• Species?do not stay the same over time; the species that we see around us today have developed over millions of years
  • This process of species change is known as?evolution
• Evolution can be defined as:
  • Changes in the heritable characteristics of organisms over generations
• Heritable?characteristics are those that can be?inherited by,?or?passed on to, the next generation
  • Changes in characteristics that are not inherited, e.g. a plant having its leaves eaten, do not lead to evolution
  • Heritable characteristics are determined by the?alleles?of?genes?that are present in an individual
  • Alleles may change as a result of?random?mutation, causing them to become more or less advantageous
• Heritable characteristics that are?advantageous?are?more likely to be passed on?to offspring, leading to a?gradual change?in a species over time
  • This is the process of?natural selection
• Changes in the heritable characteristics of organisms can also lead to the development of completely?new species
• The formation of new species via the process of evolution has resulted in a?great diversity of species?on Earth
  • Theoretically, at the origin of life on Earth, there would have been just?one?single species
  • This species evolved into?separate new species
  • These species would then have?divided?again, each forming new species once again
  • Over millions of years, evolution has led to countless numbers of these?speciation?events, resulting in the millions of species now present on Earth

Evolutionary change over a long period of time has resulted in a great diversity of species

• 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
• Speciation can result from?geographical isolation
  • This means that a species is separated into two populations by e.g. being on different islands or different sides of a mountain range
    • The ocean and the mountains in these examples are?geographical barriers
• This creates two populations of the same species who cannot interbreed due to being in different places;?as a result,?no exchange of genes,?or?gene flow,?will occur between them
• 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
  • 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

Example: 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 speciation in trees