How natural selection works at Higher Level: The same steps as SL, with different real cases: Finland's tawny owls, oak acorns and Australia's rabbits. At HL, mutation and sexual reproduction get their own statement, and human activity as a selection pressure another.
Practise this as you read
- Name each step in the guide's words: variation, overproduction, competition, survival, inheritance.
- Tie every step to the selection pressure in the question.
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It starts with differences: Natural selection can only work if individuals differ. That variation comes from the population's genetic diversity.
The points to remember
- Genetic diversity in a population gives variation: no two individuals are the same.
- New variation comes from mutation: random changes in genes (and new mixes when parents breed).
- Mutations are random: they do not happen because an animal needs them.
- Only heritable variation counts: it must be in the genes to be passed on.
Real example: a tawny owl, a woodland owl of Europe, is either grey or brown. The colour is set by its genes, stays the same for life, and passes from parents to chicks.
Not on purpose: Never write that an animal 'turned dark to hide' or 'mutated to survive'. The variation was already there, by chance; the environment then decides which variants do best.
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Every species produces more young than its environment can support. Not all of them can survive.
The points to remember
- Species overproduce: far more young are born than can survive.
- Resources are limited: food, water, space, nest sites, mates.
- So individuals compete, and many die before they can breed.
- Who wins the competition depends on their variation.
Remember it as: Too many young, too little food: only some make it.
Real example: in a good year one oak tree can drop tens of thousands of acorns. Squirrels, jays and deer eat most, and seedlings compete for light, so only a few become trees.
Use the guide's words: Name the steps as the guide does: variation, overproduction, competition for limited resources, survival and reproduction.
In the competition, the selection pressure decides who wins. Nature keeps what works best there, not the strongest.
The points to remember
- A selection pressure (predators, disease, heat, cold) acts on the population.
- Individuals with an advantageous variation are better adapted: more likely to survive.
- They also reproduce more and leave more offspring.
- Less well adapted individuals die young or breed less: selection eliminates them.
- 'Survival of the fittest': fittest = best suited to that environment.
Real example: in southern Finland, grey tawny owls survive cold, snowy winters better than brown ones. As snowy winters have become rarer, brown owls survive just as well, and they now make up a much larger share of the population.
Tie it to the pressure: In the exam, say what the pressure is in that case (the birds, the flies, the heat). A general account not linked to the question's pressure cannot get full marks.
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Winning matters only because the winners pass their genes on. Generation by generation, the population changes.
The points to remember
- Survivors pass their advantageous genes to their offspring: the variation is heritable.
- So the frequency of the advantageous gene rises in each generation.
- Over many generations the trait becomes dominant: most of the population has it.
- If the environment changes back, selection can reverse the change.
Remember it as: Survive, breed, pass it on, repeat.
Snowy winters
- Grey owls survive better.
- Grey genes passed on more.
- Grey stays most common.
Mild winters
- Brown owls survive as well.
- Brown genes are passed on too.
- Brown share rises.
Real example: decades of records of Finnish tawny owls show the brown form becoming steadily more common as winters warmed: the frequency of the brown gene rose over many generations.
Exam questions ask how a population could evolve one trait: resistance, heat tolerance, stripes. Use the same chain every time.
Remember it as: Differ, too many, compete, survive, pass it on.
Variation
- Australia's wild rabbits varied: a few carried genes that resisted the myxoma virus.
Pressure
- In 1950 the virus was released; it killed over 99% of infected rabbits at first.
Survive
- Resistant rabbits survived and, breeding fast, left many young.
Inherit
- Their young inherited the resistance.
Spread
- Within decades far fewer rabbits died of the virus: resistance was common.
Two traps: Answer the question asked: if it asks how one trait evolves, do not drift into how new species form. And end with the spread: 'over many generations the trait becomes more common in the population'.
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How this comes up: Paper 2, Section B (a): outline how a species may evolve a greater tolerance to a condition [4].
As the Mediterranean Sea warms, some populations of seagrass are exposed to water temperatures above 30 °C in summer.
Outline the processes by which a species may evolve a greater tolerance to higher temperatures.
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