Evolution and speciation — IB Diploma Biology SL
IB Biology A4.1: evolution as heritable change, evidence from sequences, selective breeding and homologous structures, and how species split.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Evolution is change in the heritable characteristics of a population. The change is genetic: allele frequencies in the population shift over generations. Characteristics acquired during an organism's life, such as larger muscles or a scar, are not coded in its genes and are not inherited, so they are not evolution. This distinguishes Darwinian evolution from Lamarckism.
Base sequences in DNA or RNA and amino acid sequences in proteins give powerful evidence of common ancestry: the fewer differences between two species, the more recently they shared an ancestor. Selective breeding of domesticated animals and crop plants gives further evidence, showing that heritable characteristics of a population can change greatly and rapidly when there is selection.
Homologous structures have the same basic structure in different species because they were inherited from a common ancestor, even if their functions now differ, as in the pentadactyl limb of humans, bats and whales. Analogous structures have the same function but different evolutionary origins. They arise by convergent evolution, as in the wings of birds and insects.
Speciation is the formation of new species by the splitting of a pre-existing species, the only way new species have appeared. A population is divided by reproductive isolation, such as a geographical barrier, and the parts are then subject to differential selection. Their gene pools diverge, as with bonobos and common chimpanzees separated by the Congo River.
Exam questions
5 exam-style questions (18 marks), each with its mark scheme.
Answer the questionsFlashcards
15 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecosystems (unity and diversity) on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Define evolution.
Show answer
Change in the heritable characteristics of a population.
What do fewer differences in the amino acid sequence of a protein between two species suggest?
Show answer
They shared a common ancestor more recently.
Name the five groups of bones in a pentadactyl forelimb.
Show answer
Humerus; radius and ulna; carpals; metacarpals; phalanges.
What two processes are needed for speciation?
Show answer
Reproductive isolation and differential selection.
Frequently asked questions
Why is Lamarckism not evolution?
Lamarckism claimed that characteristics acquired during an organism's life are inherited. Acquired changes, such as larger muscles from exercise, do not alter the genes in gametes, so they cannot be passed on. Evolution is change in the heritable characteristics of a population, so only genetic changes that are passed to the next generation count.
How does DNA provide evidence for evolution?
Comparing base sequences of DNA, or amino acid sequences of proteins, shows how related species are. Mutations accumulate in each lineage after it splits, so species with fewer differences shared a common ancestor more recently. These relationships mostly match those found from anatomy, which is independent, strong evidence of common ancestry.
What is the difference between homologous and analogous structures?
Homologous structures share the same basic structure because they were inherited from a common ancestor, but they may have different functions, like the pentadactyl limbs of humans, bats and whales. Analogous structures have the same function but different origins, produced by convergent evolution, like the wings of birds and insects.
How did bonobos and chimpanzees become separate species?
About 1 to 2 million years ago an ancestral population of apes was divided by the Congo River, which they cannot cross. With no gene flow, the populations north and south of the river faced different selection pressures, including competition with gorillas in the north. Their gene pools diverged until they became two species, common chimpanzees and bonobos.
Written and checked against the IB Biology SL specification · Updated October 2026