Sexual selection
1) Sexual dimorphism
2) Variation in mating success
3) Sexual selection
4) Female choice based on male ornaments
5) The evolution of female preference
6) Example exam questions
http://en.wikipedia.org/wiki/File:Descent_of_Man_-_Figure_16.jpg
http://en.wikipedia.org/wiki/File:Cryptopsaras_couesii_(triplewart_seadevil).pnghttp://commons.wikimedia.org/wiki/File:Male_and_female_superb_fairy_wren.jpg
1) Sexual dimorphism
Superb bird of paradisehttp://tv.yahoo.com/show/39817/photos/10
Lesser bird of paradisehttp://www.flickr.com/photos/stavenn/2666919963
http://misheli.image.pbase.com/u34/r53lanc/upload/37557333.Wilsons_BOP.jpg
1) Sexual dimorphism
http://www.flickr.com/photos/aachen/2416207985/http://www.orchidspecies.com/catasetumsacatum.htm
Catasetum saccatum orchid femalemale
1) Sexual dimorphism
2) Variation in mating success
Ÿ Matings are not apportioned randomly within species --- some individuals obtain more mates than others.
Ÿ Consider the differences between males and females of most species: males produce many tiny mobile gametes (sperm), whereas females produce comparatively large, nutrient-rich gametes (eggs).
Ÿ Consequently, reproductive success of females is limited more by her ability to produce and nourish gametes than by the number of mates obtained.
Ÿ Males are able to produce many more sperm than there are eggs. The reproductive success of males is likely to be limited by the number of matings obtained.
http://www.joachimliedtke.com/node/19Jones et al (2000)
Drosophila
pipefish
No. of
pro
geny
Mating success (no. mates)
males
males
females
females
2) Variation in mating success
In Drosophila, the relationship between number of offspring and number of mates is steeper in males than in females.
However, in the pipefish Sygnathus typhle, the relationship is reversed: it is steeper in females than in males. Males provide all of the parental care, supplying offspring with nutrients and oxygen through a placenta-like connection.
2) Variation in mating success
The result in the “typical” case is that abundant sperm compete for much less numerous eggs, creating competition between males for fertilizations.
This leads to sexual selection on traits that enhance fertilization success in males.
Mating can be costly in males too, so the above arguments don’t apply in all species. Males of many species are “choosy”.
Males are choosy in pipefish, which have reverse sexual dimorphism
Berglund & Rosenqvist 2003 Adv. Study Behav
Berglund et al 1986
choice by males
choice byfemales
2) Variation in mating success
Sexual selection in plants
3) Sexual selection
Nonrandom association between a trait and mating success
Male-male competition: combat
Traits Selectiondifferential s
Selectiongradient b
elytra length 0.39 –0.09
horn length 0.45 0.59
weight 0.38 –0.06
Connor (1988)http://www.aeaq.ca/photos/rvigneault.htm
Sexual selection in forked fungus beetle, Bolitotherus cornutus
3) Sexual selection
Male-male competition is thought to explain why males are larger than females in many species.
Waage (1979)
Robinson & Novak (1997)
Adaptations for sperm removal and sperm displacement in male damelflies
Male-male competition: sperm competition
3) Sexual selection
Male-male competition: scramble competition for pollinators
3) Sexual selection
http://www.emporia.edu/ksn/v48n1-may2002/bittacidae.htm
Female preference of males based on resource provisioning
3) Sexual selectionMate choice: resources
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3) Sexual selection
Mate choice: male ornaments
http://animaldiversity.ummz.umich.edu/site/resources/grzimek_birds/Ptilonorhynchidae/Amblyornis_macgregoriae.jpg/view.html
5) Female choice based on male ornamentsThe terms “choice” and “preference” here refer to a mating bias
(not necessarily active choice)
http://www.astrocape.org.za/entrip/?C=S%3BO=A
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After
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Darwin proposed that such traits were elaboratedin males because females preferred them
5) Female choice based on male ornaments
Yet, the preferred male traits are costly (to the male)
5) Female choice based on male ornaments
The evolution of female preference for male ornaments
Hypotheses fall into two categories
A) Preference is nonadaptive, arbitrary
Ÿ Fisher runaway process
Ÿ Latent preferences / sensory exploitation
B) Preference is adaptive
Ÿ Direct natural selection on the preference
Ÿ Indirect natural selection (”good genes”)
http://tv.yahoo.com/show/39817/photos/10
5) Female choice based on male ornaments
The Fisher process
5) Female choice based on male ornaments
1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length.
The Fisher process
5) Female choice based on male ornaments
1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length.
2. Assume no natural selection on this preference. Females preferring long tails produce no more nor fewer offspring than females who do not prefer long tails.
The Fisher process
5) Female choice based on male ornaments
1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length.
2. Assume no natural selection on this preference. Females preferring long tails produce no more nor fewer offspring than females who do not prefer long tails.
3. Males with longer tails will then experience slightly higher mating success.
The Fisher process
5) Female choice based on male ornaments
1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length.
2. Assume no natural selection on this preference. Females preferring long tails produce no more nor fewer offspring than females who do not prefer long tails.
3. Males with longer tails will then experience slightly higher mating success.
4. The sons of such matings will inherit long tails and also the genes causing a preference for longer tails. This establishes a nonrandom association in the population (a genetic correlation) between genes for tails and preferences.
The Fisher process
5) Female choice based on male ornaments
1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length.
2. Assume no natural selection on this preference. Females preferring long tails produce no more nor fewer offspring than females who do not prefer long tails.
3. Males with longer tails will then experience slightly higher mating success.
4. The sons of such matings will inherit long tails and also the genes causing a preference for longer tails. This establishes a nonrandom association in the population (a genetic correlation) between genes for tails and preferences.
5. Because of the bias in favor of longer tails, these sons will have higher than average mating success, which indirectly favors the genes for the preference.
The Fisher process
5) Female choice based on male ornaments
1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length.
2. Assume no natural selection on this preference. Females preferring long tails produce no more nor fewer offspring than females who do not prefer long tails.
3. Males with longer tails will then experience slightly higher mating success.
4. The sons of such matings will inherit long tails and also the genes causing a preference for longer tails. This establishes a nonrandom association in the population (a genetic correlation) between genes for tails and preferences.
5. Because of the bias in favor of longer tails, these sons will have higher than average mating success, which indirectly favors the genes for the preference.
6. This self-reinforcing process favors ever-longer tails and preferences for longer tails until the mating advantage to males is counteracted by the costs of the trait.
Illustration of the Fisher process at equilibrium
Males survive best that have a moderate trait value
Survival selection favors themoderate male each generation
The equilibrium trait value in themale is greater than this
Males with a large trait valueare most attractive to females
The mating advantage of thelarge trait value offsets thesurvival disadvantage, whichmaintains the male mean abovethe survival optimum
5) Female choice based on male ornaments
(1) Tail length maximizing survival
(2) Tail length preferred by females
(3) Where the two forces are balanced
Multiple equilibria possible under the Fisher process
5) Female choice based on male ornaments
Slope refers to strength offemale preference for orangein a population
of males
Male trait and female preference often evolve in tandem among populations
http://www.practicalfishkeeping.co.uk/pfk/pages/item.php?news=889
This is a prediction of Fisher’s hypothesis(although other hypotheses make the same prediction)
5) Female choice based on male ornaments
Sensory exploitation hypothesis
A) Preference is nonadaptive, arbitraryŸ Fisher runaway processŸ Latent preferences / sensory exploitation B) Preference is adaptiveŸ Direct natural selection on the preferenceŸ Indirect natural selection (”good genes”)
5) Female choice based on male ornaments
http://www.flickr.com/photos/70259473@N00/875110682/
Sensory exploitation?
5) Female choice based on male ornaments
http://www.malawianer.de/html/body_bestandsliste.html
Priapella
Xiphophorus
http://www.aqua-fish.net/show.php? http://www.livefish.com.au/index.php?
Female preference for swords in swordless species
5) Female choice based on male ornaments
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swordfemales prefers sword
no sword
*Basolo (1005) Proc. Roy. Soc. Lond. B. Biol. SciKang et al. (2013) BMC Evolutionary Biology
Latent preferences in females for male ornaments
http://grad.bio.uci.edu/ecoevo/wvantrum/Files/Color%20Banding%20in%20Zebra%20Finches.ppt
Burley found that red and black leg bands were more attractive than blue or green bands
Zebra finches
5) Female choice based on male ornaments
Chase-away hypothesis
An extension of the ideaof sensory bias
5) Female choice based on male ornaments
Evidence for a direct advantage of female preference
http://www.critterlight.com/House-Finch-6-063003.jpg
A) Preference is nonadaptive, arbitraryŸ Fisher runaway processŸ Latent preferences / sensory exploitation B) Preference is adaptiveŸ Direct natural selection on the preferenceŸ Indirect natural selection (”good genes”)
5) Female choice based on male ornaments
A) Preference is nonadaptive, arbitraryŸ Fisher runaway processŸ Latent preferences / sensory exploitation B) Preference is adaptiveŸ Direct natural selection on the preferenceŸ Indirect natural selection (”good genes”)
5) Female choice based on male ornamentsThe “good genes” model of sexual selection
Assumptions of hypotheses of direct vs indirect advantage
5) Female choice based on male ornaments
Direct:Ÿ Males vary in their “quality” or “condition”.Ÿ Males vary in a secondary sexual trait preferred by females.Ÿ Degree of male ornamentation and male quality/condition are positively correlated
(“honest indicator”, expected to evolve only if male trait is costly).Ÿ Females who choose males having higher ornamentation obtain direct benefits (higher
survival, more, plumper offspring) via his higher than average quality (fewer STDs, better paternal care).
Indirect:Ÿ Males vary in their “quality” or “condition”.Ÿ Variation in male quality is heritable (“good genes”).Ÿ Males vary in a secondary sexual trait preferred by femalesŸ Degree of male ornamentation and male quality/condition are positively correlated
(“honest indicator”, expected to evolve only if male trait is costly).Ÿ Females who choose males having higher ornamentation obtain indirect benefits
(offspring inherit good genes, and so have higher survival and reproductive success)
Evidence for an indirect advantage of female preference
Kotiaho et al (2001) Nature
5) Female choice based on male ornaments
Males of the species court by tapping females at her back using head and forelegs.
Mating mating success increases with courtship rate, and so the trait is “preferred” by females.
Left, mean s.e. of courtship rate per minute (log + 1 transformed); right, the same after five daysof manipulation of food availability. Solid symbols, constant food treatment; open symbols, no food treatment.
http://gallery.photo.net/photo/7077136-md.jpg
5) Female choice based on male ornamentsDegree of male ornamentation and male condition are positively correlated (“honest indicator”). Demonstrated by manipulating food and showing that this affected courtship rate positively. This was a non-heritable form of condition – natural variation in “condition” was not evaluated here.
Kotiaho et al (2001) Nature
Condition is measured as the standardized residual mass from a linear regression of log body mass on log pronotum width. The different symbols represent the 12 sires. The three replications within
each sire cluster represent the three dams for each sire. Sires are ranked according to the mean condition of their offspring.
3) Condition is heritable
Conclusion: females mating with high-courting males transmit high condition to their offspring
5) Female choice based on male ornaments
Kotiaho et al (2001) Nature
Natural variation in male “condition” is heritable. Demonstrated by showing that male “scrawniness” is heritable. They did not test whether this influenced offspring survival and reproduction (it is a lab population).
The evolution of female preference for male ornaments
Interim conclusion:
There are several hypotheses for the evolution of female preferences for male traits. None has been conclusively ruled out by data.
Some make similar predictions, making it difficult to tease apart the most important cause.
A) Preference is nonadaptive, arbitrary, possibly even hazardous
Ÿ Fisher runaway processŸ Latent preferences / sensory exploitation
B) Preference is adaptive
Ÿ Direct natural selection on the preferenceŸ Indirect natural selection (”good genes”)
http://commons.wikimedia.org/wiki/File:Male_and_female_superb_fairy_wren.jpg
5) Female choice based on male ornaments
6) Example exam questions
Define and give an example of sexual selection.
Distinguish briefly: natural selection and sexual selection.
Explain the difference between the sexual selection differential and the sexual selection gradient. What do they measure, and why might the numbers they yield be different?
Two types of hypotheses have been put forth to explain the evolution of an extravagant male trait by female choice: 1) the male trait and the female preference evolved jointly to an equilibrium determined by the intensities of natural and sexual selection; 2) female preference evolved as a by-product of natural selection on the female sensory system well before the male trait evolved. Devise a realistic test to distinguish between these two hypotheses. Explain your methods.
Explain why sexual selection in most species is stronger on males than on females. Under what circumstances might this trend be reversed?
In theory, how might extravagant male traits and female preferences for extravagant traits evolve in the absence of any natural or sexual selection on females?