Download Speciation - Kaikoura High School

Species
“A species is a group of similar individual
organisms that can usually breed among
themselves to produce fertile offspring.”
Ernst Mayr
 Geneticists and molecular biologists
define a species as a group of organisms
that share the same gene pool.

Speciation
 The
formation of a new species.
 Two types:
Allopatric: similar species living in a
separate geographical area.
Sympatric: species of same genus
living together in same area.
Allopatric
1.
2.
3.
4.
5.
Population moves into new area
with many ecological niches
unfilled.
Population explosion follows with
increased variation.
Migration into fringe environments.
Geographical barrier arises so
geographically isolated.
Isolated population have different
mutations
6.
7.
8.
Different populations are exposed
to different environmental selection
pressures.
Some subspecies develop genetic
and chromosomal differences that
mean they cannot breed with the
parent population.
Now are genetically isolated so
therefore a new species.
A
cline: when we move in a
particular direction through a range
of species and find a continuous
increase or decrease in some
characteristics.
Isolating mechanisims
1.
2.
3.
Geographical barriers: mountains, rivers,
continental plates moving, oceans,
deserts.
Ecological barriers: populations may
have developed genetic differences to
cope with different ecological niches or
habitats.
Reproductive barriers: under natural
conditions, sympatric populations of
different species can be prevented from
mating by pre or post zygotic isolating
mechanisims.
Prezygotic


Habitat isolation
(nocturanl/diurnal)
Temporal isolation (differences in breeding
times)

Biochemical incompatibility
sperm and egg fail to unite - due to
chemical differences, eg, hormone signals.
Prezygotic 2
 Behavioural
Differences in courtship behaviour
are a major isolating factor.

Mechanical differences
Appearance and colouration patterns can
differ enough between closely related
species to make them “unattractive” to
each other.
The fit of the mating apparatus of many
insects is very specific.
Postzygotic
These mechanisms stop gene flow
between species even if mating is
successful.
Hybrid Sterility
The resulting offspring from two different
species may be healthy, but is sterile, eg.
horses and donkeys can breed to produce
mules, which are sterile.
Hybrid inviability
The zygote fails to develop.
Hybrid
breakdown
When Isolating Mechanisms
Fail
Sometimes isolating mechanisms are
weak. When this happens, species
hybrids can occur.
 Eg:

○
○
the native grey duck and the introduced
mallard duck hybridise freely.
The endangered black stilt is interbreeding
with the pied stilt, producing a range of
intermediate hybrids.
Sympatric
 Remember
non geographical
isolation
To illustrate the following example is
imaginary:
there is a bird population on an
island. E.g
Sympatry involves natural selection
but allopatry does not
Other ways of speciation



Polyploidy – instant speciation: the abrupt
formation of a new species with more than
two sets of chromosomes. E.g. wheat.
Chromosomal rearrangement: duplications
or delations may make individuals or small
populations that are genetically
incompatible with the rest of their species.
Isolation by time.