Natural Selection

This is possibly the most well known concept in biology and explains how life has evolved on planet Earth.

The Process of Natural Selection

Specification Reference

"Understand how evolution can come about through natural selection acting on variation bringing about adaptations."
Evolution is defined as a long lasting change in the frequency of alleles of a single gene within a population. This processes is brought about through natural selection which is detailed below:
  1. Genetic variation occurs through meiosis and point mutations
  2. This results in new alleles being created
  3. This new allele may be advantageous
  4. The organism is now more likely to survive
  5. Leading to differential reproductive success
  6. The advantageous allele is passed on to any offspring
  7. The frequency of this allele increases
  8. Resulting in evolution
There are two different types of natural selection:

Directional Selection
  • Occurs when one selective pressure is at work
  • Selection favours an extreme phenotype
  • This causes the allele frequency to shift
Stabilising Selection
  • Occurs when two selective pressures are at work
  • This happens in well adapted populations where the most advantageous allele is also the most common
  • Deviations away from the optimum result in a disadvantage


Specification Reference

"Understand how organisms occupy niches according to physiological, behavioural and anatomical adaptations."
An ecological niche is a description of where a species can successfully exist taking into account abiotic and biotic factors.
  • Biotic - factors are living factors such as competition with other organisms.
  • Abiotic - factors are non-living factors such as temperature and pH.
After considering all of these factors it is possible to describe its niche on a graph.


An adaptation is any feature that allows an organism to survive, feed or protect itself more effectively within its environment.
  • Physiological adaptations - describe how an organism works, specifically regarding enzymes and biological pathways (extremophiles).
  • Behavioural adaptations - are instinctive decisions that animals make to increase their chance of survival (migration, group hunting and courtship rituals).
  • Anatomical adaptations - are features or form that can be seen from the outside or through dissection (beak size, blubber and hairs).


Specification Reference

"Understand how reproductive isolation can lead to allopatric and sympatric speciation."
This is the formation of a new species as a result of evolution and natural selection. It occurs when organisms become reproductively isolated. There are two types of speciation:

Allopatric Speciation

This occurs when populations become physically separated from each other. This is known as geographical isolation and stops gene flow between two populations. Natural selection will act on the isolated populations in different ways eventually changing them so much that they can no longer reproduce together successfully.

Sympatric Speciation

This occurs when two organisms inhabit the same area but are prevented from breeding in different ways. This can be because of a few reasons.

Pre-zygote Reasons
  • Inviable zygote
  • Hybrid sterility
Post-zygotic Reasons
  • Seasonal isolation
  • Temporal isolation (times of day)
  • Behavioural isolation
  • Gametic isolation

Evolutionary Race

Specification Reference

"Understand that there is an evolutionary race between pathogens and the development of medicines to treat the diseases they cause."
Natural selection also works on bacterial populations. This means there is an evolutionary race between pathogenic bacteria and the development of antibiotics that treat them.

  • The introduction of an antibiotic provides favourable conditions to resistant strains
  • These are more likely to survive thus giving rise to antibiotic resistant bacteria

Antibiotic Resistance

This is a major problem in the health sector and has been caused by the following factors:
  • Antibiotics are too widely prescribed
  • Lots are used to treat symptoms in the hope that one will work
  • Patients don't complete the full course
  • Lack of basic hygiene
  • No financial incentive for firms to develop new drugs
However, there are a number of things we can do to mitigate these problems:
  • Reduce use
  • Improve education
  • Develop new ways to target bacteria
  • Develop new antibiotics