three basic possibilities are defined by how many males and females actually reproduce in any one breeding season ... Breeding Sex Ratio (or BSR) = reproducing males/ reproducing females

monogamy (BSR close to 1)

polygyny (BSR << 1)

polyandry (BSR >> 1)

polygamy includes both polygyny and polyandry

a separate issue is how long individuals of the two sexes associate with each other ... sometimes called promiscuous (short associations) or nonpromiscuous relationships

species with polygynous mating systems often have sexual bimaturism (males begin to reproduce at a later age than do females) and sexual dimorphism (males and females differ in morphology, especially in size, with males larger than females)

sexual bimaturism in polygynous species partly explains the inequality in numbers of reproducing males and females (because they begin to reproduce later in life, more males than females die before reproducing) ... regardless of mating system the primary sex ratio (at birth or hatching) always equals 1

sexual dimorphism, especially in size, occurs when one sex fights for access to the other sex (usually males fight each other for access to females), because size makes a big difference in winning a fight

polyandrous species (pipefish, North Jacana) are exceptions that prove the rule ... mating is polyandrous, females compete for males, females are bigger and more brightly colored than males, and they begin to reproduce at an earlier age

in recent decades field research has shown that related species with different mating systems often differ in ecology

each individual's behavior tends to maximize its reproductive success ... examples include Red-winged Blackbirds, Northern Jacanas, Dunnock, neotropical primates, great apes

in most species, males compete with each other for mates, females are choosy about whom they mate with ... first emphasized by Darwin in Origin of Species (1859) and in Descent of Man, and Selection in Relation to Sex (1872)

Wiley and Poston (1996) synthesized much of the previous suggestions about competitive males and choosy females ... the basic issue is what limits an individual's reproductive success ... if production of gametes limits RS then individuals are choosy ... if access to mates limits RS then individuals compete with others of the same sex

in most species females (individuals who produce eggs) have RS limited by the number of gametes they can produce or nourish ... in most species males have RS limited by the number of mates they fertilize (or more precisely the number of eggs they fertilize) ... polyandrous birds (and other species with "reversed sex roles") are exceptions that prove the rule

when individuals of one sex choose mates with particular traits, then sexual selection results

we need to clarify a couple of points right away ... (1) we should use the word "choice" to refer to individuals' behavior and the word "selection" to refer to evolutionary mechanisms ... to avoid confusing proximate and ultimate explanations!

(2) sexual selection is a subset of natural selection ... it is a special case of natural selection ... natural selection (hence sexual selection) is one mechanism of evolution ... evolution occurs when allele frequencies in a population change over time

natural selection is a change in allele frequencies as a result of differences (associated with those alleles) in survival and reproduction of individuals (phenotypes)

sexual selection is a change in allele frequencies as a result of differences (associated with those alleles) in individuals' access to mates

differences associated with alleles are heritable differences ... unless differences in reproduction or survival are associated with differences in the alleles that individuals carry, no change in allele frequencies (hence no evolution) occurs

sexual selection is a special case of natural selection because it focuses just on differences in reproductive success as a result of differences in access to mates ... any other difference in reproduction is ordinary natural selection

why does this one situation warrant a separate name?   because it changes the entire dynamics of evolution!

when one sex (usually females) choose mates with certain traits, the evolution of any alleles for choosiness spread explosively in the population ... and consequently alleles for the trait do too

think of an allele (P) associated with a preference for mating with males with a particular trait (T) ... females with P create an advantage for males with T (an advantage in passing their alleles to the next generation) ... but the progeny of P females and T males carry both alleles (P allele expressed in daughters, T allele expressed in sons, but both daughters and sons carry both alleles) ... the tendency of an allele at one locus to occur more often than by chance together with (in the same individual as) an allele at another locus more often than by chance is called genetic correlation ... mating preferences produce a genetic correlation between alleles associated with the preference and alleles associated with the preferred trait

as generations pass females with P prefer males with T but also with P (because the T allele occurs with the P allele more often than by chance) ... so the P allele creates selection for itself ... the more the P allele has spread in a population the faster it spreads itself in the next generation ... the more P there is the faster it spreads ... an explosion of P happens and along with it an explosion of T

there are two important consequences ...

(1) the mathematics of this process (first worked out independently by Kirkpatrick and Lande, both graduate students, in 1980-1981) indicates that, once P or a genetic correlation between P and T reaches a certain threshold in a population, then P and T spread automatically ... it is theoretically possible for completely arbitrary P and T to evolve (ones that make males survive less well but have no other consequences for reproduction or survival)

(2) nevertheless, subsequent mathematical analysis (primarily by Pomiankowski and colleagues) has shown that female preferences that have net costs for the female cannot spread by sexual (or natural) selection ... net costs are any costs (risks or time or energy) of searching or interacting with males (such as exposure to predators or STDs) minus any benefits (either from assistance by the male or good genes from the male) ... good genes are genes that improve the survival and reproduction of all offspring (female as well as male)

there is some evidence for all of these possibilities for costs and benefits of female choice ... but no study has measured them all in the same population in order to determine the net costs of female choice

there are two kinds of behavior that count as a preference ... Wiley and Poston (1996) emphasized that a preference in these mathematical calculations can be either (1) a direct preference when females respond to stimuli from some males more than from others or (2) an indirect preference when females' behavior sets conditions for males to compete with each other and females then mate with whichever male wins

indirect preferences might include (1) behavior which conspicuously advertises that a female is ready to mate (so all nearby males must compete), (2) preferences for mating in particular locations (so males must compete to maintain positions there), or (3) preferences for mating with whoever catches or guards her (so males again compete to be there at the right moment)

we tend to think of "female choice" as direct choice ... but a lot of female behavior results in indirect choice of mates ... resulting in sexual selection

furthermore, indirect choice by females underlies all cases of male competition for mates ... female behavior always sets the conditions for male competition

consequently, a lot of female behavior and male behavior result from sexual selection ... we can say that a lot of male behave co-evolves with female behavior (and vice versa)

lek mating systems (see below) distill all the issues of mate choice and sexual selection ... males defend small territories within which they display conspicuously ... females visit males' territories only for copulation ... both males and females find food elsewhere ... females do all parental care elsewhere ... a disporportionate number of females (sometimes a large proportion of females) mate with a fraction of males (or in a fraction of territories) ... do females in these species show direct or indirect choice? ... in other words, do they respond to features of males or do they mate at a spot where other females mate so males compete to defend territories at that spot?

"lek" is a Scandinavian word for "play" or "playing around" as in courtship ... it is also the root for the name of the toys "Legos" ... in biology a lek is a place where males congregate to defend small territories that females visit only to copulate ... it is a sex smorgasbord (to use another Scandianvian term!)