Phenotypic Variation & Selection (College Board AP 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 favorable phenotype will be passed onto offspring
  • These individuals have high evolutionary fitness
• 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 favorable 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 favor 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 (Biston betularia) is an example of how natural selection leads to evolution over time 
• This species was prevalent around Manchester, UK in the 1850s
• The onset of industrialization in that area created changed conditions that caused natural selective pressures on the moths
• This led to the profile of phenotypes within the population changing over a period of time

Peppered Moth Evidence for Natural Selection Diagram

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

• These phenotypic variations in the peppered moth population caused an increase in fitness of the general population
• The fact that an allele existed that caused dark pigmentation allowed certain moths to remain less visible to predators
  • This contributed to the survival of the species, albeit with a different profile of phenotypes
• In the time since the Industrial Revolution, the population has swung back towards the light colored moths because of a drop in pollution that has been experienced
  • This has created a selection pressure on the darker moths, which are more visible against a lighter background