Evolution and speciation — IB Diploma Biology HL
IB Biology A4.1: evolution as heritable change, evidence from sequences, selective breeding and homologous structures, and how species split.
Revision notes
6 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.
Allopatric speciation happens when populations are separated geographically. Sympatric speciation happens in the same area, when reproductive isolation is behavioural or temporal. Both need reproductive isolation and divergence of gene pools. Adaptive radiation is the rapid evolution of many species from one ancestor, each using a different niche, so closely related species can coexist without competing.
Barriers to hybridization, such as species-specific courtship behaviour, and the sterility of interspecific hybrids prevent the mixing of alleles between species. A mule is a sterile hybrid because its chromosomes cannot pair in meiosis. In plants, hybridization followed by doubling of the chromosome number gives a fertile polyploid that is isolated from its parents: abrupt speciation, as in Persicaria.
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Quick check questions
Define evolution.
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Change in the heritable characteristics of a population.
What do fewer differences in the amino acid sequence of a protein between two species suggest?
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They shared a common ancestor more recently.
Name the five groups of bones in a pentadactyl forelimb.
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Humerus; radius and ulna; carpals; metacarpals; phalanges.
What two processes are needed for speciation?
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Reproductive isolation and differential selection.
HL only Why is a mule sterile?
Show answer
Its 63 chromosomes cannot form homologous pairs in meiosis, so it cannot make viable gametes.
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.
How can polyploidy create a new species in one generation?
HL only Two plant species hybridize, giving a sterile hybrid whose chromosomes cannot pair in meiosis. If the chromosome number doubles, every chromosome gains a partner, so the polyploid is fertile. Crosses with either parent give sterile offspring, so the polyploid is instantly reproductively isolated: a new species, as in Persicaria.
Written and checked against the IB Biology HL specification · Updated October 2026