5.1.5 Natural Selection Examples

• Because evolution by natural selection generally happens over millions of years, it is difficult to see it taking place, and we often have to rely on evidence from the fossil record, and evidence of common ancestry such as homologous structures and continuous variation between species
• There are, however, some examples of evolution, on a small scale, that show changes in heritable characteristics in a short time frame, e.g.
  • Finches on Daphne Major
  • Antibiotic resistance in bacteria

Finches on Daphne Major

• Daphne major is an island in the Galapagos, the wildlife of which inspired Charles Darwin to come up with his theory of evolution by natural selection
• He noticed that some of the birds of the Galapagos, identified as finches, bore a strong resemblance to each other, but that they also showed differences that were specific to each island
  • Scientists now know that Darwin’s famous finches are not technically finches at all, but they are usually still referred to as 'Darwin's finches'
• In particular, finch beak shape and size corresponded to the diet available to them on each island

Darwin noticed that Galapagos finches had beaks that were perfectly adapted to the food sources available on the island on which they lived

• Since Darwin, many evolutionary biologists have studied the wildlife of the Galapagos
• Scientists Peter and Rosemary Grant carried out a long-term study on the finch species Geospiza fortis on the island of Daphne Major
• G. fortis’ diet consists of seeds, which when weather conditions are normal are plentiful, small, and soft, but which become fewer, larger, and tougher during times of drought
• The Grants observed a wide range of beak sizes in G. fortis when weather conditions were normal, but found that during periods of drought beak size increased
  • When seeds were plentiful, small, and soft, there was no advantage for individuals with larger beaks, and so alleles for different beak sizes were passed on to G. fortis offspring in equal proportions
  • When seeds were fewer, larger, and tougher, finches with larger beaks had an advantage when competing for food, and were therefore more likely to survive and pass on the alleles for large beak size, leading to an increase in frequency of large beaks in the population

• The observation that finches with larger beaks produce large-beaked offspring while finches with smaller beaks produce smaller-beaked offspring suggests that beak size is largely determined by genes, and so is heritable
• The heritable nature of beak size means that G. fortis can adapt to a changing environment by the process of natural selection

Peter and Rosemary Grant found that average beak size in G. fortis on Daphne Major increased in drought years when seeds became larger and tougher, and decreased in wet years when seeds became smaller and softer

Antibiotic resistance in bacteria

• Antibiotics are chemical substances made by some fungi or bacteria as a defence mechanism
• They kill bacteria by targeting processes and structures that are specific to bacterial cells
  • Antibiotics are effective against bacteria but not against viruses, and usually have no effect on animal cells

• The use of antibiotics has increased significantly since they were first introduced in the 1930s, saving millions of lives
• Since their discovery and widespread use antibiotic resistance has developed in many different types of bacteria
  • Antibiotic resistant bacteria are not killed by antibiotics

• Antibiotic resistance is a heritable characteristic and so develops in bacterial populations by the process of natural selection
  • Bacteria, like all organisms, have mutations in their DNA that give rise to variation
  • A mutation may give rise to resistance to a particular antibiotic in an individual bacterial cell
  • If a bacterial infection is treated with that antibiotic, a bacterial individual with the mutation for resistance is likely to survive
    • The antibiotic in this situation acts as a selection pressure in the same way that a predator would in a rabbit population
  • The bacterial cell with the resistance mutation will reproduce by binary fission, passing on the mutation and causing antibiotic resistance to increase in frequency in the population
    • Bacterial cells are also able to transfer genes to each other by a process called horizontal gene transfer, further increasing the number of individuals with the resistance mutation

• Note that if antibiotic use stops, an antibiotic resistance mutation will no longer be advantageous, and it will not be passed on to offspring any more often than the original non-resistant form of the gene
  • Antibiotics should not be used any more often than necessary so that a selection pressure is not provided; this will reduce the likelihood of an antibiotic resistant population developing

Antibiotic-resistant bacterial populations can evolve by natural selection

• Natural selection takes place very quickly in bacterial populations because
  • Bacterial populations contain many individuals, so the chances of an advantageous mutation appearing are higher than in other types of organisms
  • They can reproduce very quickly, meaning that generation times are short and any mutations that do arise can be passed on to many offspring in a very short time
    • Bacteria can reproduce as often as every 20 minutes
• Bacteria can transfer genes horizontally, further increasing the rate at which advantageous mutations can spread

NOS: Use theories to explain natural phenomena; the theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria.

• Scientists can gather information about the world by observing events, or phenomena
• They formulate theories that seek to explain observed events
• In the case of antibiotics, it has been observed that antibiotic resistance in bacteria is on the increase
  • In particular it has been noticed that once an antibiotic starts to be used to treat a particular infection, resistance rates begin to rise
• Scientists use the theory of natural selection to explain this observation
  • Antibiotics act as a selection pressure
  • Resistant individuals are 'selected' when non-resistant bacteria are killed by treatment
  • Resistant individuals survive, reproduce, and pass on the resistance characteristic
  • Resistant individuals increase in frequency
• Understanding the mechanism by which resistance evolves means that scientists have a better chance of solving or reducing the problem
  • E.g. by reducing the selection pressure, i.e. the use of antibiotics, natural selection can be slowed down