AQA A Level Biology复习笔记7.3.7 Types of Speciation

• Evolution causes speciation: the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
• Genetic isolation between the new population and the pre-existing species population is necessary for speciation
• There are two different situations when speciation can take place:
  • Two populations of a species are separated by a geographical barrier and become genetically isolated from each other
  • Two populations of species are living in the same area (experiencing similar environmental selection pressures) but still become genetically isolated from each other

   

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 like a body of water, or a mountain range
  • It can also be man-made, like a motorway

   

• This creates two populations of the same species who are reproductively separated from each other, and as a result, no genetic exchange can occur between them
• If there are sufficient selection pressures acting to change the gene pools (and allele frequencies) within both populations then eventually these populations will diverge and form separate species
  • The changes in the alleles/genes of each population will affect the phenotypes present in both populations
  • Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally)

   

Example of Allopatric Speciation in Trees

• Imagine there is a population of trees that are all one species
• A new mountain range forms that divides the population into two
• The natural barrier prevents the two groups from interbreeding, so there is no gene flow between them
• The two populations experience different selection pressures
• Over thousands of years, the divided populations form two distinct species that can no longer interbreed

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

Sympatric Speciation

• Sympatric speciation takes place with no geographical barrier
• A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
• Something has to happen that splits or separates the two populations:
  • Ecological separation: Populations are separated because they live in different environments within the same area
    • For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering, so a population growing in soil with a slightly different pH may flower at a different time from another population, leading to reproductive separation (and eventually genetic isolation) of the two populations

     

  • Behavioural separation: Populations are separated because they have different behaviours
    • For example differences in feeding, communication or social behaviours, such as courting behaviours to attract a mate

     

   

Example of Sympatric Speciation in Fish

• A species of fish lives in a lake
• Some individuals within the population feed on the bottom while others remain higher up in the open water
• The different feeding behaviours separates the population into different environments
  • Here, behavioural separation has led to ecological separation

   

• The separated groups experience different selection pressures
  • Long jaws are advantageous for bottom-feeding whereas shorter jaws are advantageous for mid-water feeding

   

• Over time natural selection causes the populations to diverge and evolve different courtship displays
• They can no longer interbreed; they are now considered separate species

Mutations and Genetic Drift

• Both allopatric and sympatric speciation are reliant on mutations occurring within individuals in populations
• Without mutations, there are no new alleles of genes for selection to act on
• The changes in genetic material caused by mutations are important as these changes are what produce the differences in physiology, behaviour and morphology between populations over many generations, eventually leading to speciation

Genetic Drift

• In geographically isolated populations, differences in selection pressures can lead to changes in allele frequencies, and eventually to speciation due to evolution via natural selection
• However, evolution can also occur via genetic drift
  • This is when chance (instead of environmental selection pressures) affects which individuals in a population survive, breed and pass on their alleles
  • For example, when a population is significantly small, chance can affect which alleles get passed onto the next generation
  • Over time, some alleles can be lost or favoured purely by chance
  • When there is a gradual change in allele frequencies in a small population due to chance and not natural selection, genetic drift is occurring

   

• In large populations, genetic drift is less likely to have an effect because any chance variations in allele frequencies usually even out across the whole population
  • Natural selection has a much more influential role in the evolution of large populations

   

Example of Genetic Drift in Plants

• In a small population of plants growing near a playground with a rubber floor:
  • Half of the plants have alleles for blue flowers
  • Half of the plants have alleles for pink flowers

   

• By chance, most of the seeds from the pink-flowered plants end up on the rubber floor of the playground, whereas all the seeds from the blue-flowered plants land on fresh fertile soil where they are able to germinate and grow
• Over several generations, the alleles for the pink flowers may disappear from this population due to chance (because the seeds from the original population carrying pink alleles for flower colour were not able to germinate on rubber)

An example of how genetic drift can occur