The Origin of Species

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Impeding different species from attempting to mate.

Preventing the successful completion of mating.

Hindering fertilization if mating is successful.

Prezygotic barriers maintain reproductive isolation and include:

Temporal, Habitat, Behavioral, Mechanical, and Gamete Isolation.

Reproductive Barriers Between Species

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Reproductive Barriers Between Species

Prezygotic Barriers

Habitat

Isolation

Individuals

of

different

species

Temporal

Isolation

Behavioral

Isolation

Mating

attempt

Mechanical

Isolation

Gametic

Isolation

Fertilization

Reduced

Hybrid Viability

Reduced Hybrid

Fertility

Postzygotic Barriers

Hybrid

Breakdown

Viable,

fertile

offspring

(a)

(b)

(d)

(c)

(e)

(f)

(g)

(h)

(i)

(j)

(l)

(k)

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Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers.

PreZygotic Reproductive Barriers

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Habitat Isolation

Water-dwelling Thamnophis

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Habitat Isolation

Terrestrial Thamnophis

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Temporal Isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes.

Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers.

Mechanical isolation: Morphological differences can prevent successful mating.

Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species.

PreZygotic Reproductive Barriers

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Behavioral Isolation Occurs Without Appropriate Mating Rituals

Courtship ritual of blue-footed boobies

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Mechanical Isolation:

Bradybaena with shells spiraling in opposite directions

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Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:

Reduced hybrid viability -- weak offspring

Reduced hybrid fertility -- sterile offspring

Hybrid breakdown.

PostZygotic Reproductive Barriers

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