Natural Selection (Edexcel A (SNAB) A Level Biology)

• Variation exists within a species population
  • Variation refers to the differences between individuals. It can be interspecific (between different species) or intraspecific variation (between individuals of the same species)
• Some of this variation is due to random mutations, which introduce new alleles into a population
• This means that some individuals within the population possess different phenotypes due to genetic variation in the alleles they possess
• Environmental factors affect the chance of survival of an organism; they are said to act as a selection pressure
  • Predation, disease and competition are all examples of selection pressures, which make it difficult for all individuals in a population to survive
• Selection pressures increase the chance of individuals with a specific phenotype surviving and reproducing over others
  • The advantageous alleles that code for the favourable phenotype will be passed on to offspring
• When selection pressures act over several generations of a species, they have an effect on the frequency of alleles in a population through natural selection
  • Natural selection is the process by which individuals with a favourable phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations
• Individuals that do not possess the advantageous alleles will most likely not survive long enough to reproduce, therefore reducing competition for resources
  • This will decrease the frequency of these non-advantageous alleles over time in a population
• These changes in allele frequency will ultimately lead to evolution within the population

Evidence for evolution by natural selection

• The theory of evolution by natural selection was first suggested by Charles Darwin
  • As evidence mounted in favour of this theory, it became increasingly accepted by scientists
  • In science, evidence provides confidence in theories until the point where enough evidence is gathered in order to accept the theory as an accepted scientific explanation
• The peppered moth is an example of how natural selection leads to evolution over time 

The peppered moth is a well-known example of how natural selection leads to evolution