17.2.3 Natural Selection: Changes in Allele Frequencies

• Evolution involves changes in allele frequencies over time
• This can be caused by natural selection
  • Selection pressures (caused by the environment) increase the likelihood that certain individuals with specific alleles survive to reproductive age, enabling them to pass on their alleles to their offspring
• In addition to natural selection it is also possible for allele frequencies to change as a result of chance; this can occur due to a process known as genetic drift
• Other processes that can cause changes in allele frequencies due to chance events include
  • The founder effect
  • The bottleneck effect

Natural selection

• When a new allele arises in a population or a change in the environment occurs then directional selection can happen
• Directional selection produces a gradual change in allele frequencies over several generations
  • There is always phenotypic variation within a population
  • There is a selection pressure in the environment, e.g. the presence of a predator
  • Some individuals in a population may have a phenotype that aids their survival in the presence of a selection pressure
    • The phenotype is produced by particular alleles
  • Individuals with the favoured phenotype are fitter and so more likely to reproduce and pass on the advantageous alleles to their offspring
  • Those who do not possess the advantageous allele or phenotype are less likely to survive and pass on their alleles to their offspring
  • So over time and several generations the frequency of the advantageous allele increases and the frequency of other alleles decreases

Genetic drift

• When a population is very small chance can affect which alleles get passed on to the next generation
  • Meiosis results in haploid gametes, meaning that a fertilisation event only passes on half of the alleles of an individual; the half that gets passed on is the result of random fertilisation, and the other half of the alleles may be lost to the next generation
• Over time some alleles can be lost or passed on purely by chance; this is genetic drift
• Genetic drift is more likely to affect allele frequencies in a small population
  • E.g. if a coin is tossed 10 times it is reasonably likely that heads will not come up at all, whereas if a coin is tossed 100 times and heads didn't come up at all you would think you had a dodgy coin!
  • In a similar way the chances of a certain allele simply being lost by chance as a result of random fertilisation is much greater if only 10 pairs of birds are breeding than if there were 100 pairs of birds breeding

Example of genetic drift in plants

• In a small population of five plants growing near a playground with a rubber floor, three of the plants have blue flowers and two of the plants have pink flowers
• By chance most of the seeds from the pink flowered plants end up on the rubber floor of the playground while all the seeds from the blue flowered plants land on fertile soil where they are able to germinate and grow
  • Note that the seeds from the pink flower do not fall on the impermeable surface because of any disadvantageous allele in the plant's genome, but purely by chance, e.g. because of a gust of wind or a passing animal
• If this happens by chance over several generations the allele for the pink flowers may be lost from this population 

The founder effect

• The founder effect occurs when a small number of individuals from a large parent population start a new population
  • The founder effect can come about as the result of chance
    • E.g. a chance event such as a storm may separate a small group of individuals from the main population
• As the new population is made up of only a few individuals from the original population only some of the total alleles from the parent population will be present
  • In other words, not all of the gene pool is present in the smaller population
• Because the population that results from the founder effect is very small it is more susceptible to the effects of genetic drift

The founder effect in lizards

• Anole lizards inhabit most Caribbean Islands and they can travel from one island to another via floating debris or vegetation
• A small number of lizards may be separated from the main population on a larger island and carried away to a smaller island by a chance event such as a large ocean wave or a storm
• The lizards arriving at a new island may only carry a small selection of alleles between them, with many more alleles present in the lizard population on the original island
  
  • E.g. the lizards on the original island could display a range of scale colours from white to yellow and the two individual lizards that arrived on the island may have white scales
    • This means that the whole population that grows on that island might only have individuals with white scales
    • In comparison the original island population has a mixture of white and yellow scaled individuals.
• If the yellow allele were recessive and present as a single copy in the original two lizards that arrived on the island, the chance of it being lost as a result of genetic drift is increased due to the small size of the gene pool

The founder effect on lizards and their scale colour.

Bottleneck effect

• The bottleneck effect is similar to the Founder effect
• It occurs when a previously large population suffers a dramatic fall in numbers
• A major environmental event can greatly reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost
• The surviving individuals end up breeding and reproducing with close relatives

Example of the bottleneck effect

• A clear example of a genetic bottleneck can be seen in cheetahs today
• Roughly 10,000 years ago there was a large and genetically diverse cheetah population
• Most of the population was suddenly killed off when the climate changed drastically at the end of the Ice Age
• As a result the surviving cheetahs were isolated in small populations and lots of inbreeding occurred
• This meant that the cheetah population today has a lack of genetic variation
• This is problematic for conservation as genetic variation within a species increases the likelihood that the species is able to respond in the event of any environmental changes
  • Remember the environment exerts a selection pressure on organisms

The bottleneck effect in cheetahs after the Ice Age.

Processes that Cause Allele Changes Table