1. Sexual Selection
Christina Saremi
PSYC141

2. What is Sexual Selection?
Proposed in 1871 by Charles Darwin
Acts on an organism's ability to obtain or copulate with a mate.
Organisms have traits which will provide individuals with advantages in gaining access to mates. 
Both humans & animals

3. Intrasexual Selection
Individuals will compete with their own sex for access to the opposite sex
Develop weapons to compete with each other
Large teeth, horns, bigger muscles, aggression
Male-male competition

5. Intersexual Selection
Members of one sex will attempt to impress members of the other sex (usually females, since they’re the ones that need to be impressed)
Evolution of sexual ornamentation
Brighter colors, plumage, courtship display in males
Female choice

6. Anole Lizard

7. Thought to be the 7th largest in the world. Bull moose is 7 years old.

8. Ornamentation or Defense?
If defense were the driving force in their evolution, why wouldn’t females evolve these traits as well?
Male stag beetles. Winner gets a lady.

9. Sexual Selection and Natural Selection
Do they differ? Debated.
Male weapons, like large horns, are used both to attract mates and ward off predators. How would you differentiate between these two?
Male ornamentation tells us that natural selection and sexual selection are separate.
Ornamentation is not used to ward off predators.

10. Theories of Sexual Selection
Runaway selection- Male ornamentation is driven by female choice for attractive features. Male features are NOT a direct indicator of genetic advantage. Natural selection keeps it in check.
Parasite theory- Adornments evolve to show females that the males are free of parasites. These males will pass disease resistance to their offspring. Males are honest signallers- ornaments correlate with parasite resistance rather than simply appearing to do so.
Handicap hypothesis- Male ornamentation developed as impediments to show females their ability to survive.
Amotz Zahavi
Runaway- looking for the best genes in a group. Feature selected by females will become progressively exaggerated in the gene pool

11. Female Choice and Parental Investment
Parental investment: males provide the sperm; the smallest cell in the human body
Females have limited reproductive potential… they will be more choosy.
A research study in 2009 done by Finkel and Eastwick supported that in humans, males are choosier than females in certain social situations.
Males that adopt a long term mating strategy can be choosy because they are limited in the number of offspring their relationship can produce.

12. Why Can’t We All Just Be Asexual? Cons of Sexual Reproduction
Sex is costly (John Maynard-Smith and George Williams):
Cost of meiosis- throwing half of your genes away on each offspring
Cost of producing males (Some males do not reproduce at all)
Cost of courtship
Sex only wins out when it doubles the number of offspring produced.
Asexual- 100% of genes passed on
Sexual- 50% of genes passed on.

13. Why Can’t We All Just Be Asexual?
Muller’s ratchet- Harmful mutations that arise in individuals in an asexual population will be passed on. The mutations will accumulate. In sexual populations, only half of the offspring will inherit the mutation.
Raffle analogy- In a game of survival and reproduction, sex leads to variation, and each offspring is like a new ticket with a new number. Being asexual means giving each offspring the same number. There is not much of a chance to win.
Tangled bank hypothesis- In any given environment where there exists competition for space, food, and resources, it is advantageous to diversify.
The Red Queen- Parasites and hosts are in a perpetual ‘genetic arms race’ in which one must survive.  The host must produce genetically variable offspring in retaliation to parasites so that some of them will have resistance through variability.

14. Humans: Culture & Sexual Attraction
Culturally determined reproductive behavior. Variability in weight, hair color, height, etc…
Universally, males prefer good looks in women. Some traits are large eyes, good teeth, shiny hair, full lips, small jaw, and the hourglass figure.
All features are indicators for youthfulness and fertility- signals correlate with levels of estrogen and progesterone.
Women prefer good financial resources and high social status in men. More often than not, these men are older.
Studies show that women place greater emphasis on physical attractiveness in men in parts of the world where parasites are more common.

15. Fashion and Fertility
Li & Haselton, studied the women’s choice of dress across ovulatory cycles.
Women wear more fashionable clothing during the fertile window of their cycle.
Low fertility (A) and high fertility outfits (B).

16. Female Body Shape
Human beings focus their attention on the female body as a source of pleasure, attraction, fertility, and reproduction. Across cultures, there is a difference in what is considered an ideal body shape.
The waist-to-hip ratio (WHR) is the ratio of the circumference of the waist to that of the hips. It is an indicator of health and a measure of attractiveness. 0.7 is preferred overall, whereas it is 0.6 in China and 0.8 or 0.9 in South America and Africa .

17. Symmetry
In 1993, Gangstead and Thornhill claimed that both males and females prefer mates who are symmetrical.
Results showed correlation between attractiveness rating and overall symmetry.

18. Averageness
1990: Langlos and Roggman examines whether if mathematical averageness is linked to facial attractiveness.
Computer-processed composites of each image. Averaged by pixels in 2, 4, 8, 16, and 32-face composites.
Results showed the 32-composite face is rated as the most attractive.

19. Composite Miss Germany 2002
Virtual Miss Germany 2002
The real Miss Germany 2002
Composite Miss Germany 2002
Is sexual selection silently working in the background and influencing our choices?