IB Biology HLEvolution & SpeciationPaper 1 & 2~11 min read
Evolution
Species do not stay the same. Give a population enough generations and it will change — not because anything is trying to improve it, but because some of its members leave more offspring than others.
📚 What you need to know
Evolution is a change in the heritable characteristics of organisms over generations.
Only heritable characteristics matter — ones controlled by the alleles of genes and passed to the next generation.
New alleles appear by random mutation, which may make a characteristic more or less useful.
In natural selection, individuals with advantageous characteristics are more likely to survive and reproduce, so their alleles become more frequent.
Enough change can produce new species, which is why Earth has such a huge diversity of them.
Lamarck suggested acquired characteristics are inherited. He was wrong, because those changes are not in the DNA.
Natural selection is a theory: it explains a very wide range of observations and is unlikely to ever be falsified, but it cannot be formally proved.
What evolution actually means
Evolution has a definition you should be able to write out word for word.
Definition
Evolution is the change in the heritable characteristics of organisms over generations
Three words in there do a lot of work.
Heritable — the characteristic must be able to be passed on to the next generation. Heritable characteristics are controlled by the alleles of the genes an individual carries.
Organisms, plural — evolution happens to a population, not to one individual. No single organism evolves during its lifetime.
Over generations — the change is measured by comparing one generation with the next, not by watching one animal grow.
So a plant that has its leaves eaten by a caterpillar has definitely changed. But that change is not in its DNA, so its seeds are unaffected and nothing has evolved.
If a question asks whether something is evolution, ask yourself one thing: could this change be passed on in a gamete? If the answer is no, it is not evolution, no matter how dramatic the change looks.
Where the variation comes from
Natural selection cannot act on a population where every individual is identical. It needs raw material, and that raw material is variation.
Variation comes from random mutation. A mutation changes an allele, and that new allele may make its owner slightly better suited to the environment, slightly worse, or make no difference at all. Crucially, the mutation happens first, without any reference to what the organism needs. The environment does not order the mutation; it only decides what happens to it afterwards.
Natural selection, step by step
Charles Darwin worked this out after observations made on a round-the-world voyage, backed up by years of experiments and argument. The logic is only four steps long, and every exam answer about natural selection is built from them.
Green stands for one allele, grey for another. The population is not being redesigned — the mix of alleles inside it is shifting.
Written out in full, Darwin’s theory says:
Individuals in a species show a wide range of variation, caused by random mutations in their DNA.
Individuals must compete to survive, because of selection pressures such as shortage of food, predators or disease.
Individuals with characteristics best suited to the environment have a higher chance of survival, so they are more likely to reproduce.
Their advantageous alleles are passed to their offspring.
Over many generations those alleles become more frequent in the population.
The one condition. Natural selection only works if the characteristic is heritable. A helpful feature that cannot be passed on dies with the individual and changes nothing.
Lamarck: the famous wrong answer
Before Darwin published, the French scientist Jean-Baptiste Lamarck had already suggested that species change over time. His mistake was in the mechanism.
Lamarck thought that changes an organism picks up during its lifetime — called acquired characteristics — could be inherited. A body part used a lot would get bigger and stronger; one that was never used would fade away; and either way, the change would be handed on to the young.
His classic example was the giraffe. A short-necked ancestor stretches up for high leaves, its neck lengthens a little, and its offspring inherit that longer neck. Repeat for many generations and you get today’s giraffe.
The two ideas make the same prediction about giraffe necks getting longer. They completely disagree about where the change starts.
Lamarck’s ideas were incorrect because they miss heritability. Acquired characteristics are not passed on to offspring: stretching your neck, or lifting weights, changes your body but leaves the DNA in your gametes untouched.
One honest complication. The newer field of epigenetics shows that some chemical tags on DNA can occasionally be passed on. This is a real exception, but such changes are unlikely to be major drivers of natural selection, so it does not rescue Lamarck.
Why we still call it a theory
Students often hear “it’s only a theory” and think that means “it’s a guess”. In science it means almost the opposite.
Scientists gather information by observing events, then build theories that explain what they observed.
The theory of natural selection explains an enormous range of observations, from fossils to antibiotic resistance, and is widely accepted. No other reasonable explanation has been proposed, so it is likely to remain the accepted account of how species change.
A few small parts of Darwin’s original version have been corrected. He assumed evolution is always slow — but antibiotic resistance can spread through a bacterial population in months. It was also once claimed the fossil record could not support evolution, yet many fossils show intermediate forms.
Those corrections are updates to the theory, not a falsification of it.
Because evolutionary change happens over geological timescales, it is not possible to formally prove that natural selection produced today’s species. That is why the word “theory” stays.
Worked examples
WE 1
Heritable or not?
A gardener prunes a hedge into a neat shape every year for twenty years. Explain why the seeds of the hedge do not produce neatly shaped plants. (3 marks)
Point 1: name the type of change
Pruning changes the shape of the plant during its lifetime, so it is an acquired characteristic.
Point 2: link to DNA
Acquired characteristics do not change the alleles in the plant’s gametes, so the change is not heritable.
Point 3: the conclusion
Only heritable changes can be passed to offspring, so the seeds grow into normally shaped plants.
Not in the DNA, so not inherited — and so not evolutionthe marks are for the words “acquired” and “not heritable”, not for describing the hedge
WE 2
Natural selection in bacteria
Explain how a population of bacteria becomes resistant to an antibiotic. (4 marks)
Point 1: variation first
A random mutation gives a few bacteria an allele for resistance, before the antibiotic is used.
Point 2: the selection pressure
The antibiotic is the selection pressure. Non-resistant bacteria are killed.
Point 3: survival and reproduction
The resistant bacteria survive and reproduce, passing the resistance allele on.
Point 4: change in frequency
Over many generations the resistance allele becomes far more frequent, so the whole population is resistant.
Mutation first, antibiotic second — never the other way roundnever write that the antibiotic caused the mutation; that is a Lamarckian answer and loses marks
WE 3
Why Lamarck was wrong
Outline why Lamarck’s theory of evolution is not accepted. (3 marks)
Point 1: what he claimed
Lamarck proposed that characteristics acquired during an organism’s lifetime are passed to its offspring.
Point 2: the flaw
Such changes are not caused by alleles, so they lack heritability and cannot be inherited.
Point 3: what replaced it
Darwin’s theory works because the variation it acts on is already genetic, so it can be passed on.
No heritability, no evolutionyou can mention epigenetics as a minor exception, but say it is not a major driver of selection
💡 Exam tips
Learn the definition of evolution as a sentence: change in the heritable characteristics of organisms over generations.
Always start a natural selection answer with variation caused by random mutation. It is usually the first marking point.
Use the phrase selection pressure when you name what is doing the killing or the choosing.
Say alleles become more frequent, not “stronger” or “better”.
Populations evolve, individuals do not. Never write that an animal evolved a feature because it needed one.
If asked about the word “theory”, explain that it explains many observations but cannot be formally proved over geological time.
⚠ Common mistakes
Saying organisms mutate because they need to. Mutations are random and come first; the environment only selects afterwards.
Saying animals “adapt” during their lifetime. An individual can acclimatise, but adaptation of a population happens over generations.
Forgetting reproduction. Surviving is not enough — the alleles only spread if the survivor breeds.
Treating “theory” as “guess”. A scientific theory is a well-supported explanation.
Writing that the fittest are the strongest. Fitness means best suited to the current conditions, which may mean smallest, dullest or fastest to hide.
Up next: Evidence for Evolution — where the proof actually comes from, including DNA sequences, selective breeding and the bones in your own arm.
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