Speciation

How Species Form

Species

• How are new species defined?

• Used to be on basis of structure

• These are different species:– Top: Grevy’s zebra

(endangered)– Bottom: Plains zebra

(widespread in Africa)

Speciation

• When some members of a sexually reproducing population change so much that they are no longer able to produce viable, fertile offspring with members of the original population

Reproductive Isolation

• Can produce new species if there is no gene flow between two populations

• Many isolating mechanisms; some which occur before fertilization and some after

Pre-zygotic Isolating Mechanisms• Behavioural

– Any special signals (bird song, pheromones, mating rituals, etc.) prevent interbreeding

– Ex. Eastern and Western meadowlark look the same, have overlapping ranges but have very different songs

Pre-zygotic Isolating Mechanisms• Habitat

– Two species may live in the same general area but have different habitats

– Eg. The common garter snake is commonly found near water but the Northwest garter snake prefers open meadows

Pre-zygotic Isolating Mechanisms• Mechanical

– Many species are separated by temporal (timing barriers)

– Diurnal vs. nocturnal; mate or flower at different times

– Eg. Tropical orchids each flower in response to weather stimuli

Pre-zygotic Isolating Mechanisms• Mechanical

– Some species fail to mate because they are anatomically incompatible

– Lock and key genitalia (dogs, insects, etc.)

– Plant structure may impede pollination

– Dog breeding

(Great Dane vs. Chihuahua)

Post-zygotic Isolating Mechanisms• Hybrid Inviability

– Genetic incompatibility between two species

– Stops development of hybrid zygote

– Eg. Sheep/goat crosses usually die in early development (otherwise we’d have shoats…or geep!)

– Doesn’t always fail

Post-zygotic Isolating Mechanisms• Hybrid Sterility

– Can mate and produce hybrid offspring which are sterile

– Failure of meiosis due to chromosome number

– Eg. Horse and donkey make a mule

Modes of Speciation

1. Allopatric• Most common• 2 populations geographically

separated from each other (physical barriers)

• Species evolve separately in reproductive isolation

Modes of Speciation

2. Sympatric• A population may split into separate gene pools,

even within same geographic area

There’s a video for that!

• http://www.youtube.com/watch?v=2oKlKmrbLoU