Lecture 2: Sex Differences 1: Evolutionary Theory and Sexual Selection

- focuses on sex differences in non human species

Animal Model System

Using animals to investigate biological principles

• people study accessible animals to extrapolate that knowledge to other species, including humans

The Naturalistic Fallacy

The idea that what is natural (appears in nature) is good and/or inevitable

• We don’t want to do this

• rather, think “nature is witless” “it is not kind, cruel, not red in tooth and claw, nor benign in its ministrations. it is utterly absolutely impartial”

Natural Selection

• The individual is the unit of selected

• The environment is doing the selecting

• How wisely do you respond to your environment

• differential reproduction

• currency of success is number of offspring

• traits that provide an advantage in surviving

Differential Reproduction

Some individuals leave more offspring behind than others

Proximate levels of causation

IMMEDIATE triggers

• an immediate cue that causes things to happen

• easier to do

• ex. less daylight triggering hormonal changes causing birds to migrate

• easier to do 

Ultimate levels of causation

Always concerns reproductive success

• harder to do

Genetic Determinism

• aka sociobiology controversy

• the belief that some behaviours are solely caused by biological/genetic factors. The idea that nature (biology) trumps/overrides nurture (learning, environment)

True or False: When speaking of a sexually reproducing species, both sexes need similar survival traits because they both have faced similar environmental challenges

True

True or False: Evolutionary theory predicts relatively few differences between males and females

True

Primary Sexual Characteristics

Traits that are involved in the act of reproduction

Secondary Sexual Characteristics

Traits that vary between the sexes but are not necessary for the physical act of reproduction (mostly males have more exaggerated of these features)

• costly to produce (take a lot of energy)

• Make an individual more conspicuous to predators

Sexual Selection

• traits that provide an advantage in reproduction

• Intrasexual competition (male-male competition)

• Intersexual selection (usually female choice)

• basic mammalian pattern

Intrasexual Competition

Members of one sex competing

• usually male-male competition

• males compete to: exclude rivals, mate more often

• Certain secondary sexual characteristics give some males an advantage over other males when physically competing for a mate

• Think of ELEPHANT SEAL video

• Results in ‘weapons’ or size (dimorphic)

What drives male-male competition?

Access to mates (this is the main limiting factor on male reproductive success)

Natural selection or Sexual Selection?: If both sexes have large canines

Natural Selection

Natural selection or Sexual Selection?: If only males have large canines

Sexual Selection

Intersexual Selection

• usually female choice

• Some secondary sexual characteristics have evolved bcs members of one sex (females) prefer in members of the other sex 

• Results in ‘ornaments’ on males or displays by males 

Courtship rituals/displays

Sexually selected displays motivated by female choice

1. Sage-Grouse: 

• Females choose most attractive males based on acoustic and visual displays to mate with

• Only a few males get to mate bcs of female choice vs Elephant seals who use size & male-male competition to mate

2. Bowerbirds

• Male bowerbirds use a bunch of skills to impress female birds

• decorating/dancing/singing

What are females ‘looking for’ at the ultimate level (reproduction)

• ‘Good genes hypothesis’

• Exaggerated displays may indicate good health, good genes may lead to better offspring