Males and females evolved after sex

Isogamy Anisogamy

Anisogamy was the result of disruptive selection

Further sexual dimorphismOften we see species that have some bizarre looking traits that don’t seem to have any logical function. As a matter of fact, in many cases these traits would even seem to hurt the organisms survival

Peacock Peahen

Typically, these elaborate traits are seen in themales of the species, but not the females

A special form of natural selection explains the evolution of these puzzling traits

In other words, Sperm is cheap!

Sexual selection is a consequence of anisogamy

Male vs. Female investment in offspring

Female gametes cost more per unit than male gametes

Female reproduction will be limited by access to resources

Male reproduction will be limited by access to mates

Relative Parental Investment (RPI):

Mating Strategies and Reproductive Success

Natural selection will favor males that attempt to mate often

•Males should compete for females

A female’s reproduction is limited by the number of eggs she can make--> a single mating may fertilize all her eggs

•Females should be choosy about which male she mates with

Intrasexual selection… Male-Male Competition

Male bighorn sheep

Humans?

Marine Iguanas (Galapagos Islands)

Males are bigger than survival rates would predict

Male Iguanas compete for territories

N Average size (Snout-vent length)

Males that copulated

253 401

Males that tried to copulate

343 390

Larger males with better territories were more likely to reproduce

In northern elephant seals, successful males have “harems” of females. Fewer than 1/3 of the males mate. Male-male competition is vicious and often lethal.

In species where not so much is at stake less lethal forms of male competition may be selected for.

Intrasexual selection… Sperm Competition

1. Larger ejaculates2. Mate Guarding3. Copulatory Plugs4. Other specialized

structures

Intrasexual selection… Infanticide

When male lions take over a pride of females, they kill any non-weaned cubs fathered by previous males

Advantage for males: females return to breeding condition 8months earlier than they would otherwise

The role of female choice

Darwin recognized that females were typically the choosers, and males competed tremendously with each other to be chosen.

Victorian science was not very PC.Darwin often caricatured his observations by stating that females were coy by nature and males were eager, or aggressive, when it came to mating.

Intersexual selection… Female Choice

Females may select for males with traits that indicate their quality

Male widowbirds with longer tails may advertise “good genes”

Courtship displays by males

Intersexual selection… Female Choice

Male hanging flies providetheir mate with a “nuptial gift” of a dead insect treat

Male redback spiders place themselves in position to be eaten by the female

Females may select males that provide her with a reward

Sexual selection can lead to the evolution of bizarre traits

Selection on stalk length also increases female preference

Gender roles depend on relative investments of males vs. females

In birds, about 90% of species show “monogamy”, but only 20-25% not promiscuous

Females who choose males that will stick around will do better, but cheating with a flashy male may get your offspring better genes

Sex role reversals

Gender roles are not set in stone

Male Jacana birds incubate eggs, guard offspringFemales mate with several males- “harem”

Are humans the product of sexual selection?

Are we solely the product of our genes?

Social Darwinism

BUT Evolutionary biology tells us: that sex differences are not universal that sex differences are not inevitable

Our brains tell us:

that sex differences are not always desirable that biological differences do not justify social inequality