Evolution: core idea
Evolution = change in the heritable characteristics of a population over time.
Only genetic / heritable changes count as evolution; acquired characteristics do not.
This helps distinguish Darwinian evolution from Lamarckism.
In exam answers, link evolution to populations, inheritance, and change over generations.
Natural selection theory is strongly supported because it predicts and explains a wide range of observations.
Evidence for evolution
DNA / RNA base sequences and amino acid sequences provide strong evidence for common ancestry.
The more similar the sequence, the more closely related the organisms are likely to be.
Selective breeding in domesticated animals and crop plants shows that inherited traits can change markedly over relatively short times.
This is evidence that evolutionary change can occur when selection acts on heritable variation.
Strong exam line: molecular evidence + artificial selection both support evolution.
Homologous structures
Homologous structures have a similar underlying anatomy because of common ancestry, even if their functions differ.
Required example: pentadactyl limbs.
Classic exam idea: same basic bone plan, different functions in different organisms.
Example uses: human arm, whale flipper, bat wing, dog forelimb.
Exam tip: if asked why homologous structures support evolution, say they show descent from a common ancestor.
This diagram shows that very different limbs still share the same basic structural pattern, supporting common ancestry. It is ideal for revising homologous structures and the pentadactyl limb idea. Source
Analogous structures and convergent evolution
Convergent evolution occurs when unrelated organisms evolve similar features because they face similar selection pressures.
Analogous structures have similar functions but different evolutionary origins.
This means similarity does not always mean close evolutionary relationship.
Exam contrast:
Homologous = same origin, may have different function.
Analogous = different origin, similar function.
Example of analogous features: wings of birds and insects.
This image helps you quickly see the difference between same ancestry and same function. It is especially useful for avoiding the common exam mistake of confusing homology with analogy. Source
Speciation: how new species form
Speciation = the splitting of one pre-existing species into two or more species.
It is the only way new species appear.
Speciation increases total species number; extinction decreases it.
Gradual evolutionary change within one species is not speciation.
Speciation usually happens gradually as populations become increasingly different.
Key exam phrase: reproductive isolation + differential selection drive speciation.
Reproductive isolation and differential selection
Reproductive isolation prevents or reduces gene flow between populations.
Once populations are isolated, different environments can impose different selection pressures.
This leads to divergence in traits over time.
Required example: bonobos and common chimpanzees became separated by the Congo River.
The river acted as a geographical barrier, allowing differential selection to produce divergence.
Exam tip: always explain both parts: isolation first, then selection causes divergence.
This image clearly shows allopatric speciation: isolation stops gene flow, then populations diverge. It matches the syllabus focus on geographical isolation in speciation. Source
HL only: allopatric vs sympatric speciation
Allopatric speciation: populations become isolated by a geographical barrier.
Sympatric speciation: speciation occurs without geographic separation.
In both cases, reproductive isolation must evolve.
Forms of reproductive isolation required by the syllabus:
Geographic isolation
Behavioural isolation
Temporal isolation
Similarity: both produce separate gene pools and eventually new species.
Difference: allopatric starts with physical separation; sympatric does not.
This diagram is useful because it shows how sympatric speciation can happen without a physical barrier. It also directly supports the HL requirement for different forms of reproductive isolation. Source
HL only: adaptive radiation
Adaptive radiation = rapid divergence of a common ancestor into multiple closely related species.
It happens when there are vacant niches and reduced competition.
It increases biodiversity because closely related species can coexist by using different niches / resources.
Think: one ancestor -> many species, each adapted to a different way of life.
Common revision example: Darwin’s finches.
This image shows how closely related species can diversify to exploit different ecological niches. It is a classic example of adaptive radiation and biodiversity increase. Source
HL only: barriers to hybridization and plant speciation
Barriers to hybridization prevent the mixing of alleles between species.
Example: courtship behaviour can stop mating between animal species.
Even if hybridization occurs, hybrids may be sterile.
Required example: mule = sterile hybrid.
In plants, speciation can occur abruptly by hybridization and polyploidy.
Polyploidy = extra sets of chromosomes; this can instantly create reproductive isolation.
Required plant example: knotweed / smartweed (Persicaria).
Exam tip: plant speciation can be much faster than animal speciation.
Exam-ready comparisons
Evolution vs speciation:
Evolution = heritable change in a population.
Speciation = splitting of one species into new species.
Homologous vs analogous:
Homologous = common ancestry, similar structure.
Analogous = convergent evolution, similar function.
Allopatric vs sympatric:
Allopatric = geographic isolation.
Sympatric = no geographic barrier.
Hybridization vs polyploidy:
Hybridization = crossing between species.
Polyploidy = extra chromosome sets that can create instant reproductive isolation.
Checklist: can you do this?
Define evolution and explain why acquired characteristics are not evolution.
Use DNA / protein sequence similarity as evidence for common ancestry.
Distinguish homologous and analogous structures and give one example of each.
Explain speciation using reproductive isolation and differential selection.
Compare allopatric and sympatric speciation, and explain polyploidy in plants.
Fast exam traps to avoid
Do not define evolution as change in an individual; it is change in a population.
Do not say all similarities are due to common ancestry; some are due to convergent evolution.
Do not confuse evolution within a species with speciation.
Do not forget that reproductive isolation is central to speciation.
Do not ignore molecular evidence; sequence data is one of the strongest forms of evidence.
One-paragraph exam summary
Evolution is the change in heritable characteristics of a population over time. Evidence includes DNA / RNA and amino acid sequence similarities, selective breeding, and homologous structures such as pentadactyl limbs. Analogous structures arise by convergent evolution and show similar function without common origin. Speciation occurs when a pre-existing species splits, usually because reproductive isolation prevents gene flow and different selection pressures cause divergence. At HL, students must also know allopatric vs sympatric speciation, adaptive radiation, and rapid plant speciation by hybridization and polyploidy.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.