Natural Selection and Adaptation - TCE Biology (Tasmania)
“Evolution: life on Earth has changed and diversified over time; mutation is the ultimate source of genetic variation; natural selection occurs when environmental selection pressures confer a selective advantage on a specific phenotype, enhancing its survival and reproduction”
Natural selection is the process where individuals with heritable variations that suit the environment survive and reproduce more than others, passing on those favourable alleles. Over generations this increases the frequency of helpful traits, producing populations that are well adapted to their environment.
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.
An adaptation is an inherited feature that increases an organism's chance of surviving and reproducing in its environment. Adaptations are the product of natural selection acting over many generations.
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.
A bacterial population varies in resistance to an antibiotic. When the antibiotic is applied (the selection pressure), non-resistant bacteria die but a few resistant individuals survive. These survivors reproduce, passing on the resistance alleles, so the next generation has a higher proportion of resistant bacteria. Over repeated exposure the whole population can become resistant. This is natural selection observed within a human lifetime.
The environment selects individuals based on their observable traits (phenotypes). Because many traits are heritable, this changes allele frequencies in the population over generations, but selection cannot directly choose alleles, only the organisms that carry them.
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.
A common error is to write that an organism develops a feature because it needs it, or that individuals adapt during their lifetime and pass on the change. Individuals do not choose to evolve. Variation already exists by chance through mutation and meiosis, and the environment selects which variants survive. Acquired characteristics, like a muscle built through exercise, are not inherited.
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.
Natural selection is part of criterion 8 (Section E of the exam), alongside mutation, gene flow and genetic drift, and the micro and macro evolutionary change covered on population genetics and speciation. The variation that selection acts on comes ultimately from mutation, reshuffled by meiosis and fertilisation.
Exam-style questions
Questions in the style of TASC 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.
Original7 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.
Original5 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.