Key terms:
Species
A biological species
is:a group of organisms
that can interbreed and
are reproductively
isolated from other
such groups.
Each of these
butterflies is a different
species (there are
thousands of different
species of butterfly
which do not
interbreed)
Populations
A biologist defines a population
as:
the total number of one
species in a particular area.
Continuous distribution
Populations can be very large
and occupy a large area, with
fairly continuous distribution.
Example: human
population, Arctic
tundra plant species
Populations may also be limited
in their distribution and exist in
isolated pockets or “islands”,
cut off from other populations of
the same species.
Fragmented distribution
Example: Some frog
species
Gene Pool
A gene pool is defined as
the sum total of all the
alleles for all the genes
present in a population at
any one time.
AA
AA
aa
AA
Aa
Not all the individuals will
be breeding at a given
time.
The population may have
a distinct geographical
boundary.
Each individual is a carrier
of part of the total genetic
complement of the
population.
aa
Aa
aa
Aa
AA
AA
aa
Aa
AA
A gene pool made up
of 16 individuals
Aa
Gene Pool
Geographic boundary
of the gene pool
Individual is homozygous
recessive (aa)
AA
Aa
AA
AA
aa
Aa
Aa
Individual is
homozygous
dominant (AA)
aa
aa
AA
Aa
aa
Aa
Aa
AA
AA
aa
Individual is
heterozygous (Aa)
A gene pool made up of 16 individual organisms
with gene A, and where gene A has two alleles
How could a gene pool
change
Geographic boundary
of the gene pool
Individual is homozygous
recessive (aa)
AA
AA
AA
aa
Aa
Aa
Individual is
homozygous
dominant (AA)
Aa
aa
aa
AA
Aa
aa
Aa
Aa
AA
AA
aa
Individual is
heterozygous
(Aa) A gene pool made up of 16 individual organisms
with gene A, and where gene A has two alleles
Gene flow
• Movement of alleles in and out of a gene
pool due to individuals moving between
populations
Genetic equilibrium
• is when the gene pool is stable, the allele
frequencies of a population remain
unchanged from one generation to another
• This occurs when there is a
• Large population which is
• Isolated (no migration or gene flow)
• No mutations
• Random mating
• No natural selection
Demes
• A species usually exists as distinct populations
may be separated geographically. These local
interbreeding populations are called demes.
• Organisms mostly interbreed within the deme
rather than with members of other populations,
therefore, demes often develop slightly different
allele frequencies, giving each different
characteristics.
Species tricky to define
• Boundaries of a species gene pool can be
unclear .
For example: closely related species of the
dog family can interbreed
Also, species can show a gradual change in
phenotype over a geographical area. This
gradual change is called a cline. This often
occurs over the length of a country or
continent.
Species
The boundaries of a species gene pool can be sometimes unclear, such as the genus
to which all dogs, wolves, and related species belong:
Coyote–red wolf hybrids
Coyote
Canis latrans
Red wolf
Canis rufus
Interbreedin
g
Interbreeding
Domestic dog
Canis familiaris
Interbreedin
g
Black-backed jackal
Canis mesomelas
Gray wolf
Canis lupus
No interbreeding
Side-striped jackal
Canis adjustus
No interbreeding
Dingo
Canis familiaris dingo
Golden jackal
Canis aureus
Clines
Species can show a gradual change in phenotype over a geographical
area. This gradual change is called a cline. This often occurs over the
length of a country or continent.
Ring species – a special type of cline
C
B
D
A
E
If a cline forms a ring,
(eg. across a continent)
demes A and E may be
unable to breed when
they meet, although,
the intermediate forms
can still interbreed.
Are A and E still the
same species or two
separate species?
Sub-species
• These arise when populations show
characteristics that are different from nearby
populations. Sub-species can interbreed but
this often occurs less frequently. This is
normally because of geographical isolation.
Changing Allele Frequencies
Mutation: Spontaneous mutations can alter alleles
frequencies and create new alleles.
AA
A’A
Gene flow: Genes can be exchanged
with other gene pools as individuals
move between them.
Small population size: Allele
frequencies can change randomly
from generation to generation.
Natural selection: Selection pressure against
certain alleles combinations may reduce reproductive
success.
Non-random mating: Individuals seek out
particular phenotypes with which to mate.
Changing Allele Frequencies
aa
Emigration
AA
aa
AA
Aa
Aa
AA
Aa
Aa
Immigratio
Aa
Aa
AA
aa
AA
AA
AA
Natural
selection
Mate selection
(non-random
mating) Aa
Aa
Aa
Aa
Aa
aa
A’A
AA
Boundary
of gene
pool
Mutation
Gene flow
Aa
Aa
Aa
Aa
Aa
Geographical
barrier
AA
aa
Aa
aa
Aa
aa
aa
Genetic