evolutionary choices : sex and life histories

differences between a male and a female

females have larger gametes than males

• more invested in reproduction than males

costs of sex

nearly all populations have equal numbers of males and females, they both contribute 50% in the next generation

• half individuals make only sperm (males), cutting the rate of population increase. this means that an asexual mutant should outcompete sexual forms

• organisms have to find a mate and then there is conflict — finding mates and investing in reproduction cuts energy they can use for producing offspring

• risk of reducing fitness by mixing genomes

two-fold cost of sex

the evolutionary disadvantage that sexual populations grow at half the rate of asexual populations due to the fact that half offspring produced are males and they :

• do not give birth to children themselves

• so contribute less to population growth

benefits of sex

• genetic variability — spreads risk as offspring will have a range of phenotypes and can give rise to disease resistance (example)

• allows large scale evolutionary change under selection

• allows organisms to adapt more rapidly

what are the costs of sex outweighed by?

the benefits of genetic variability which allows species to have high levels of Darwinian fitness, in unpredictable environmental conditions and in response to pathogens, which are also evolving under natural selection

difference between human and octopus life histories

octopus :

• lays many eggs

• shorter juvenile stage as octopus babies grow more rapidly than humans

• reproduces once (semelparous)

• dies before eggs hatch

humans :

• few offspring

• longer juvenile stage

• born at long intervals

• parental care

• females can live long after they can reproduce

what are life-history trade offs

the evolutionary risk of using energy for once purpose, such as growth, and its ability to reduce the energy available for other purposes such as reproduction and defence against disease

• leads to trade-offs between function such as number/size of offspring, or reproducing now or later

some life history traits

• size at birth

• timing of growth over the lifespan

• age and size at sexual maturity

• number and size of offspring

• length of life

• sex

what can vary between species related to their lifespan?

survivorship varies between species

type 1 survivorship curve

• convex curves

• high survival in early/middle life, followed by a rapid decline

• typical of species that care for few offspring (including humans)

type 2 survivorship curve

• diagonal curves

• mortality rate/survival probability is independent of age giving an exponential decline

• birds and lizards often follow this pattern

type 3 survivorship curve

• concave curves

• have the greatest mortality early in life

• relatively low rates of death for those surviving this bottleneck

• characteristics of species than produce many offspring (eg plants/trees)

the chromosomes that determine whether birds are males or females

birds have different nomenclature

they have chromosomally determined sex, but females are ZW and males ZZ

how is sex determined in reptiles

reptile sex is determined by the temperature of embryonic development, and can be controlled by the mother as she incubates her eggs

can control sex ratio of their offspring

how is sex determined in tetrapods

vertebrate sex is established either chromosomally or by the environment (temperature sex determination)

birds and mammals have fixed chromosomally determined sex (XY or ZW)

why can fish change sex during their lifetimes

they do not have chromosomally determined sex because of SEQUENTIAL HERMAPHRODITISM :

• where an organism changes its sex multiple times during its life time

• can be beneficial to transition from male into female when larger body sizes are able to produce more eggs

• or beneficial to be larger to attract more mates