Post on 18-Jan-2016
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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