11 sexual selection - department of zoology, ubcbio418/11 sexual selection.pdf · this leads to...

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

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


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


http://www.flickr.com/photos/aachen/2416207985/http://www.orchidspecies.com/catasetumsacatum.htm

Catasetum saccatum orchid femalemale



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


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.


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


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

[placeholder page for video]

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

Before

After

Me

an

nu

mb

er

of

ne

sts p

er

ma

le5) Female choice based on male ornaments

Darwin proposed that such traits were elaboratedin males because females preferred them

Yet, the preferred male traits are costly (to the male)


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


The Fisher process


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


(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


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)


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”)


http://www.flickr.com/photos/70259473@N00/875110682/

Sensory exploitation?


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


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


Chase-away hypothesis

An extension of the ideaof sensory bias


Evidence for a direct advantage of female preference

http://www.critterlight.com/House-Finch-6-063003.jpg




5) Female choice based on male ornamentsThe “good genes” model of sexual selection

Assumptions of hypotheses of direct vs indirect advantage


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


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.


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



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


11 sexual selection - department of zoology, ubcbio418/11 sexual selection.pdf · this leads to...

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