• The?theory of evolution?states that?species?do not stay the same, but change over time; this can lead to the process of?speciation
• Speciation can be defined as?the development of new species from pre-existing species over time
• In order for speciation to occur two populations of the same species must be?isolated?from each other in some way
  • When this happens, there can no longer be an?exchange?of?genes?between the two populations
  • The exchange of genes is sometimes known as?gene flow
• Isolation of populations may occur as a result of
  • Geographical isolation
    • This leads to a type of speciation known as?allopatric speciation
  • Random mutations?that?prevent them from interbreeding?with each other
    • This leads to a type of speciation known as?sympatric speciation
• Populations that are isolated from each other may face different?selection pressures?in their environment e.g. different predators or sources of food
• The different environmental conditions for the two populations might mean that?different alleles are advantageous, so different alleles are more likely to be passed on and become?more frequent?in each population; this is the process of?natural selection
  • The?allele frequencies?in the two populations change over time
  • Note that a process known as?genetic drift?can also affect allele frequencies
• Over time the two populations may begin to differ?physiologically,?behaviourally?and?morphologically?to such an extent that they can no longer interbreed to produce?fertile offspring; speciation has occurred

Allopatric speciation

• Allopatric speciation occurs as a result of?geographical isolation
  • It is the most common type of speciation
• Allopatric speciation occurs when populations of a species become?separated?from each other by?geographical?barriers
  • The barrier could be?natural?e.g. a body of water or a mountain range
  • It can also be?man-made?e.g.?a motorway
• This creates two populations of the same species between which no?gene flow?is taking place
• Allele frequencies in the?gene pools?of the two populations may change in different ways due to
  • Different?selection pressures?acting on them
  • The?accumulation of random changes?resulting from genetic drift
• Changing allele frequencies will lead to changes in the?phenotypes?of the two populations
• If enough allele frequency differences arise between the two populations then they will eventually no longer be able to?breed with each other?and?produce fertile offspring, and can be said to be?separate species

E.g. allopatric speciation in trees

• A population of trees exists in a mountainous habitat
• A new mountain range forms that?divides the species?into?two populations
• The geographical barrier prevents the two populations from?interbreeding?so there is?no gene flow?between them
• The two populations experience?different environments, so different?alleles?become?advantageous
• Different alleles are therefore more likely to be?passed on?in each population
• Different alleles become?more frequent?in each population
• Over thousands of years the divided populations?form two distinct species?that can no longer interbreed to produce fertile offspring

The geographical barrier of a mountain range can lead to allopatric speciation in trees

Sympatric speciation

• Sympatric speciation takes place with?no geographical barrier
• Isolation instead occurs when random changes in the?alleles?and therefore?phenotypes?of some individuals in a population?prevent?them from successfully?breeding?with other individuals in the population
• Examples of phenotype changes that can lead to isolation include
  • Seasonal?changes
    • Some individuals in a population may develop different?mating?or?flowering?seasons to the rest of the population i.e their?reproductive timings?no longer match up
  • Mechanical changes
    • Some individuals in a population may develop changes in their?genitalia?that prevent them from?mating?successfully with individuals of the opposite sex i.e. their?reproductive body parts?no longer match up
  • Behavioural changes
    • Some individuals in a population may develop changes in their?courtship behaviours?meaning they can no longer?attract?individuals of the opposite sex for?mating?i.e. their methods of attracting a mate are no longer effective
• The populations may still?live in the same habitat?but they are isolated from each other in the sense that they?do not interbreed
• The lack of gene flow between the two populations means that allele frequencies in the gene pools of the two populations may change in different ways
• Changing allele frequencies will lead to changes in the phenotypes of the two populations
• If enough allele frequency differences arise between the two populations then they will eventually no longer be able to?breed with each other?and?produce fertile offspring, and can be said to be?separate species

E.g. sympatric speciation in fruit flies

• A population of fruit flies exists in a laboratory
• A random allele change resulting from mutation?divides the species?into?two populations
  • The allele changes leads to a change in phenotype e.g. food preference
• The difference in phenotype prevents the two populations from?interbreeding?so there is?no gene flow?between them
• Different alleles are therefore?passed on?in each population
  • This could be due to difference in selection pressure e.g. certain enzymes are advantageous for the digestion of different foods or due to random passing on of different alleles
• Different alleles become?more frequent?in each population
• Over time the divided populations?form two distinct species?that can no longer interbreed to produce fertile offspring

Isolation mechanisms other than geographical isolation can also lead to speciation

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