II. Reproductive Isolation

A. Prezygotic Isolation Mechanism B. Postzygotic Isolation Mechanism

Reproductive Isolation

barriers that impede two species from producing viable, fertile, offspring

Prevents gene flow

Hybrids

offspring of crosses between different species

Prezygotic Barriers

• Operate prior to zygote formation- prevent fertilization

1. Prevents Mating

2. Preventing completion of mating 

3. Hindering fertilization if mating is successful  

Habitat Isolation

• Overlapping geographic range but live/breed in different areas 

• -> rarely interact, no opportunities to mate

Temporal Isolation

• Species that breed at different times of day, different seasons, or different years 

Behavioral Isolation

Courtship rituals and other behaviors unique to a species are effective barriers

Mechanical Isolation

Morphological difference can prevent successful mating

Gametic Isolation

Sperm of one species may not be able to fertilize eggs of another species ex. Sea urchins

Postzygotic Isolation Mechanism

Prevents the hybrid zygote from developing into a viable or fertile adult

Reduced Hybrid Viability

Genes of the different parent species impair the hybrid’s development

Reduced Hybrid Fertility

Hybrid’s viable but sterile or low fertility

Often due to problems during meiosis

Ex. mule = female horse (2n=64) + Male Donkey (2n=62)

Hybrid Breakdown

1st generation hybrids viable and fertile (usually seen in plants rare in animals)

Hybrids can mate with each other or parent species

Subsequent offspring (f2) infertile or frail