Natural Selection
- Junessa Masaya
- Apr 15
- 3 min read
Updated: May 20
HSC Biology | Free Study Notes
In this lesson
what variation is
what selection pressures are
how survival and reproduction affect populations
how allele frequencies change over time
What is natural selection?
Natural selection is the process by which individuals with favourable characteristics are more likely to survive and reproduce in a particular environment.
Over time, these favourable characteristics can become more common in the population.
Key idea
Natural selection does not happen because organisms “try” to change. It happens because:
variation already exists in the population
some individuals survive and reproduce more successfully than others
Variation
Variation means differences between individuals in a population.
Where variation comes from
Variation can arise from:
mutation
sexual reproduction
recombination of alleles
HSC marking guidelines note that mutation is a source of variation, and that sexual reproduction produces much more genetic variation through processes such as random assortment and crossing over.
Why variation matters
Without variation, natural selection cannot occur.
If all individuals were identical, there would be no advantage for one individual over another when conditions change.
Selection pressures
Selection pressures are factors in the environment that affect survival and reproduction.
Types of selection pressures
What selection pressures do
Selection pressures do not create variation. Instead, they act on the variation already present in the population.
They make some characteristics more advantageous than others in a given environment.
Survival and reproduction
Natural selection depends on differences in survival and reproductive success.
Survival
Individuals with favourable traits are more likely to survive long enough to reproduce.
Reproduction
If those individuals reproduce successfully, they pass their alleles on to offspring.
Why reproduction matters most
A trait only affects evolution if it influences reproductive success. An organism may survive well, but if it does not reproduce, its alleles are not passed on.
Allele frequency change
An allele frequency is how common an allele is in a population.
How natural selection changes allele frequencies
If an allele gives an advantage:
individuals with that allele are more likely to survive and reproduce
that allele is passed to more offspring
the allele becomes more common over generations
If an allele reduces survival or reproductive success:
it may become less common over time
Why this matters
A change in allele frequency in a population means evolution has occurred.
HSC marking guidelines state that changes in the gene pool are the basis of evolution, and that when the gene pool changes, evolution has occurred.
Natural selection step by step
Basic sequence
Variation exists in a population.
A selection pressure affects the population.
Some individuals are better suited to the environment.
Those individuals survive and reproduce more successfully.
Their alleles are passed on more often.
Over time, allele frequencies change.
Example of natural selection
Antibiotic resistance in bacteria
A bacterial population may contain variation because of mutation.
If antibiotics are used:
most bacteria may be killed
bacteria with resistance survive
resistant bacteria reproduce
the resistant allele becomes more common
Cane toads in Australia
Cane toads in Australia as an example of population change under selection pressures over time.
Natural selection and biodiversity
Natural selection helps explain biodiversity because it contributes to:
adaptation to different environments
changes in populations over time
speciation over long periods
This is why Module 3 links natural selection to the diversification of life and to speciation.
Important distinction
Natural selection is not the same as:
mutation
genetic drift
gene flow
Natural selection is specifically about differential survival and reproduction due to environmental pressures acting on variation.
Worked example
Exam-style question
Explain how natural selection can lead to a change in allele frequency in a population.
Worked answer
Variation exists in a population because individuals carry different alleles. If a selection pressure acts on the population, individuals with a favourable allele are more likely to survive and reproduce. They pass this allele to more offspring, so over generations the frequency of that allele increases in the population.
Why this works
This answer:
includes variation
includes selection pressure
links survival and reproduction to allele frequency change
Common mistakes
Saying natural selection creates new traits because organisms need them.
Forgetting that variation must already exist before selection occurs.
Mixing up natural selection with genetic drift.
Explaining survival without mentioning reproduction.
Saying individuals evolve, rather than populations changing over time.
Quick quiz
What is variation?
What is a selection pressure?
Why is reproduction important in natural selection?
What is an allele frequency?
How does natural selection lead to evolution?
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