Sexual Selection

The difference between natural and sexual selection…

Sexual selection is non-random differential reproduction (because traits are related to attracting mates or mating oppurtunities)

“Sex” can refer to the act of sexual reproduction (i.e. mating)

“Sex” can also refer to the “a collective set of phenotypic and genotype traits of an organims related to reproduction

There is more than one definition of “sex”, and this is constantly being refined

• gamete type, chromosomal inheritance, genetic expression, gonad. presence/absence, anatomy/morphology, physiology, hormones, (and more) can all be use to ‘define’ SEX

• however, not all of these traits are linked, and so defining sex as a true binary eludes the complexity of biology

Sex is not binary or bimodal, it is actually multimodal

Secondary sex characteristics are features of animals which are unique to one sex in a species, but don’t directly contribute to reproduction

Examples; horns, long feathers, and coloration

Sexual dimorphisms are any characteristics that make two sexes of the same species look different

• could include primary and/or secondary sex characteristics

These differences often come about as a result of sexual selection producing secondary sex characteristics

Intrasexual selection- (intra- within a sex) members of one sex fight other members of their sex for access to to the other sex

• competition for mating opportunities

Intersexual selection- (inter- outside a sex) an individual’s offspring gets half of their genes from the other parents, thus, the choice of a partner affects an individual’s fitness

• mate choice affects fitness

  • through an increase in fecundity (some partners could produce more offspring)

  • through an increase in survivorship (some partners could produce offspring with higher survival)

Why are females typically the “choosy sex”?

Only in some animals

Males

• sperm is smaller, cheaper, and abundant

• cost of mating is relatively low

• parental care is optional

• fitness may be maximized by mating with multiple females

Female

• eggs are larger, costlier, and more rare

• cost of mating is relatively high

• parental care is not optional

• fitness is maximized by getting the best alleles for offspring

• females are more selective

How are organisms choosing mates?

Assessment of mates (mate “choice”) is not a conscious choice ( only in intersexual selection)

• reflex

• driven by physiology and environment

Mate choice assessments

1. Courtship behavior

   • fixed sequence of behaviors preformed by an individual when approaching a potential mate, direct benefits: none, indirect benefits: indicative of alleles that constitute to survival (immune system)

2. Nuptial gifts

   • material gift given to a potential mate (food, sticks, nesting, material, etc.), necessary to trigger mating behavior, direct benefit: may provide energy for mate (if food gift), indirect benefit: may be indicative of alleles that contribute to survival of the offspring

3. Territory

   • area defended by a potential mate, direct benefit: indicates quality of resources available to the choosy sex and their offspring, indirect benefit: may be indicative of parental care or alleles that contribute to offspring survival

4. Appearance

   • appearance correlates with other measures of quality in Barn Swallows, tail length is correlated with parasite load, parasite load may correlate with immune function

   • assessed directly, meaning that the choosy individual is determining whether to mate with their partner again based on care given to first clutch/liter

   • indirectly assessed

5. Parental care

   • behaviors performed by an individual that promote survival of offspring

   • direct benefits: parental care can be critical for success of raising young, indirect benefits: parental care can be

Mate choice increases reproductive success, another way to increase relative reproductive success, is sexual conflict

Selection does not act “for the good of the species”

Traits that increase fitness will be favored by selection

An individual can have a higher relative fitness if it has more offspring, better surviving offspring, or decreasing the fitness of other individuals of its sex

Typical sex roles

• typically males engage in sexual conflict behavior or traits because males generally benefit more than females from mating with multiple partners

Sexual conflict

Seen in insects, birds, mammals

• forced copulation

• traumatic insemination

• toxic semen

• infanticide

Three main mating systems

1. monogamy

   • two individuals, typically male and female

   • pair bond

   • sometimes no sexual dimorphism

     • stronger selection on traits that make monogamy advantageous (e.g. providing parental care) than appearance traits

   • extra-pair copulations: a behavior where one partner in a monogamous pair mates with a different individual

     • males: increased reproductive success

     • females: mate with genetically superior individual

2. polygamy

   • pair bonds

   • 3 different types of polygamy

     • polygyny- several females

     • polyandry- several males

     • polygynandry- several males, several females

3. promiscuity

   • NO pair bonds ( no lasting relationship, organisms are just together during copulation)

   • many individuals

How many individuals are involved?

Do we see pair bonds?

Pair bonds is a relationship that forms between 2 or more members of a mating group, it has to last for some amount of time after mating

Why do different mating systems evolve?

• often driven by the needs of the young

• if two parents are needed for protection/food —> monogamy

• if only on parent or no parents needed —> promiscuity or polygamy

• which sex provides (more or any) parental care can vary

• in mammals, female provides milk —> often polygynous

• where males provide —> often polyandrous

Evolution via sexual selection

• Occurs when we have

  • heritable genetic variation in traits that contribute to reproductive success only

• Occurs via

  • intrasexual selection

  • intersexual selection