A species facing a changing climate has three options: move, re-time, or change. The first two filled the last two pages. This page is the third one – and the useful thing about it is that we can watch it happening, in a population of owls, over a few decades.
📚 What you need to know
Natural selection is driven by selection pressures in the environment.
A selection pressure is any feature of the environment that limits the survival chances of an individual.
Climate change introduces new selection pressures, so it can drive evolution by natural selection.
Tawny owls show polymorphism: alleles give rise to different phenotypes, or morphs – grey and brown.
A long-running Finnish study found the frequency of brown owls rose from around 30 % to around 50 %.
The explanation is milder winters and less snow, which change which morph is better camouflaged.
Selection pressures
Natural selection does not happen in a vacuum. It is driven by selection pressures – and the definition is worth having ready, because questions often open by asking for it.
Definition
A selection pressure is a feature of the environment that limits the survival chances of an individual.
Examples you already know: the presence of a predator, a shortage of food, antibiotics killing bacteria. Each of these means some individuals survive and others do not, and which ones survive depends on the characteristics they happen to have.
Climate change matters here because it introduces new selection pressures, or changes the strength of existing ones. When the environment changes, the characteristics that were advantageous may stop being advantageous – and evolution by natural selection follows.
Tawny owls: evolution you can watch
Tawny owls show polymorphism. That means their alleles give rise to more than one distinct phenotype within the same species – these alternative forms are called morphs. Some tawny owls are grey; some are brown.
A decades-long Finnish study has tracked the proportions of the two morphs, and found an increase in the frequency of brown owls in the population, from around 30 % to around 50 %. That change is thought to be the result of natural selection.
Percentages, not numbers of owls. The bar is the whole population in each period.
Why grey used to be better
In a snowy environment, pale grey owls are less visible against the background. Being harder to see makes them more successful, and gives them a better chance of surviving and reproducing.
Here is a detail worth including if you want a top-band answer: it is not known whether being harder to see helps mainly with catching prey, or mainly with avoiding predators, or both. Camouflage cuts in two directions, and the study has not separated them. Saying so shows you understand the difference between the observed pattern and the proposed mechanism.
Why brown is winning now
Global warming has brought milder winters, which means less snow lies on the ground. Against a snowless winter background, brown owls have increased success – they are the better camouflaged morph now. Brown owls are therefore more likely to survive, reproduce, and pass on their alleles for brown feathers to the next generation.
Nothing here required a new mutation. The brown allele was already present in the population; the environment simply changed which allele was the advantageous one.
Use this four-step shape for any natural selection question: variation, pressure, differential survival, change in allele frequency.
Why this example is on the syllabus. Evolution is usually taught with fossils and timescales of millions of years. The tawny owl study is here because it shows natural selection producing a measurable change in allele frequency within a few decades, in response to a pressure we created ourselves.
One caution before you write about this in an exam. Evolution needs existing variation to work with, and it needs enough generations to act. A species with little variation, or a long generation time, may not be able to evolve fast enough to keep up with a changing climate. The owls are a success story. They are not proof that everything will adapt.
Worked examples
WE 1
Define and identify
Define the term selection pressure, and state one selection pressure acting on tawny owls in Finland. (2 marks)
Step 1: the definition
A selection pressure is a feature of the environment that limits the survival chances of an individual.
Step 2: the example
The amount of snow cover, which determines how visible each morph is against the background.
A feature of the environment that limits survival, plus a named one“the weather” is too vague – name the specific factor that affects survival
WE 2
Explain the change in morph frequency
Explain how global warming has caused an increase in the frequency of brown tawny owls in Finland. (4 marks)
Point 1: variation
Tawny owls show polymorphism, so grey and brown morphs already existed in the population.
Point 2: the changed environment
Global warming has brought milder winters and less snow cover.
Point 3: differential survival
Brown owls are less visible against a snowless background, so they have greater success in surviving and reproducing than grey owls.
Point 4: allele frequency
Brown owls pass on their alleles for brown feathers to more offspring, so the frequency of brown owls has risen from about 30 % to about 50 %.
Existing variation plus a new pressure equals a shift in allele frequencynever write that the owls changed colour to suit the snow – the variation came first
WE 3
Evaluate the conclusion
The researchers suggest that pale grey owls survive better in snowy conditions because they are less visible. Suggest a limitation of this conclusion. (2 marks)
Point 1: the ambiguity
It is not known whether reduced visibility improves success in catching prey or in avoiding predators, or both.
Point 2: why that matters
The data show a correlation between snow cover and morph frequency, but do not identify the mechanism producing the survival advantage.
The pattern is clear; the reason behind it is not fully establishedthis is the same correlation-and-causation thinking from the first page of the unit, applied to a different data set
💡 Exam tips
Learn the definition of selection pressure word for word.
Use polymorphism, morph, allele and phenotype correctly – they are all in this one example.
Always start a natural selection answer with variation already exists.
Quote the figures: around 30 % rising to around 50 %.
Finish with a statement about allele frequency, not just about which owls do better.
Mention the uncertainty over prey capture versus predator avoidance if asked to evaluate.
⚠ Common mistakes
Saying the owls adapted to become brown. Individuals do not adapt; populations change in composition.
Saying a mutation appeared because of the warming. The brown allele was already present.
Writing that grey owls died out. They are still around half the population.
Confusing morph with species. Both morphs are tawny owls.
Stopping at “brown owls survive better”. The mark is for the change in allele frequency that follows.
Assuming every species can evolve fast enough. It depends on existing variation and generation time.
That completes Climate Change. The five pages run in one line of argument: we strengthened the greenhouse effect, that warming is now doing measurable damage to ecosystems, some of the carbon can be put back where it came from, and in the meantime species are responding in the only ways available to them – shifting their range, shifting their timing, or shifting their allele frequencies.
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