Evolution of Species

• Mutations can arise naturally or they can be increased by mutagenic agents, e.g. radiation, X-rays, UV radiation or chemicals such as mustard gas.
• Mutagenic agents can be used by geneticists to try to create new mutant varieties of organisms that are useful for humans.
• Mutations may be neutral, confer an advantage or a disadvantage to survival. Over time advantageous mutations result in variation within a population.
• Advantageous mutations result in an improvement in

  the organisms functioning or appearance.
• Advantageous mutations are very rare but very important and can lead to the evolution of new species.
• Disadvantageous mutations cause the organism harm, or makes it harder for the organism to survive.
• An adaptation can be structural, behavioural or physiological.
• Behavioural Adaptations are how species respond to environmental stimuli in different ways, e.g. plants grow towards light, woodlice move away from light.
• Physiological Adaptations are changes within the cells or tissues of an organism in response to environmental stimuli resulting in an improved

  ability to cope with changing environment.
• Variation within a population can be very beneficial to organisms living in a changing environment. If the allele they possess gives them a better chance of survival, it will allow them to survive long enough to pass this onto the next generation. This will lead to a population evolve over time to become better adapted to their new environment.
• Natural selection is one of the basic mechanisms of evolution.
• When a population grows too big, selection pressures will act to control its size.
• Examples of selection pressures are

  
  

  Temperature: some organisms might be better adapted to colder temperatures

  
  

  Predators: some organisms might be better adapted to camouflaged and avoid predators
• Natural Selection:

  
  

  More offspring are produced than the

  environment can sustain.

  
  

  Variation exists between members of

  a species.

  
  

  The best adapted survive.

  
  

  Those who survive reproduce.

  
  

  Favourable alleles that confer a

  selective advantage are passed on.

  
  

  Over many generations the alleles

  increase in frequency within the

  population.
• The total of all the different genes in a

  population is the gene pool.
• There are 5 stages in speciation.

  
  

  A large interbreeding population of one species exists- sharing the same gene pool.
• Stage 2

  
  

  Random mutations in each sub-population give rise to new variation within each group (but not shared by both groups).
• Stage 3

  
  

  Natural selection selects for different mutations in each group due to different selection pressures.
• Different selection pressures act on each sub-population

  depending on local conditions such as:

  
  

  Climate

  
  

  Predators

  
  

  Disease.
• Stage 4

  
  

  Over a very, very long time……

  
  

  Stage 3 repeats causing the two gene pools to become so altered that the groups become genetically distinct & isolated.
• Stage 5

  
  

  If the original barrier is removed, the two sub-populations are no longer able to interbreed. Speciation has occurred.
• Speciation is the formation of two or more species from the original one.
• Populations first need to be isolated from each other so that gene pools can diverge.
• The isolated populations develop different characteristics & eventually will no longer be able to breed with each other.
• Ecological Barriers


  e.g. changes in temperature, humidity,

  pH, salinity, altitude
• Behavioural Barriers

  Examples include:

  
  

  -Active at different times of the day.

  
  

  -Breeding seasons out of sync.

  
  

  -Different courtship behaviours

  
  

  >Bird Song

  
  

  >Mating Dance or Ritual