Key terms:

SpeciesA 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)

PopulationsA biologist defines a population as:

the total number of one species in a particular area.

Populations can be very large and occupy a large area, with fairly continuous distribution.

Populations may also be limited in their distribution and exist in isolated pockets or “islands”, cut off from other populations of the same species.

Example: human population, Arctic

tundra plant species

Continuous distribution

Example: Some frog species

Fragmented distribution

Gene PoolA gene pool is defined as the sum total of all the alleles for all the genes present in a population at any one time.

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.

A gene pool made up of 16 individuals

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Gene PoolGeographic boundary

of the gene pool

A gene pool made up of 16 individual organisms with gene A, and where gene A has two alleles

Individual is homozygous dominant (AA)

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Individual is heterozygous (Aa)

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Individual is homozygous recessive (aa)

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How could a gene pool changeGeographic boundary of the gene pool

A gene pool made up of 16 individual organisms with gene A, and where gene A has two alleles

Individual is homozygous dominant (AA)

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Individual is heterozygous

(Aa)

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Individual is homozygous recessive (aa)

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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.

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DingoCanis familiaris dingo

CoyoteCanis latrans

SpeciesThe 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

Interbreeding

Inter-breeding

Interbreeding

Inter-breeding

Domestic dogCanis familiaris

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Interbreeding

Inter-breeding

Interbreeding

Gray wolfCanis lupus

Red wolfCanis rufus

Black-backed jackalCanis mesomelas

Golden jackalCanis aureus

Side-striped jackalCanis adjustus

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

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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.

Mutation: Spontaneous mutations can alter alleles frequencies and create new alleles.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 againstcertain alleles combinations may reduce reproductive success.

Non-random mating: Individuals seek outparticular phenotypes with which to mate.

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Changing Allele Frequencies

Boundary of gene

pool

Gene flow

Emigration

Mate selection (non-random

mating)

Immigration

Natural selection

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Mutation

Geographical barrier

Genetic drift

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