Natural Selection

This lesson covers: 

  1. Selection pressures and how they influence organisms
  2. The steps involved in natural selection
  3. An example of natural selection in bacteria
  4. An example of natural selection in insects

Selection pressures

All organisms encounter selection pressures, which are environmental factors that affect their survival and reproduction. The organisms that are best adapted to these selection pressures have a higher chance of passing on their traits to future generations.


Selection pressures include:

  • Predation
  • Competition for resources
  • Climate change
  • Disease


Fitness describes an organism's ability to survive and reproduce. Those with higher fitness are more likely to transfer their genes to the next generation.

How natural selection works

An allele is a variant form of a gene. Different alleles can result in different observable traits.


Genetic diversity is the total number of different alleles in a population. This leads to variation as different combinations of alleles mean different proteins are produced, resulting in different characteristics.


Genetic diversity influences natural selection as organisms with advantageous traits are more likely to survive, reproduce, and pass on their alleles, influencing the gene pool. The more successful an organism is at reproducing, the more likely its alleles will be passed on to the next generation, increasing the allele frequency.

The process of natural selection includes these key steps:

  1. There is variation in characteristics within a species.
  2. More genetic variation emerges within a population due to random mutations.
  3. Individuals with alleles that code for traits that are advantageous for survival are more likely to reproduce.
  4. These advantageous alleles are passed down to offspring.
  5. Over time, these beneficial alleles become more common in the population.


This mechanism leads to populations becoming more adapted to their environment over generations.

Modern examples of evolution

Natural selection is an ongoing process that enables organisms to adapt to environmental changes.

Antibiotic-resistant bacteria

The development of antibiotic resistance in bacteria, such as MRSA, is a clear example of natural selection in action.


This resistance emerges through natural selection as follows:

  1. Some bacteria develop random mutations that provide resistance to antibiotics.
  2. When antibiotics are used, only the resistant bacteria survive, while the others die off.
  3. The resistant bacteria reproduce, passing on resistant alleles to their offspring.
  4. Over time, the proportion of resistant alleles increases, leading to mostly resistant bacteria.


This resistance complicates treatment options, increases healthcare costs, and raises mortality rates in humans.

Pesticide-resistant insects

In agricultural settings, populations of pests like insects can develop resistance to the pesticides that could previously control them.


This resistance emerges through natural selection as follows:

  1. Some insects randomly develop a mutation providing resistance to a pesticide, so they survive while the majority die.
  2. The resistant individuals reproduce, passing on their resistance alleles to their offspring.
  3. The frequency of resistance alleles increases over generations, increasing the population's overall resistance to the pesticide.
  4. Eventually, the pest population evolves to a point where the pesticide is no longer effective.


Understanding the potential for pre-existing adaptations in pests is crucial for anticipating and managing the evolution of resistance, which unchecked can lead to increased pesticide use, environmental damage, and crop losses.