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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
– Some bees carry pollen on
legs, some on backs, some
on wings
Pre-zygotic Isolating Mechanisms
• Gametic
– Ensures that sperm and egg
will rarely form a zygote
– Sperm may not be able to
survive in the internal
female environment
– Pollen grains of one species
fail to germinate on stigma
of another species
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
Post-zygotic Isolating Mechanisms
• Hybrid Breakdown
– F1 generation of hybrid
offspring is viable
– If two F1 hybrids mate then
F2 offspring are sterile or
weak
– Eg. Some cotton plants
produce fertile hybrids, but
their offspring die as seeds
Modes of Speciation
1. Allopatric
• 2 populations geographically separated from each
other (physical barriers)
2. Sympatric
• A population may split into separate gene pools,
even within same geographic area