Natural selection and selection pressures: WACE Year 12 Biology
“Explain how natural selection acts on variation through selection pressures to change populations”
Natural selection is the process by which individuals with phenotypes better suited to the environment survive and reproduce more than others. Variation exists in a population; a selection pressure such as predation, disease or limited food gives an advantage to some phenotypes; those individuals leave more offspring; and over generations the advantageous alleles become more common, changing the population.
What this dot point is asking
SCSA wants you to lay out the logic of natural selection as a clear sequence, define a selection pressure, and apply the model to real examples to explain how a population changes over time. A strong answer keeps cause and effect explicit and uses correct allele frequency language.
The conditions for natural selection
Natural selection requires four things, and a complete answer states all of them.
- Variation: individuals in a population differ in their inherited traits.
- Heritability: at least some of that variation is genetic and can be passed on.
- Selection pressure: an environmental factor that affects survival or reproduction.
- Differential reproduction: individuals with favourable traits survive and reproduce more, passing on more of their alleles.
What a selection pressure is
A selection pressure is any environmental factor that influences which individuals survive and reproduce. Examples include predation, competition for food or mates, disease, drought, temperature and the availability of shelter. The same trait can be favoured or disfavoured depending on the pressure, so selection is always relative to the current environment.
Natural selection acts directly on the phenotype (the trait you can see), but its lasting effect is to change the frequency of alleles in the population. Over generations, alleles that build advantageous phenotypes become more common and disadvantageous alleles become rarer.
Fitness
In biology, fitness means reproductive success, not physical strength. A fit individual is one that leaves more surviving, fertile offspring. A trait that helps an organism survive is only favoured if it also leads to more offspring; survival without reproduction does not increase fitness.
Worked Australian example
Follow the selection pressure over generations
When the myxoma virus was released in Australia to control rabbits, it killed most of the population at first. But rabbits varied in their resistance, and the few with alleles for greater resistance survived and bred. Over generations the resistant allele became more common, and the rabbit population recovered. The virus was the selection pressure; differential survival of resistant rabbits changed the population's allele frequencies.
How a population changes over time
Putting the steps together: a population already contains variation produced by mutation and reshuffled by meiosis. A selection pressure makes some phenotypes more successful at surviving and reproducing. Those individuals pass on more of their alleles. After many generations the population is dominated by the favoured alleles, and the population has changed, or evolved. If the environment changes again, a different phenotype may be favoured, and the direction of selection can reverse.
Directional, stabilising and disruptive selection
Selection can push a population in different ways: directional selection favours one extreme (such as larger size), stabilising selection favours the average and removes extremes, and disruptive selection favours both extremes over the average. Recognising the type helps explain how a trait distribution shifts.
Natural selection: differential survival and reproduction based on heritable variation. Selection pressure: an environmental factor affecting survival and reproduction. Fitness: an individual's reproductive success. Adaptation: an inherited trait that improves fitness in a given environment.
Why this drives continuity and change
Natural selection links the variation generated by mutation and meiosis to the long-term survival of a species. It is not random: while the variation is random, the survival of the fittest phenotypes is non-random and directional. This is how populations stay matched to their environment, and how, over very long times, the accumulation of selected changes produces new species.
- Saying organisms change to suit the environment
- Individuals do not change their genes to adapt; selection acts on variation already present, favouring some individuals over others.
- Equating fitness with strength
- Fitness is reproductive success, not physical power.
- Calling selection random
- The variation is random, but which individuals survive and reproduce is non-random.
- Forgetting heritability
- Only heritable variation can be selected; an acquired trait is not passed on.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksA bacterial population is repeatedly exposed to an antibiotic. Over time the population becomes resistant to the antibiotic. Explain this change using natural selection, and explain why it is incorrect to say the bacteria became resistant because they needed to.Show worked answer →
A 6 mark answer needs the selection sequence plus the correction of the misconception.
- Pre-existing variation
- The bacterial population already varied in resistance because of different alleles arising from random mutation. The resistance alleles existed before the antibiotic was applied.
- Selection pressure
- The antibiotic is the selection pressure: it kills susceptible bacteria but resistant bacteria survive.
- Differential reproduction
- Surviving resistant bacteria reproduce and pass on the resistance alleles, so the next generation has a higher frequency of resistance.
- Over generations
- Repeated exposure removes susceptible cells each time, so the resistance allele frequency rises until the population is predominantly resistant.
- Why "needed to" is wrong
- The bacteria did not change their genes in response to need. Mutation is random and occurred regardless of the antibiotic; the antibiotic only selected the variants that already happened to be resistant.
Markers reward pre-existing random variation, the antibiotic as selection pressure, differential reproduction and the explicit rejection of purposeful adaptation.
WACE 2023-style4 marksExplain the biological meaning of fitness, and explain why a trait that helps an individual survive is not necessarily favoured by natural selection.Show worked answer →
A 4 mark answer needs the definition plus the survival-reproduction distinction.
Fitness. In biology, fitness is reproductive success: the number of surviving, fertile offspring an individual leaves, not physical strength.
Survival is not enough. Natural selection favours traits that increase the passing on of alleles. A trait that aids survival only raises fitness if it also leads to more offspring. An individual that survives but does not reproduce passes on no alleles, so its survival does not increase its fitness or change the gene pool.
Markers reward fitness as reproductive success and the point that selection acts through reproduction, not survival alone.