How does natural selection lead to adaptation in populations?
Explain how natural selection acts on variation to produce adaptation over generations.
How natural selection acts on heritable variation, the role of selection pressures, and the structural, physiological, and behavioural adaptations that result, for TCE Biology Unit 4.
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What this dot point is asking
The mechanism of natural selection
Natural selection, first explained by Charles Darwin and Alfred Russel Wallace, depends on a small number of conditions all being met:
- Variation: individuals in a population differ in their characteristics.
- Heritability: some of that variation is genetic and can be passed to offspring.
- Overproduction: organisms produce more offspring than the environment can support.
- Selection pressure and competition: limited resources, predators, disease, and other factors mean not all individuals survive and reproduce.
- Differential reproductive success: individuals whose traits suit the conditions leave more offspring, so the alleles for those traits become more common over time.
The key measure is fitness, which in biology means reproductive success, not physical strength. An individual is fit if it passes on more of its alleles to the next generation.
Selection pressures
A selection pressure is any environmental factor that affects an organism's chance of survival and reproduction, and therefore drives natural selection. Examples include predation, competition for food or mates, climate, availability of water, and disease. When a selection pressure changes, the alleles that are favoured can change too, which is why populations can shift over time.
Types of adaptation
Adaptations are usually grouped into three categories:
- Structural: physical features of the body, such as the thick fur of an alpine mammal or the streamlined shape of a fast fish.
- Physiological: internal or biochemical processes, such as the production of venom, or kidneys that conserve water in a desert animal.
- Behavioural: actions or patterns of behaviour, such as migration, nocturnal activity to avoid heat, or huddling for warmth.
Selection pressure changing over time
If the environment is stable, selection tends to keep a population well adapted. If conditions change (a new predator arrives, the climate warms, a food source disappears), the traits that confer fitness change, and the population evolves in response, provided suitable variation already exists. Populations with more genetic variation are better able to adapt to change, which is one reason genetic diversity matters for survival.
Artificial selection
Humans can act as the selecting agent, choosing which individuals breed to favour particular traits. This is artificial selection, seen in farm crops, livestock, and dog breeds. It works by the same principle as natural selection (selecting heritable variation) but with a human, rather than the environment, deciding which traits are favoured. It demonstrates how quickly populations can change when selection is strong.
Exam-style practice questions
Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
TCE 20237 marksA population of bacteria is exposed to an antibiotic. Most are killed, but a few survive and the population eventually becomes resistant. Explain, step by step, how natural selection produces antibiotic resistance in the bacterial population, and explain why describing the bacteria as deliberately becoming resistant is incorrect.Show worked answer →
A 7 mark answer gives the natural-selection sequence and corrects the Lamarckian misconception.
- Step 1 - variation
- Within the bacterial population there is genetic variation; by chance, a few bacteria carry a mutation giving resistance to the antibiotic.
- Step 2 - selection pressure
- The antibiotic is a selection pressure. It kills the non-resistant bacteria, but the resistant ones survive.
- Step 3 - differential survival and reproduction
- The surviving resistant bacteria reproduce (rapidly, by binary fission), passing the resistance allele to their offspring.
- Step 4 - change in allele frequency
- Over generations the proportion of resistant bacteria rises until most of the population is resistant.
- Why not deliberate
- The bacteria do not choose to become resistant in response to the antibiotic. The resistance mutation already existed by chance before exposure; the antibiotic only selects which bacteria survive. Variation is random, selection is not.
Markers reward the variation/selection/differential reproduction/frequency-change sequence and the correction that mutation is random and pre-existing, not a deliberate response.
TCE 20215 marksDistinguish between structural, physiological and behavioural adaptations, giving an Australian example of each, and explain what is meant by a selection pressure.Show worked answer →
A 5 mark answer defines the three adaptation types with examples and defines selection pressure.
- Structural adaptation
- A physical feature. Example: the thick fur or large ears of some marsupials, or a cactus-like reduced leaf area in arid-zone plants.
- Physiological adaptation
- An internal/chemical process. Example: a desert animal producing very concentrated urine to conserve water, or kangaroos lowering metabolic rate in drought.
- Behavioural adaptation
- A learned or instinctive action. Example: nocturnal activity in desert animals to avoid daytime heat.
- Selection pressure
- An environmental factor (such as predation, climate, drought or competition) that affects an organism's chance of surviving and reproducing, so it determines which variations are favoured.
Markers reward correct definitions and a valid example for each adaptation type plus a correct definition of selection pressure.