Tuesday, 4th September - Sexual Selection Flashcards
Natural selection is about ______ ________.
Sexual selection is about ________ _______.
Sexual reproduction has costs as well as ______.
Personal Survival
Reproductive success
Benefits
What is ‘Anisogamy’ ?
What are the 2 main forms of sexual selection?
- (Gametes of different sizes) means males usually invest less per offspring than females – affects male vs female behaviour.
- male combat & female choice.
Note
They affect behaviour and morphology differently.
- Understanding reproductive behaviour depends on taking the gene’s eye view of the process. It is about differential transmission of heritable characters – NOT the preservation of the species
Costs of sexual reproduction
1. Cost of meiosis?
2. Cost of finding a mate?
1.
Production of gametes by meiosis breaks up parental genome
- Splits gene combinations that were successful in parents as individuals. - Dilutes personal contribution to next generation.
(Remember, meiosis just mixes up the gene pool; genes are not lost by being split up, only by not being passed on)
2.
- Searching time, investment in courtship. Time is money!
- Risk of injury from close association.
Costs of sexual reproduction
1. Cost of producing “useless” males?
‒ “cost” in this context is not in energy, but in lost opportunities
- In polygamous sp, few males are needed to fertilise a group.
(Usually one extremely successful male who will mate with mulitple females which is good for passing his genetic material on for future generations. If you produce a scrawny male it is essentailly a genetic dead end as he will likely not be a successful mate in cimparrison to big strong animals)
Benefits of sexual reproduction
Explain these terms:
- ‘recombination’ ?
- ‘variable environments’ ?
- ‘stable environments’ ?
- Recombination increases chances of finding right combination of genes to favour offspring – like a raffle
- In variable environments, what worked this year may not again
- In stable environments, continuous longterm biotic adaptation still needed
- Red Queen hypothesis
- important in evolution - defense against predators and parasites
Benefits of sexual reproduction
1. What is ‘Muller’s ratchet’?
2. Asexual lineages have _______ variation?
1.
(accumulation of bad genes)
• If the best genes are lost from a population, they can be recovered from immigrants by recombination in sexual populations.
2.
• Asexual lineages have minimal variation = less for natural selection to work on.
Recombination has allowed massive increase in biodiversity over evolutionary time. This is a consequence and not an advantage to a specific species but it is important for shaping ecosystems as a more diverse array of organisms can exploit more environmental neiches.
• As a consequence (not necessarily an advantage) of sex.
Define these terms:
Isogamy and anisogamy
Isogamy = gametes of equal size
- Both sexes similar
- Only in bacteria, fungi & algae
Anisogamy = gametes of unequal size
– Two sexes different
- Females produce a few large, sessile eggs
- Males produce many small, motile sperm
– All higher animals and plants
- This is fundamental reason for many differences in sexual, parental and social behaviour
Consequences of Anisogamy?
(Female vs male)
Females invest more per gamete than males
– especially if also providing large-yolked eggs or parental care
– but applies even if neither parent cares for young
So most male courtship aimed at exploiting female investment
Consequences of Anisogamy
Lifetime reproductive success (LRS) of each sex determined differently.
Females limited by _______ __ _____
• LRS for female = _______ _______
– Males limited by ______ __ _______ unless monogamous (rare)
Females limited by production of young
Fertility - Losses
Access to females
Consequences of anisogamy
Variance in lifetime reproductive success of each sex different
Most females breed
– so variance low; can afford to be selective
Not all males breed
– so variance high; increases effect of sexual selection
– consequences explain patterns of courtship and parental care
- If variance (chances of failure) is high, pressure of sexual selection increases.
• Many more male than female elephant seals fail to breed, so effects of sexual selection on males much stronger.
Male elephant seals can breed only when at prime age and size: 8-12 years, but >50% adult females breed from 2 years (Andersson 1994:118)
- Sexual selection favours adaptations which increase breeding success (fitness) - even if the effect is to decrease individual survival.
- Sexual selection may be opposed by natural selection!
- But limited by natural selection in long term.
– E.g. antlers, long feathers, and loud noises can become too dangerous. – Hence mating costs can be very high even though sperm is cheap.
Male túngara frogs produce airborne vibrations to attract females
(= adaptation)
– Noisier males win mates. –
***They also incidentally produce waterborne vibrations that predatory bats can detect***
• Only males with best genes for loud singing achieve breeding success
(= fitness)
BUT
they suffer greater mortality from predation. • Therefore, there is a limit to how loud males can chirp to increase their reproductive success.
Two mains forms of sexual selection?
Process of selecting characteristics that favour access to mates takes two main forms:
• Intrasexual selection: males fight each other, females stuck with winner
• Intersexual selection: males display, females choose
Many forms of competition: before, during and after mating – different consequences for morphology and behavior.
- Male-male competition before mating, winner takes all (deer, seals, goats).
- Strongest males are successful, so huge sexual dimorphism in size and weapons.
- Females not passive; may incite males to test their quality
(= fitness of their genes).
- Dominant male wins access to mate. Normally “winner takes all”…
But not always!
Consequences of intrasexual selection
Male-male combat sometimes leads to alternative reproductive strategies
Explain how this is applied to dung beetles?
- Male dung beetles either mate-guard or sneak.
- Guard males are large have big horns – Sandneak males are small and no horns.
- Horns are helpful if you need to guard and fight – but are a nuisance if you need to sneak down small tunnels.
Co-evolutionary “arms race” between males and females
Mate guarding in seed beetles has driven harmful adaptations, what are they?
- Fitness costs to females results in coevolution of tougher reproductive tracts.
- Intrasexual selection has evolutionary consequences for both sexes!
Post intrasexual selection
Never give up!
It’s not too late for sperm competition
• Name 4 ‘First-male advantage’ ?
• Most ‘fertile male advantage’?
First male advantages
Reduces success of later males
– Copulation plugs
(mammals, reptiles, arthropods)
– Spiders: female stores sperm, first lot have priority
• Most fertile male advantage
– in groups where all males mate, favours male that injects more, larger, faster sperm.
– Males can also increase their sperm production and courtship effort if female smells of other males (e.g., flour beetles).
Post intrasexual selection
If all else fails, what extreme measure can still be taken?
Infanticide
- An extreme form of sexual interference.
- following new male takeovers of group of breeding females (intruding lions, Chacma baboons).
- Ensures only the new male’s genes are passed on.
- Induces mate guarding - by resident male.
Intersexual selection (female choice)
- Concept proposed by Darwin but was even more contentious than natural selection.
- Depends on linkage between male signal and female preference.
- Raises question of truth in advertising – Zahavi’s handicap hypothesis: sexually selected traits come at a cost. Only the strongest males can carry the most extreme sexually selected traits.
What are the two main forms of female choice sexual selection?
Two main forms:
Material benefits
• E.g., food (sometimes the male!), territory, and sperm
Displays that indicate quality
• E.g., size, strength, colour, courtship performance
Consequences of female choice
1. Material benefits
– Courtship feeding (nuptial gifts) to potential mates (many birds and mammals)
– Hanging flies distract female with a gift, mate while she eats it
– Largest gifts give him enough time (20 minutes) to complete sperm transfer
– When done, he will try to get it back, to offer to another female!
Consequences of female choice
1. Material benefits
- Food and territory are normally in short supply, hence allowing males to convince females.
- Sometimes sperm is limited too…
- Sperm of male Drosophila melanogaster is depleted after multiple copulations.
- Therefore, females prefer to mate with virgin males.
Consequences of female choice
2. Indicators of quality
- Leads to strong sexual dimorphism in colour, shape and size.
- E.g., colouration and shape (peacock jumping spider); calls and plumage (bird of paradise).
- With or without exaggerated courtship behaviour
(e.g., bowerbird vs. house finch).
- Can be modified by early experience, e.g. by cross-fostering.
- Inappropriate signals learned from foster parents is then misunderstood by potential mates.
- E.g. black robin chicks raised by tomtits
Honest signaling without exaggerated courtship behaviour
Male house finches can’t synthesise carotenoids.
- Their colour varies from pale yellow to bright red.
- Correlated with foraging ability and nutritional status.
• Reddest males feed chicks fastest.
• This skill is heritable, so females prefer redder mates.
Post intersexual selection
Cryptic female choice
– females choose mates based on their sperm.
- Females can maximise fitness of offspring by ‘collecting’ and choosing among males.
- Occurs via mechanisms such as sperm rejection, evolution of sperm storage organs, destroying sperm, and sabotaging ‘sperm plugs’ from males.
