IB Biology · Theme A: Unity and diversity · SL and HL
A4.1 Evolution and speciation
A one-page summary of A4.1 Evolution and speciation, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What is the evidence for evolution?
How do analogous and homologous structures exemplify commonality and diversity?
What A4.1 covers
The evidence for evolution
- A4.1.1Change in heritable characteristics
- A4.1.2Evidence from sequences
- A4.1.3Selective breeding: crops
- A4.1.3Selective breeding: animals
- A4.1.4Homologous structures: the pentadactyl limb
- A4.1.5Analogous structures: convergent evolution
How new species form
- A4.1.6Speciation is splitting
- A4.1.7Isolation and differential selection
- A4.1.7Chimpanzees and bonobos
Keeping species apart, and speciation in one step
- A4.1.8 · HLAllopatric and sympatric
- A4.1.9 · HLAdaptive radiation
- A4.1.10 · HLBarriers to hybridization
- A4.1.10 · HLSterile hybrids
- A4.1.11 · HLAbrupt speciation by polyploidy
- A4.1.11 · HLKnotweeds and smartweeds: Persicaria
A4.1 Evolution and speciation: summary
Evolution
- Change in heritable characteristics of a population.
- Acquired changes are not evolution (not Lamarckism).
Evidence
- Sequences: fewer differences → more recent common ancestor.
- Selective breeding: crop varieties and breeds show rapid change.
Homologous vs analogous
- Homologous: shared ancestry (pentadactyl limb).
- Analogous: same function, different origins (convergent evolution).
Speciation
- Splitting of a species: the only source of new species.
- Needs reproductive isolation + differential selection (Congo River apes).
HL · Modes and radiation
- Allopatric (geographic) vs sympatric (behavioural, temporal).
- Adaptive radiation: species coexist in different niches.
HL · Keeping species apart
- Courtship barriers; sterile hybrids such as the mule.
- Hybridization + polyploidy: new plant species at once (Persicaria).
Key terms
- Evolution
- change in the heritable characteristics of a population over generations.
- Selective breeding
- humans choosing which individuals with desired traits reproduce.
- Homologous structure
- shared because it was inherited from a common ancestor.
- Analogous structure
- same function, different evolutionary origin.
- Convergent evolution
- unrelated lineages evolving similar features under similar pressures.
- Speciation
- the splitting of a pre-existing species into two or more.
- Reproductive isolation
- any barrier that stops gene flow between populations.
- Differential selection
- different selection pressures favouring different alleles in each population.
- Allopatric speciation
- speciation after a geographical barrier separates populations.
- Sympatric speciation
- speciation in the same area, with no physical barrier.
- Adaptive radiation
- rapid diversification of one ancestor into species filling different niches.
- Niche
- the role of a species in its ecosystem: its habitat, food and interactions.
- Prezygotic barrier
- stops fertilisation between species, e.g. courtship behaviour.
- Postzygotic barrier
- acts after fertilisation, e.g. a sterile hybrid such as the mule.
- Hybridization
- interbreeding between two different species.
- Polyploidy
- having more than two complete sets of chromosomes.
Sample exam questions
Three of the 49 multiple-choice questions for A4.1. Try each one before opening the answer.
Question 1. Which statement best defines evolution?
- A change in an individual during its lifetime
- A change in a population's heritable traits
- The deliberate breeding of desired traits
- An increase in the number of organisms
Show the answer
Answer: B. Evolution is defined as change in the heritable (inherited) characteristics of a population across generations, not change in a single individual.
Question 2. A weightlifter develops large muscles through training. Why can this trait not be passed on to offspring by evolution?
- Large muscles are always harmful to survival
- Only traits gained after reproduction pass on
- Muscle cells do not contain any DNA at all
- It is acquired, not a change in alleles
Show the answer
Answer: D. Only heritable (genetic) variation is passed on. Muscle gained by training is an acquired characteristic and does not change the alleles carried in the gametes.
Question 3. In a hospital, the percentage of a bacterial population resistant to an antibiotic rose from 6% to 71% over several years of treatment. This is direct evidence that:
- individual bacteria learned to resist the drug
- individual bacteria altered their own DNA in response to the drug
- no genetic variation was present at the start
- the population's heritable traits changed
Show the answer
Answer: D. Resistant bacteria survived and reproduced more, so the resistance allele became more common. That is an observable change in the population's heritable characteristics — evolution.
Linking questions
Questions that connect A4.1 to other parts of the course, the kind that come up in Paper 2.
- How does natural selection change the frequency of alleles in a population over generations? (see D4.1)
- How do mutations generate the heritable variation on which evolution and speciation depend? (see D1.3)
- How are similarities in DNA base sequences and protein amino acid sequences used to work out evolutionary relationships? (see A1.2, A3.2)
- How is the classification of organisms into a hierarchy of taxa used to reflect their evolutionary history? (see A3.1, A3.2)
Practise A4.1
Study notes, every question and full markschemes for A4.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.