Transcript Isolating Mechanisms - NCEA Level 3 Biology

Isolating
Mechanisms
Describe the mechanisms which
isolate individuals of a population
and allow speciation to occur.
Geographical Barriers
• Shifts in the continental plates –
NZ separated from the
supercontinent Gondwanaland
at a very early stage before the
evolution of mammals.
• Our birds, reptiles, insects and
plants evolved without the
pressure of grazing, browsing or
predator mammals.
Geographical Barriers
• As a result they did not evolve
protective adaptations to
counter the attacks of
mammals later introduced by
settlers.
• The separation from
Gondwanaland prevented
gene exchange between NZ
and the vast continent.
Geographical Barriers
• The only native NZ mammal is
a bat thought to have blown
over to NZ from Australia.
• Barriers include
– water – oceans, rivers changing
course.
– Desserts and canyons.
– Mountain ranges.
Ecological Barriers
• Populations may have
developed genetic differences
to cope with different
ecological niches or habitats.
– E.g. different temperatures.
Reproductive Barriers
• Under natural conditions,
sympatric populations of
different species are
prevented from mating by
isolating mechanisms. These
can be divided into 2
categories:
– Prezygotic
– Postzygotic
Reproductive Barriers
• Premating / Prezygotic
These take effect before
fertilisation.
– Habitat Differences – the 2
species may never meet, e.g.
one living on the top of a tree
and the other on the ground.
One may be nocturnal and the
other diurnal.
Reproductive Barriers
– Differences in Breeding Times
– the species may be sexually
receptive at different times of
the year.
– Mechanical Differences – e.g.
the genitalia of different insect
species will not fit together so
breeding is prevented. Some
plants have flowers that can
only be pollinated by certain
insects.
Reproductive Barriers
– Behaviour Patterns – there are
many examples of courtship
behaviour which takes place in
a stepwise ritual, only members
of the same species have the
behaviour patterns to complete
the ritual.
e.g. in birds, the songs and
displays will seem wrong to a
different species.
Postmating Isolation
• Postzygotic Isolation
• These isolations prevent
successful reproduction after
fertilisation.
– Hybrid Inviability – a zygote is
formed but does not develop
properly.
– Hybrid sterility – a hybrid
forms but it is sterile
Postmating Isolation
– Hybrid Breakdown – the hybrid
offspring are fertile but produce
many infertile or non-viable
offspring.
Polyploidy
• This is the abrupt formation of
a new species. It is an
important sympatric speciation
as it does not involve
geographical isolation.
• In one generation the parent
and the offspring can belong
to a different species.
• More common in plants.
Isolation by Time
• A species that disappeared
one million years ago
obviously can not interbreed
with a species living today.
• Although individuals living close together
within each species can interbreed, as
distance increases, fertility decreases,
indicating increasing genetic divergence as
the habitat and climate change. These frogs
are now considered to be four different
species: 1) Rana pipiens, 2) R. blairi, 3) R.
utricularia, and 4) R. berlandieri. The mating
calls of each species differ, and defective
embryos occur between certain
combinations, indicating that both pre-mating
and post-mating isolating mechanisms exist.