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IB DemystifiedMYP Sciences
Variation, natural selection and evolution
Evolution is not only in fossils: it happens in hospitals, farms and gardens today. Differences between individuals, and which ones survive, are enough to change a species.
Recommended for MYP 5 · eAssessment priority · About 4 lessons · Criteria A, B, C and D
Figure 1. Natural selection in action: an antibiotic leaves only resistant bacteria to reproduce.
describe continuous and discontinuous variation, and explain where new variation comes from
explain evolution by natural selection, step by step
explain antibiotic and pesticide resistance as natural selection
evaluate evidence for evolution, including fossils and DNA
explain how new species form, and why species become extinct
compare natural selection with selective breeding
Before you start
You will use these skills. If any feel shaky, review them first.
genes, alleles and mutations (see DNA, genes and inheritance)
how bacteria reproduce by dividing
percentages and percentage change
Key vocabulary
Variation
Differences between individuals of the same species, caused by genes, environment, or both.
Natural selection
The process by which individuals better suited to their environment survive and reproduce more, passing on their alleles.
Selection pressure
A factor, such as a predator, disease or shortage of food, that affects which individuals survive.
Adaptation
An inherited characteristic that increases the chance of surviving and reproducing.
Species
A group of organisms that can interbreed to produce fertile offspring.
Evolution
The change in the inherited characteristics of a population over many generations.
Understanding evolution
What is it?
Evolution is the change in a population's inherited characteristics over generations. Natural selection is the main process that drives it: variation, a selection pressure, survival and reproduction of the best suited, and inheritance of their alleles.
Why does it happen?
Mutations create new alleles at random. More offspring are produced than can survive, so individuals whose alleles suit the environment are more likely to survive and breed. Their alleles become more common. Individuals do not change to survive; populations change because some individuals survive and others do not.
How do we know?
Fossils in rocks of different ages show life changing over time. DNA shows closely related species share more similar sequences. And evolution has been measured directly: finch beaks deepened after a drought, peppered moths changed colour as pollution rose and fell, and bacteria evolve resistance within years.
Why does it matter?
Antibiotic resistance is one of the biggest threats to modern medicine. Evolution also explains pesticide resistance, why flu vaccines change each year, and why species with low genetic variation are at risk of extinction.
What does it connect to?
Evolution unites all of biology: genetics, ecology, classification and disease. It connects to Earth science through fossils and rock layers, and to mathematics through exponential growth and statistics.
Evidence and new species
Fossils: rock layers of different ages preserve changing forms of life.
DNA: the more closely related two species are, the more similar their DNA. Humans and chimpanzees share a recent common ancestor; chimpanzees are relatives, not ancestors.
Observed evolution: antibiotic resistance, pesticide resistance, and the Galápagos finches.
New species form when populations are isolated, for example on different islands. Different mutations and selection pressures act on each, until they can no longer interbreed to produce fertile offspring.
Selective breeding works the same way, except that humans, not the environment, choose which individuals breed.
Evolution in the real world
Doctors track which antibiotics still work against local bacteria before prescribing. Conservationists manage the genetic variation of endangered species through breeding programmes. Farmers rotate pesticides and plant refuges of unsprayed crops to slow the evolution of resistant pests.
Worked examples
Example 1: explaining an adaptation
Explain how Arctic hares came to have white winter fur.
Variation: in an ancestral population, fur colour varied because of different alleles.
Selection: in snowy conditions, hares with paler fur were harder for predators to see, so more survived.
Inheritance: they reproduced and passed on alleles for pale fur; over many generations, white winter fur became the norm.
Example 2: percentage change in a population
In a population of 400 beetles, 60 are green. Ten years later, 150 of 500 are green. Has the proportion of green beetles increased?
At first: 60 ÷ 400 × 100 = 15%.
Later: 150 ÷ 500 × 100 = 30%.
The proportion doubled, from 15% to 30%, which could suggest that green beetles are being favoured by selection.
In the eAssessment
Evolution questions often give an unfamiliar organism and ask you to apply natural selection. Expect:
Explain: use the full sequence: variation (from mutation), selection pressure, survival, reproduction, alleles passed on, over many generations.
Interpret: population data over time, such as resistance percentages or proportions of a form.
Evaluate: whether data prove a cause, and whether a claim (“humans evolved from chimpanzees”) matches the evidence.
Discuss: antibiotic use, conservation and resistance, weighing health, economic and ethical points.
Common ways to lose marks: saying individuals adapt or “get used to” something; suggesting organisms change because they need to; leaving out inheritance of alleles; and saying people become resistant to antibiotics (it is the bacteria).
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: does camouflage affect survival?
Partially guided investigation · about 40 minutes · outdoors, whole class
Research question
Does the colour of model “prey” (green or red cocktail sticks) affect how many are found by “predators” (students) on a grass area in 30 seconds?
Scientific background
Many predators hunt by sight. Prey that blend into their background are harder to find, so they are more likely to survive and reproduce.
Hypothesis
Write your own hypothesis, with a justification using natural selection.
Variables
Identify the independent and dependent variables, and at least three control variables, explaining how and why you will control each.
Apparatus
50 green and 50 red cocktail sticks, a marked-out square of grass (for example 5 m × 5 m), stopwatch, tally chart.
Method
A student who is not a predator scatters all 100 sticks randomly across the square.
Each predator in turn has 30 seconds to pick up as many sticks as they can, one at a time.
Record how many of each colour are found, then return them and re-scatter.
Repeat with at least five different predators.
Calculate the mean number of each colour found, and the percentage of each colour that “survived”.
Safety. Check the area for hazards first, pick up sticks carefully (they have points), and collect every stick at the end so none are left behind.
Then evaluate: in what ways is this model like real predation, and in what ways is it not?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
