Starter Outline how variation, adaptation and selection are major components of evolution [4] Variation must exist before evolution can occur Environment can select those variations that give an advantage Selection pressure Those with advantage survive and reproduce Next generation has that advantageous characteristic Over time species becomes better adapted to its environment

Natural selection

Learning Objectives Outline how variation, adaptation and selection are major components of evolution Discuss why the evolution of pesticide resistance in insects and of drug resistance in microorganisms has implications for humans Success criteria To understand modern day examples of evolution

Evolution in action Drug resistance in microorganisms Pesticide resistance in insects

Video

Antibiotic resistance Staphylococcus aureus is a bacterium that can be found on the skin and in the throat. It causes a range of illnesses from minor skin infections to life-threatening diseases such as meningitis and septicaemia.

Antibiotic resistance and MRSA Some strains of Staphylococcus aureus have evolved become resistant to one or more of the commonly used antibiotics including methicillin. These are termed methicillin-resistant Staphylococcus aureus (MRSA). Methicillin Susceptible Staphylococcus aureus (MSSA). This form is still contained by the use of the antibiotic Methicillin. MRSA is especially prevalent in hospitals: Here patients tend to be more vulnerable to the infection i.e. older, sicker and weaker. People live together and are examined by doctors and nurses that have just touched other patients. Many antibiotic strains are used, any resistant strains therefore have an advantage.

This image shows the increase in the increased frequency of MRSA from samples in USA hospitals The genome for S. aureus was completed at the Sanger Institute and published in June 2004.Sanger Institute and published in June Comparison of the genomes for the two forms shows significant differences between MRSA and MSSA.

Antibiotic resistance

Evolution by natural selection: Applying an antibiotic to a population of bacteria applies a very strong selection pressure in favour of mutants able to resist the antibiotic. This resistance is then inherited by offspring or it can be passed on via plasmids between bacteria. The resistance gene increases in frequency in the population The species has evolved into two new forms – resistant (MRSA)and non-resistant (MSSA).

Antibiotic use Antibiotics are used very widely, sometimes unnecessarily through patient pressure. If used appropriately when a disease has been diagnosed then the infection may be eradicated. However, the patient must complete the whole course of antibiotic. Sometimes patients stop taking the antibiotic as soon as they feel well. Doing this aids the development of resistance, as some of the bacteria may still be alive at this point, and these are going to be the very ones that are most resistant to the drug’s effects. Antibiotics are sometimes used to treat viral diseases, because they may help prevent secondary bacterial infections to which the patient may be vulnerable.

Pesticide Resistance Your turn now, using texbook review pesticide resistance in insects

Plenary Questions 1. Explain why evolution occurs in a shorter time in populations of microorganisms than in populations of mammals 2. Explain why evolution tends to happen in short bursts Microorganisms have short life cycle Every generation can have mutations arise Selection pressure can change species in just a few generations In mammals life cycle is longer and mutations less frequent When environment changes this applies new selective pressures. If variations are advantageous these are selected, species changes. Once species is well adapted to new environment changes slow or stop.