5.24 Isolation & Speciation

• 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 genetic drift; the 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