Sexual selection 1 Flashcards
Sexual selection definition
Selection that “…depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.” (from Darwin 1871)
Teleological descriptions of evolution
*A way of incorrectly representing the evolutionary process as acting for a particular purpose
*With behavioural traits, easy to begin to slip into talking about individuals behaving consciously
Evolution is not a goal directed process
Example 1 of teleological description:
1)Male grasshoppers call at a higher pitch near roads to avoid acoustic interference
2)Grasshopper males from populations that live near noisy roads have evolved higher pitched calls to deal with acoustic interference
Compared to in the context of natural selection:
1)Male grasshoppers vary in the pitch of their calls, and this variation is heritable
2)At field sites near roads, noise from cars masks lower pitch calls. As a result, females can only recognize males calling at higher pitches
3)Over time, since males with higher pitch calls leave more offspring, populations near roads consist of males that call at higher pitches, on average
Example 2 of teleological description
- Red deer males have evolved to fight battles against other males and thereby obtain exclusive access to females (and consequently, mating opportunities).
Compared to in the context of natural selection:
- Red deer males with higher fighting ability and fitness are more likely to win in competition for mates, resulting in more mating opportunities so that this behavioural trait is passed from father to son.
Variation is heritable
- Selection of fitness and traits
- Reproduction with a high frequency of this selected alleles (in offspring)
Components of fitness
*Number of matings
*Number of successful fertilisations
*Number of offspring
*Offspring viability (# offspring surviving to adulthood)
*Number of grand-offspring
*Lifetime reproductive success (not possible in long-lived species e.g. galapagos tortoise)
How could we test the hypothesis that sexual selection led to the elaboration of peacocks’ trains?
Petrie (1994, Nature) at Newcastle uni looked at variation in eyespot coverage on tail feathers
Mean area of eyespots correlates with no. Of offspring and survival proportion
A correlation between level of ornamentation and male fitness
How to identify causation in sexual selection?
*Evolutionary biologists don’t stop at just observing patterns…
They ask - what processes produce differential fitness?
Mechanisms of sexual selection:
Affiliative Agonistic
Intrasexual Same-sex clubs Competition
(within sex)
Intersexual Mate choice Sexual Conflict
(between sex)
Example of same sex clubs:
Bonobo females
females engage in genito-genito rubbing (sexual interaction)
The networks formed help to organise access to parental care of young and distribution of food
House mice females
Females in preferred partner dyads (with females they associate with) were more likely to reproduce and successfully wean pups than females in non-preferred ones (ones they had not met)
Leks of males (e.g. Black Grouse, Tetrao tetrix)
–Males gathered together to perform courtship displays
–In lek species males do not provide any parental care
-coorporation creating a group display draws more attention from females than individual displays
Benefits of cooperation between males:
*More conspicuous to females
*Subordinate individuals that stick around may eventually gain dominant positions in the leks
Same sex competition (combat)
Male-male combat
weapons:
horns, tusks and antlers
e.g. deer, boar, moose and longhorn beetles
Sperm competition:
Dung beetles (Onthophagus taurus)
–Females mate with multiple males
–Selection favors males with larger testes
In a selection experiment tracking traits over many generations
Monogamous (single male and female maintianed over generations)
vs. polygamous experimental lines (single female many males over generations)
Results over 20 generations:
Showed testes size of polygamous line increases over generations but not in monogamous line
Sperm competition effect on testes size example 2: primates
Larger individuals have proportionally larger testes but the interesting thing about this graph is white points are from monogamous species and black points are from polygamous females and we see larger testes occur in polygamous species
In damselfly copulation the mating male scoops sperm from previous males out of the female it is mating with.
What about females?
Why are females understudied?
It is more difficult and less studied throughout history so far.
Mate choice
Mate choice
Occurs “when a choosing individual assesses more than one potential mate and when variation in some trait of potential mates influences the chooser’s reproductive decisions”
*Opposed to “panmixia”, or random mating
*Note that “choice” does not imply that the process is conscious or purpose-driven
*Drury & Gowaty 2010, Encyclopedia of Animal Behaviour
Mate Choice decisions:
* To copulate or not to copulate?
* To inseminate or not to inseminate?
* To allow insemination or not to allow insemination?
* To allow fertilization or not to allow fertilization?
* To invest in parental care or not to invest in parental care?
Fitness benefits to (to choosers) of discriminating between mates varies:
*No benefits
*Direct benefits
*Indirect benefits
Sensory bias
(no clear benefit of mate choice)
*Pre-existing bias in the perceptual system of one sex favors members of the other sex who display a particular trait
*No benefits necessary
e.g. pre-existing preference for ‘chuck’ sound in Tungara frog females when choosing a mate
e.g. Swordtail characins attract females with an ornament fin resembling food, bias is to choose ant-shaped ornaments (with no clear benefit)
–Ornaments look more like ants in populations where females eat a lot of ants
Direct benefits from mate choice
*Fitness benefits that arise from pairing/copulating with a particular mate
*E.g.: nuptial gifts, offspring provisioning, access to a good territory
^ e.g. male hanging fly provides prey item to the female which she eats during copulation – a nuptial gift
e.g. direct benefit example – nuptial feeding (cardinals)
Indirect benefits from mate choice
*Fitness benefits arising from mate choice that manifest in the genotypes of offspring
Good genes model: Choosers pick mates who are in better condition (e.g., healthier) to confer better condition to offspring
good genes model example: link between colour intensity and health in stickleback males
Example: 3 spined sticklebacks – males with brighter red bellies have higher health levels so selecting these males increases chances of having fit and healthy offspring
-positive relationship between female choice and colour intensity – when there is a red filter in the water a loss of correlation occurs
Mate choice indirect benefits: Zahavi’s Handicap Principle
Zahavi’s Handicap Principle – The costs of producing a signal ensure that the signal is honest.
e.g. having long plumage puts you more at risk of predation, a male that survives to adulthood with long tail feathers clearly has a high level of fitness – better chance of healthy offspring
Experiment:
Group 1: shortened tail (cut feathers)
Control 1: cut off tail re-adhered
Control 2: natural tail length no interference
Group 2: Elongated tail - adding the cut tails to the length of this group of males – resulting in more copulation
^The cost of producing a longer tail is less in a male with high fitness thus it requires an energy investment. A lower fitness male would not have the spare energy to do this/ would experience negative impacts from redirecting energy in this way – thus the signal is true.
“If a given signal requires the signaler to invest more in the signal than it would gain by conveying phony information, then faking is unprofitable and the signal is therefore credible” -Zahavi & Zahavi 1997
Runaway selection
Runaway selection
*A genetic association between an ornament and a preference lead to positive feedback over evolutionary time
*Indirect benefit because offspring will have exaggerated ornaments (“sexy sons”)
Also see: Fisherian Runaway selection graphs in notes
Runaway sexual selection example : Drosophila sperm connected to female drosophila genitalia
*Breeding experiments demonstrate a genetic correlation between male sperm length and the seminal receptacle in females.
*Many sexually selected traits exhibit positive allometry: signal traits get larger faster than body size (within and between species).
*Among species studied so far, nowhere is this more pronounced than in the sperm of fruit flies.
*Breeding experiments demonstrate a genetic correlation between male sperm length and the seminal receptacle in females.
*Evidence for the Fisherian runaway process
