Sexual SelectionChristina SaremiPSYC141

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

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

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

Anole Lizard

Ornamentation or Defense?

If defense were the driving force in their

evolution, why wouldn’t females evolve these

traits as well?

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.

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

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.

Why Can’t We All Just Be Asexual? Cons of Sexual

Reproduction

Sex is costly (John Maynard-Smith and George Williams):

1. Cost of meiosis- throwing half of your genes away on each offspring

2. Cost of producing males (Some males do not reproduce at all)

3. Cost of courtship

4. Sex only wins out when it doubles the number of offspring produced.

Asexual- 100% of genes passed on

Sexual- 50% of genes passed on.

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.

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.

http://youtu.be/8qfMNES8gEk

Fashion and Fertility

Li & Haselton, 2008- 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).

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 .

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.

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.

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?