- signals are traits that have evolved to modify another’s behaviour in way that benefits signaller
- for a signalling system to be evolutionarily stable, on av. both signaller & receiver should benefit
costs of signalling
- conspicuous to predators
- time spent signalling rather than feeding
- growth of structure
- inhibits locomotion
sexy sons hypothesis
- females choose males with heritable attractive traits
=> maximising their fitness through reproductive success of their ‘sexy sons’ - attractive traits can be arbitrary features that are not necessarily signals of quality
-
runaway selection: a trait in 1 sex becomes genetically linked to preference for that trait in other sex
=> coevolutionary feedback loop between trait & trait preference
deceptive signalling
- most of time signals are honest % benefit both signaller and receiver
- however, signallers may benefit by deceiving receivers
- as result of behaviour of signaller, receiver registers a certain situation that is not in reality occurring…
- as result of the interaction, signaller benefits (while receiver pays a cost)
when can deceit be stable?
(in deceptive signalling)
- when cost of ignoring an honest signal is high
- when cost of being deceived is low
- BUT most signals are actually honest -> lies are expensive when they are found out!
Deceitful sexual signalling is hugely costly for females…
-
low quality males produce
dishonest signal of quality - females deceived in mate choice
- females invest in low quality offspring
good genes hypothesis
- extravagant ornaments/displays are not arbitrary traits
- females choose males based on extravagant/costly traits because they are honest signals of genetic quality/viability
what makes a signal of male quality informative for females?
indices (can’t be faked)
- dishonesty not possible
- eg. male bird with bright plumage: index of genetic quality as brightness of feathers reflects ability to acquire sufficient nutrients, correlates with overall genetic fitness
handicaps
- costly for low quality individs to fake
- either through production of traits / social costs
- costly/conspicuous/cumbersome ornaments or weapons
- dishonesty possible but unprofitable
- eg. peacock tail is handicap as energetically costly to grow & maintain. Male with large, colorful tail shows he has sufficient resources & genetic fitness to overcome this cost, making him an attractive mate
another index eg.
- red deer stags display fighting ability to opponents by roaring
- to roar well, stag must be in good condition
- body size is causally related to resonant frequency of roars
- signals of fighting ability are indices: they can’t be faked
- stags that roar more = more success in fighting
- fighting is costly: roaring signall benefits signaller & receiver
carrying on with Red deer stags
(LRF)
- larger stags produce lower resonant frequencies (LRF)
- females use LRF as signal of body size
- females prefer larger males
- stags with LRF have greater reproductive success
the handicap principle
signals show strong condition dependence:
- high quality individuals can afford to produce costly signals
- low quality individuals could produce signal but cost would outweigh the benefit
Female choice and the Lek Paradox
- fixation of male traits is fundamental problem
- female preference for traits = reduced genetic variation
- without trait variation: no longer benefit to females being choosy
- ornaments/displays & mate choice are costly
- if cost of mate choice outweighs benefit; female choosiness will not be maintained
How is genetic trait variation & female choosiness maintained?
many aspects of trait variation:
- sneaky/satellite mating
- parasites/disease -> parasites attracted to Hawaiian field crickets “singing” -> silent “satellite” males take advantage
- environment
- Red Queen hypothesis
what would prevent the Lek paradox?
sources of variation in male signalling
eg. changing predation pressure
changing predation pressure eg.
- more dangerous predators of guppies downstream
- male sexual signalling is less conspicuous downstream
