16.4 reproductive isolation is reinforced when diverging species come into contact

Once a barrier to gene flow is established...

reproductive isolation will begin to develop through genetic divergence

reinforcement

enhanced reproductive isolation between populations due to natural selection for greater isolation.

prezygotic isolating mechanisms

prevent hybridization from occurring

postzygotic isolating mechanisms

reduce fitness of hybrid offspring

what leads to reinforcement of prezygotic isolating mechanisms?

selection against hybridization from postzygotic isolating mechanisms

types of prezygotic isolating mechanisms

mechanical, temporal, behavioral, habitat, gametic

mechanical isolation

differences in size and shape of reproductive organs makes mating impossible

temporal isolation

When two species breed at different times of day, season, or years.

behavioral isolation

differences in courtship rituals or pollinator attraction that prevent them from interbreeding

habitat isolation

populations live in different habitats and do not meet

gametic isolation

the sperm of one species may not be able to fertilize the eggs of another species

types of postzygotic isolating mechanisms

low hybrid zygote viability, low hybrid adult viability, hybrid infertility

low hybrid zygote viability

zygotes fail to mature or have severe abnormalities

low hybrid adult viability

hybrid offspring may have lower survivorship than non-hybrid offspring

hybrid infertility

hybrid offspring are sterile or have low fertility

if reinforcement is occuring, in what populations are species expected to evolve more effective prezygotic reproductive barriers?

sympatric populations

hybrid zone

a region in which members of different species mate and produce hybrids

hybrid zones often contain

recombinant individuals resulting from many generations of hybridization

why do narrow hybrid zones persist?

strong selection against hybrids and adults do not move over long distances but both species move short distances into zone