a mating system is the pattern of relationships between the sexes in a population -- there are three basic kinds of mating system . . .

monogamy

approximately equal numbers of males and females produce offspring in a breeding season -- one male mates with one female

polygyny ("many females")

more females than males reproduce in a breeding season -- some males have many mates, others none

polyandry ("many males")

more males than females reproduce in a breeding season -- some females have many mates, others none

frugivorous (fruit-eating) birds of tropical forests illustrate the evolution of mating systems in relation to requirements for raising offspring . . .

(1) those that feed insects to their young have nesting behavior like that of strictly insectivorous birds -- monogamous pairs work together to feed their offspring -- insects hide and are hard to find so two parents can provision twice as many young -- tanagers (over 250 species in tropical America) include many examples (although few have been well studied)

(2) those that feed fruit to their young and nest in cavities also form monogamous bonds and work together -- cooperation of male and female is needed to defend the nest hole from competitors of the same and many other species -- toucans and parrots are examples

(3) those that feed fruit to their young and make hidden or cryptic nests are polygynous -- examples include some of the birds-of-paradise (confined to New Guinea except for three in Australia), most of the manakins (throughout American tropics), and some of the cotingas (throughout American tropics, including the Cock-of-the-rock Rupicola rupicola)

in these species, males spend most of their time displaying and challenging rival males at leks (aggregations of small territories for mating) -- males often have elaborate plumage -- females are drab and visit the display territories only for copulation -- they then lay one egg in a hidden nest -- with only one parent visiting the nest there is less chance of predation -- even so, 90% of nests are lost to predators -- a female can quickly collect enough fruit for herself and her offspring (fruit is conspicuous and easy to find because plants "want" their seeds dispersed) -- she spends most of the time sitting on her nest

because females can raise their offspring without assistance from males, many females can choose the same mate -- thus males compete for access to females -- female choice of mates in polygynous species results in strong sexual selection

sexual selection is a difference in the spread of alleles caused by differences in mating success of phenotypes associated with those alleles (as opposed to any other component of fecundity, for instance clutch size)

sexual selection occurs when females mate preferentially with males that have particular traits -- females with such preferences leave offspring that include sons with those traits and daughters with those preferences -- all offspring (both daughters and sons) tend to carry alleles for both the female preference and the male trait -- in other words there is genetic correlation between alleles for the mating preference and alleles for the preferred trait -- genetic correlation occurs when particular alleles at each of two genetic loci occur together in the same individual more often than randomly

as sexual selection continues generation after generation, female preferences not only spread alleles for the male trait but also spread alleles for the preference itself -- as alleles for the preference spread, the advantage of the male trait increases -- sexual selection is thus self-promoting -- it produces accelerating evolution of male traits and female preferences -- females' preferences and males' traits tend to evolve together

eventually advantages of greater mating success by males are balanced by disadvantages of greater vulnerability to predators as a result of elaborate structures or vulnerability to disease as a result of allocation of energy to displays -- in other words, advantages of greater fecundity are balanced by greater risks of mortality -- when this balance is reached, evolution of male traits stops

calculations show that sexual selection can occur even when females prefer male traits that have no redeeming advantages aside from attractiveness to females (female preferences for arbitrary male features)

the chances for sexual selection increase, however, if females prefer male traits (for instance, efficiency in feeding or resistance to disease) that increase survival or reproduction of all offspring, female as well as male (female preferences for good genes)

female preferences can also evolve when females prefer males that provide more parental care or protection (female preferences for direct benefits for the female herself and her progeny

if a male trait does not have any genetic basis (no correlation with any allele at any locus), self-promoting evolution of female preferences would not occur

female preferences can influence male features either directly (for instance, females choose males on the basis of individual differences in some anatomical feature) -- or indirectly (for instance, females mate at a particular location, so that males compete for access to that location and evolve features like large size and coloration that increase success in competition with other males -- remember the Bluehead Wrasse)

in any case, sexual selection produces sexual dimorphism (differences in morphology between the sexes) in features that serve to attract mates or to improve success in competing with rivals for mates