Sexual Selection

Sexual Selection

1. The costs of sex- the cost of recombination; this causes the break- up of successful genotypes and the recombination of their elements. The cost of mating; in all species which sexually reproduce, much time and energy is invested in securing mates. All behaviour and elaborate ornamentation associated with sexual reproduction could be dispensed with n an asexual species.

2. The Red Queen Theory; Antagonistic pairs of species, such as predator and prey, or parasite and host, react to each others ongoing evolution by evolving counterplots. Disease is the biggest threat to most species; WWI casualties approx. 8 million, Spanish Flue pandemic 1918-19 killed between 25-50 million. sexual recombination may provide a defence against rapidly evolving pathogens.

3. Sexual selection; a problem for natural selection. if these traits are beneficial for survival, why don't females have them too? sexual selection; process by which secondary sexual traits become elaborated because their incense the owners ability to gain mates.

4. Natural selection vs sexual selection. Natural selection= 'survival of the fittest' sexual selection= 'survival of the fittest'. fundamentally these forces work in the same way. The distinction; sexual selection focuses its attention on the selective consequences of sexual interactions within a species. Natural selection operates in the context of other environmental factors (such as climate, disease, adaptations for food finding etc).

5. Bateman (1948) studied the fertility of male and female fruit flies as a function of the number of mates. Main findings; Variance in male reproductive success much greater than female. Male reproductive success depends upon number of mates; additional matings may have a reproductive payoff. A females reproductive success does not depend upon number of mates. The sex which invests more in the young becomes a resource for which other members of the jess parental sex compete. The sex with the higher potential reproductive rate (usually males because sperm are cheap to produce and male investment often ends at conception) compete for the sex with the lower potential reproductive rate. Batemans Principle.

6. Sexual selection is generated by two processes; intra-sexual competition; competition between members of the same sex over mates. Inter-sexual choice; choice by members of one sex for particular mating partners of the opposite sex.

7. when heritable variation in the factors that cause differences in success in intra-sexual competition and inter-sexual choice... generates selection on the competing/ chosen sex sex increase body size, weapons and sexual displays.

8. Sexual selection and sex differences; males compete for access to females in the majority of species; Females choosy, males competitive, but not always e.g. spotted sandpiper. this competition ;ears to sex differences in both morphology and behaviour. As a generalisation, the bigger the difference in parental investment between sexex, the more different the sexes are..

9. size dimorphism across species; increasing polygyny reflects strong male-male competition. Stronger competition leads to increased sexual dimorphism, males bringer larger/brighter. Holds true for most mammal and bird species.

10. Sexual displays; bring sexes together for mating and influence outcome; intra-sexual competition, inter-sexual choice. information is transferred via signals (and more static cues) that may be visual, acoustic or olfactory (chemical/ pheromonal)

11. Key forms of intra-sexual competition (usually male-male competition), direct combat; males with large body size and weaponry selected. Sperm Competition; males whose spree successful in fertilising eggs selected.

12. Direct combat; displays and intra-sexual competition. When individuals compete for mates opponents are rarely equally matched. In particular different in a ability to acquire or defend the resource Resource holding potential (RHP).

13. Displays and intra-sexual competition. Evolutionary models; have shown that when fighting is costly, for example the risks of injury are high or fighting has high energetic costs, individuals should monitor the RHP of their opponent and withdraw without fighting if they would be likely to lose the fight. if this is actually happening we should see animal contexts often being settled by displays of strength during which individuals assess the RHP of their opponents, This sort of assessment can only be maintained by selection if the displays give a reliable indication of fighting ability; if weak individuals are able to, by bluffing, imitate the displays of strong individual, there would be selection to detect bluffers and use of the bluffable cue for assessment cue for assessment would be abandoned.

14. Roaring in Red Deer; Glutton-Brock and Albon (1979) found that fights were more frequently preceded by roaring contests where the approacher roared more frequently than approach than other way round. Also played back recorded the presence of rival stag. Stags would increase their roaring rate to match that of playback- unless the challenger simulated by the playback was roaring so fast that they couldn't match him.

15. Novel local production in Koalas; Charlton et al., 2013; Velum (soft palate) is more massive than vocal folds and acts as an alternative source for the vulcanisation- enables very low fundamental frequency (27Hz)

16. male- male assessment based on croak pitch in common toads. larger males have larger local folds and therefore lower pitched rocks. Davies & Halliday; introduced male much likely to attack when played the high pitched croak than when played the low pitched croak. Visual cues and tactic cues were also important. Honest because of relationship between larynx (voice box) and body size- larger males, more massive vocal folds.

17. Spem competition; Male- male aggression not the only form on intrasexual competition, however. Perhaps the greatest constraint on male promiscuity is post-copulatory competition; sperm competition. mate guarding, sperm displacement,blocking the females reproductive tract. Cryptic female choice. complexity of female tract to select best super, positive correlation between frequency of remaining and complexity.

18. Sperm competition and body size dimorphism in primates; testis size and body size dimorphism linked to mating system in primates. Multi-male/multi-female groups (chimps) provide a lot of opportunity for spern competition.

19. female choice; females prefer males with secondary sexual traits, the idea to well accepted in Darwins time. Now widely accepted as a major force in evolution.

20. Displays and inter-sexual choice; displays also function to attract mating partners of the opposite sex- and specifically to provide a means by which individuals can choose a mate of the right species and also the best mate out of those available at the time.

21. Choice for mate of the right species; choosing the right species is crucial hybrids are often infertile, and female preferences have clearly been shaped wth respect to this. e.g. work by Gerhardy on grey tree frogs.

22. Female preference for elaborate feature in Africa widow bird (Andersson 1982), small birds with very long tails employed during courtship. widow birds defend territories- mean nests number the same across the 4 groups. After treatment males held territories, but females preferred males with long tails.

23. Female choice; females may prefer males with secondary sexual traits. An idea not well accepted in Darwins time. Now widely accepted as a major force in evolution. So females prefer exaggerated male traits, but why? Thee main models; Runway selection, Good Genes, Sensory exploitation.

24. Key parameters in inter-sexual choice (usually female choice). Runaway selection; (through a process of Fisherman selection); females choice males who are highly attractive (rather than more viable) and as a result have more attractive sons who are chosen by females in the next generation. Good genes; females choose males with characteristics that indicate high genetic quality and as a result obtain genes for their offspring that confer high viability (offspring grow and survive better); for this to be evolutionary stable, chosen characteristics must be honest indicators of male genetic quality. Sensory exploration; females choosing males that exploit their innate sensory biases.

25. Runaway selection (fisher); some variation in females preference for a male trait. Tons of these females inherit the traits; daughter inherit the preferences; genetic covariance. positive feedback in selection; females prefer males iwht the trait because they enjoy an advantage.

26. Runaway selection; a circular argument? yes, but that is why it works. Origin of preferences; fisher originally envisioned a functional advantage to male traits which can 'set the ball rolling' But perhaps males traits actually signal male quality.

27. Direct and indirect benefits- Females can gain directly from mating; nutrition e.g. spermatophores in arachnids and insects, Adelie penguins trade rocks for sex. But in may cases they don't, but are still choosy; paradox of the lek, choosing genes?

28. Good genes; sometimes females choose even though they down seem to gain anything other than sperm. Catenoid- dependent traits; colouration in birds, fish.

29. In sedge warbles (work by Catchpole & Colleagues) females prefer males who sing more songs types (larger song repertoires). these males; get mates first, are more stimulating to females in the lab. Evidence that males with large repertories provide food genes, may also sire more attractive sons.

30. Degree of symmetry in male ornaments may also indicate fitness; female choice for low levels of fluctuating Asymmetry (FA) in length to outermost tail-feathers in barn swallows; Moller, 1992 & 3. Fluctuating Asymmetry (FA)= absolute deviation from perfect symmetry on characteristics that are (on average) symmetrical on the two sides of the boyd. Evidence that FA reflects individuals propensity to withstand stress during development and predict males: THornhill 1992

31. Handicap Principle; Zahavi (1975)- secondary sexual characteristics may be attractive because they are detrimental to survival. This idea was controversial because it was originally based on verbal argument. MAthmatatical modelling showed formally that it could work.

32. Examples of the handicap principle; Stotting- ritualised jumping seen in gazelles and other quadruplets, occurs during predator pursuit. predator more likely to give up chase, Honest signal of health and speed? Testosterone- dependent traits; immunocompetence handicap hypothesis; bird song, morphology (peacocks tail and human body).

33. Sensory exploitation; pre-existing female choice. Xiphophorous species (Basolo 1990); swordtail- females prefers males with larger swords, Platyfisj- same preferences even though no swords.

34. Human Mate choice; Humans prefer symmetrical faces (good genes), females are more choosy than males. Females prefer faces of males with high levels if testosterone. Females also attend to other secondary sexual traits e.g. voice pitch. competition e.g. mate guarding.

35. Summary; sex is a very widespread form of reproduction, although explaining its advantage of asexual reproduction is not easy. the two sexes are defined by anisogamy, which is probably maintained by disruptive selection, and exists in equal numbers because of ESS maintains by frequency- dependent selection. Sexual selection in the competition for mates. According to Batesman's principle, males usually compete for females, who are the choose sex. there are 3 main models of sexual selection, which are not mutually exclusive; Fisherman runaway, Good genes, Sensory exploitation.