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Behavioural Ecology

Sexual selection and mate choice

QuestionAnswer
What is sexual selection? ‘the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction’
What is sexual selection? = Adaptations generated by intraspecific interactions and competitions = Adaptations associated with the struggle to reproduce
Sexual selection and natural selection can be distinct Traits that are attractive to mates because they constrain survival and only occur in the competing sex Traits associated with enhanced survival can also be attractive to mates
Why does it occur? - Bateman principle - Parental investment - Potential reproductive rate variance
INTRA- (within) SEXUAL SELECTION = INDIVIDUALS OF THE SAME SEX (USUALLY MALES) COMPETE FOR MATINGS
INTER- (between) SEXUAL SELECTION = INDIVIDUALS OF ONE SEX (USUALLY FEMALES) CHOOSE MATING PARTNERS FROM THE OTHER SEX
INTRA- AND INTER-SEXUAL SELECTION CAN BE INTER-RELATED Males may compete intra-sexually for an inter-sexual result ie. Red deer stags compete intra-sexually during the rut and establish dominance hierarchy for territories Hinds choose males and territories inter-sexually
MATE CHOICE WITHIN INTER-SEXUAL SELECTION Selecting direct and indirect benefits
Primary direct benefit - conspecific recognition Parental investment Mate choice & discrimination evolves more rapidly in sympatric (overlapping) than allopatric (isolated) Drosophila species pairs Avoidance of hybridisations which may be less fertile/infertile
Sexual selection for male sacrifice in redback spiders Male redback spiders often actively sacrifice themselves while mating Females benefit from an extra meal Males feed their own offspring, and achieve greater sperm transfer
Does courtship effort predict fertility? - to insure against partner infidelity?
Indirect benefits? Why have signals and mate choice evolved when males provide NO OBVIOUS DIRECT benefits?
1. Runaway selection FEMALE PREFERENCES FOR PARTICULAR TRAITS GENETIC LINK BETWEEN MALE TRAIT AND FEMALE PREFERENCE SONS CARRYING THE TRAIT GAIN SUCCESS TRAITS FIX IN POPULATION AND FEMALE GENES FOR TRAIT CHOICE PERSIST RUNAWAY EXAGGERATION OF THE TRAIT
2. Good gene models Costly traits - handicap CARRYING THE HANDICAP IS AN HONEST SIGNAL OF INDIVIDUAL MALE ‘QUALITY’ HANDICAP IS CONDITION-DEPENDENT AND BETTER MALES CARRY BIGGER HANDICAPS BEING FATHERED BY MALES THAT CAN CARRY THE HANDICAP WILL CAPTURE BETTER GENES
3. Parasites PARASITE LOAD IS A MAJOR & GENERAL PROBLEM SIGNAL IS LINKED TO PARASITE RESISTANCE BEING FATHERED BY MALES WITH EVIDENT SIGNALS WILL CAPTURE BETTER RESISTANCE GENES
4. Compatible genes MATE CHOICE BENEFITS IN CHOOSING GENOTYPES THAT COMPLEMENT SELF GENERAL BENEFIT TO AVOID OUTBREEDING GENERAL BENEFIT TO AVOID INBREEDING SPECIFIC BENEFIT AT PARTICULAR LOCI / COMPLEXES (MHC)
The theories are linked CO-EVOLUTION OF LINKED FEMALE PREFERENCE IMPORTANT IN RUNAWAY MODELS COST OF SIGNAL MORE IMPORTANT IN HANDICAP MODELS SPECIFICITY OF SIGNAL TO IMMUNITY IMPORTANT IN PARASITE RESISTANCE MODELS COMPATIBILITY FOCUSED ON MALE-FEMALE RELATEDNESS
The theories are linked ALL FUNDAMENTALLY SIGNAL GOOD GENES: Sexy sons General quality Parasite resistance Compatibility
1. Runaway selection example: correlational evidence Male ornamentation varies across different populations of Trinidadian guppy (affected by predation)
1. Runaway selection example: correlational evidence Male ornamentation varies across different populations of Trinidadian guppy (affected by predation) Observational comparison of male trait and ‘naïve’ female preference across 7 different populations
1. Runaway selection example: correlational evidence Naïve females prefer their own population’s male ornamentation characteristics = Evidence that female preference genes co-evolved with male trait!
1. Runaway selection example: experimental evidence Male dimorphism in eye stalk lengths Males with bigger eye stalks win in competition for resources and mates Artificial selection to increase or decrease male eyespan in different lines,
1. Runaway selection example: experimental evidence Test female preference in those lines for that ‘short’ or ‘long’ eye span males = Evidence that female preference genes experimentally co-evolved with male trait expression
2. Good genes example evidence Peacock tails are extravagant and composed of eyespots which vary in size Females prefer bigger males carrying bigger eyespots
2. Good genes example evidence Peafowl experimentally single-paired to males with different tail eyespot traits Peacocks provide nothing to females apart from sperm/genes
2. Good genes example evidence Chicks grew bigger when fathered by males carrying bigger eyespots Chick groups sired by big-eyed fathers had better % survival
3. Parasite mediated sexual selection evidence Offspring from males with (naturally) longer tails have fewer mites – even when translocated to other nests
4. Compatible gene evidence Heterozygosity at the MHC confers an immune advantage in mice Experimental observations that when mice are allowed to mate, there are more heterozygous offspring than you would expect from random mating – suggesting mate choice for heterozygous offspring
SEXUAL SELECTION COULD ALLOW COSTLY SEX TO PERSIST? Sexual selection could help to purge mutation load (bad genes) from lineages if…. Mutations influence male mating success in the face of competition and choice
SEXUAL SELECTION COULD ALLOW COSTLY SEX TO PERSIST? Mutations influence male mating success in the face of competition and choice OR Mutations are more deleterious in males than females because of sexual selection
High sexual selection backgrounds ... resist extinction!
Created by: rose.coo
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