Evolution is change in the gene pool of a population over many generations. Comparing DNA sequences shows how closely species are related, and separating a population into groups that cannot interbreed explains where new species come from.
The theory of evolution states that new species form from pre-existing species over time, as a result of changes to gene pools from generation to generation, driven by mutation, natural selection and genetic drift. DNA sequence data give evidence of relationships: mutations accumulate at a roughly steady rate, so the fewer the differences between the base sequences of two species, the more recently they shared a common ancestor. A species is a group of organisms that can interbreed to produce fertile offspring. Speciation needs genetic isolation, so that there is no gene flow between two populations. In allopatric speciation a geographical barrier separates them; in sympatric speciation they live in the same area but are separated ecologically (different habitats or food) or behaviourally (different courtship or breeding times). Different mutations, selection pressures and drift then change each gene pool until the populations can no longer interbreed.
Fig. 1: an evolutionary tree built from the number of base differences, as in the simulator; fewer differences means a more recent common ancestor
Section 01
Reading relationships in DNA.
Compare the five 20-base sequences. Highlighted bases differ from the human sequence, and the table counts differences for every pair. Press Build the tree.
Evolution: the gene pool of a population changes over many generations. When the changes are large enough that a population can no longer interbreed with the original one, a new species has formed from a pre-existing species.
Why DNA works as evidence: all species inherit DNA from their ancestors. After two lineages split, mutations accumulate separately in each, at a roughly constant rate, so the number of base differences increases with the time since divergence (a molecular clock).
Reading the data: in the simulator, human and chimpanzee differ by 1 base, gorilla by 2, orangutan by 4 and mouse by 9. Human and chimpanzee share the most recent common ancestor; mouse branched off longest ago.
Other molecular data: amino acid sequences of proteins such as cytochrome c or haemoglobin, and mitochondrial DNA (inherited through the mother, with a fast mutation rate) are compared in the same way.
Advantage: molecular data are objective and quantitative, and can show relationships that appearance hides, for example where similar features evolved separately in unrelated groups.
Examiner trap
Humans did not evolve from chimpanzees. The tree shows that humans and chimpanzees share a common ancestor more recently than either does with gorillas; both lineages have changed since the split.
Section 02
Barriers to gene flow.
Choose allopatric and press Run 2000 generations: a river forms and the two halves change colour separately. Watch CAN INTERBREED? change from yes to less often to no: new species. Then choose sympatric.
Feature
Allopatric speciation
Sympatric speciation
where
populations in different places
populations in the same area
isolating mechanism
geographical barrier: river, mountain range, sea, desert
ecological (different habitat, host or food) or behavioural (courtship, breeding time)
example
populations on different islands
insects feeding and mating on different host plants
Step 1, isolation: a barrier stops gene flow, so the two populations no longer share a gene pool.
Step 2, divergence: different mutations arise in each; different environments give different selection pressures; genetic drift acts separately, especially in small populations.
Step 3, reproductive isolation: allele frequencies change so much that, even if the populations meet again, they cannot interbreed to produce fertile offspring, for example because courtship, breeding season, genitalia or chromosomes no longer match.
Sympatric example: if some insects feed and mate only on one host plant and others only on another, there is little gene flow between the groups even though they share the area, so they can diverge into separate species.
Examiner trap
Separation alone does not make a new species. The populations must become reproductively isolated: unable to interbreed to produce fertile offspring. Two populations separated briefly that can still interbreed are still one species.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the simulator's difference table, how many bases differ between the human and gorilla sequences?
Quick check+10 XP
When you press Build the tree, which pair is shown sharing the most recent common ancestor?
Quick check+10 XP
In the allopatric simulation, what stops gene flow between the two halves of the population?
Quick check+10 XP
In the sympatric simulation, the population stays in one area. What separates the two groups?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
The amino acid sequence of cytochrome c differs by 1 amino acid between species P and Q, by 10 between P and R, and by 11 between Q and R. Which conclusion is best supported?
Exam style+20 XP
Which is essential for speciation to occur?
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Two populations of crickets live in the same field. One sings and mates in spring and the other in late summer. Over time they become separate species. This is an example of:
Exam style+20 XP
How do two isolated populations come to have different gene pools?
Skill unlocked
Evolution and speciation, mastered.
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