Presentation on theme: "Males and females evolved after sex IsogamyAnisogamy."— Presentation transcript:
Males and females evolved after sex IsogamyAnisogamy
Anisogamy was the result of disruptive selection
Further sexual dimorphism Often 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 the males 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 NAverage size (Snout-vent length) Males that copulated Males that tried to copulate 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… 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 provide their 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 offspring Females 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