5.1 Essential ideas

5.1.2 Natural selection

It is undeniable that species change over time, and that some species have gone extinct. The theory of natural selection provides an explanation for the general mechanism driving evolution. In order to understand natural selection, we must accept the following three conditions:

  • Species tend to produce more offspring than the environment can support.
  • There is natural and inheritable variation in the offspring produced.
  • The inheritable variation produces traits that are either advantageous or non-advantageous in that environment at that time.

When the above conditions are met, natural selection acts in the following way:

  • Individuals displaying advantageous traits are able to exploit the environment more efficiently and produce more offspring. Offspring will also display the advantageous trait.
  • Over time, more advantageous traits become more frequent in a population, and less advantageous traits become less frequent.

Natural selection is just like artificial selection (See: Page 5.1.1), but in place of a person directing and selecting mating, the environment selects the most adapted individuals by giving them a reproductive advantage – that is, more viable offspring.

Selection in action – antibiotic resistance

Antibiotics are a class of drugs used to treat bacterial infections. They function either by interfering with cell walls or membranes, thereby killing bacterial cells directly; or by interfering with protein synthesis and gene expression, thereby preventing reproduction. Within these two broad classes, there at least 100 different antibiotics, each working on a specific metabolic pathway in bacteria.

Examples of antibiotics that break down bacterial cell walls Examples of antibiotics that interfere with protein synthesis/gene expression
Penicillin, Carbapenem, Aztreonam, Bacitracin Tetracycline, Floxacin, Chloramphenacol


Since there are a number of different ways that antibiotics kill or inhibit bacteria, not all antibiotics are effective against all bacteria. In fact, during the last three to four decades, many antibiotics have become less effective against the types of bacteria they were once able to treat effectively. This is an example of natural selection in action.

This is how some bacteria have evolved resistance to antibiotics:

  • Within a population of bacteria, there is natural genetic variation – some bacteria are naturally resistant to antibiotics, some are not.
  • Before bacterial infections were treated with antibiotics, the frequency of resistant bacteria was low because there was no advantage to being resistant.
  • When antibiotics are prescribed, the non-resistant bacteria die, but the resistant bacteria reproduce. The change in the environment selects resistance.
  • The relative frequency of the resistant trait is higher in the next generation. At the same time, new variation by way of mutation is introduced. Mutation rates in bacteria are high because generation time is so short.
  • After many generations, antibiotics are no longer effective because the majority of bacteria carry the gene for resistance. There may be more than one mutation for resistance by this time.

natural selection chart

Figure 5.1.2a - Natural selection in action – a simple mechanism for the evolution of antibiotic resistant strains of bacteria.

Penicillin

Figure 5.1.2b – Penicillin: world's first widely used antibiotic

Language tools

Non-scientists often use misleading terminology when discussing evolution. Think about why the following phrases should be avoided:

Did you know?

The first widely used antibiotic was penicillin. It was a very effective treatment for syphilis and was given the nickname ‘Magic Bullet’.

International mindedness

Search ‘MRSA’ (methicillin-resistant Staphylococcus aureus) in your web browser to discover some of the devastating effects of antibiotic resistance on human lives.