A4.1 Evolution and speciation

Question 1

A4.1.1 Evolution as change in the heritable characteristics of a population

Answer 1

What evolution is – evolution is defined as a change in the heritable characteristics of a population over time.
This means that only traits encoded in DNA and passed on via gametes count as evolutionary change; if the change is not heritable, it does not qualify as evolution.

Distinguishing Darwinian evolution from Lamarckism –
In Darwinian evolution, changes in allele frequencies arise from mutation, sexual reproduction, and natural (or sexual) selection acting on genetic variation.
Lamarckism claims that traits acquired during an organism’s life (e.g., muscles from exercise, tanning, or stretched necks) can be passed to offspring; modern genetics shows these changes are not written into the DNA and therefore are not inherited.
So acquired, non‑genetic changes are not regarded as evolution, even if they affect an individual’s survival, because they don’t change the heritable gene pool.

Evolution as a scientific theory (NOS) – the theory of evolution by natural selection explains and predicts a vast range of observations (fossil record, biogeography, embryology, molecular genetics), so it is strongly supported and unlikely ever to be falsified in its core claims. However, in science, theories are never “proven” with absolute certainty by correspondence to reality; they are well‑supported, testable explanatory frameworks. That is why evolution is called a scientific theory—it is a pragmatic truth, meaning it is accepted because it best explains the available evidence and continues to make accurate predictions, not because it has been “proven true” in a philosophical sense.

Question 2

A4.1.2 Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins

Answer 2

Sequence data as evidence –
DNA and RNA base sequences, and the amino acid sequences of proteins produced from them, are inherited and therefore reflect evolutionary history. When the same gene or protein is compared across species, similarities and differences in the sequence indicate how closely related the species are: closely related species have very similar sequences, more distantly related species show more differences.

Common ancestry and homologous molecules –
For example, humans and chimpanzees share very similar DNA and amino acid sequences in many genes (e.g., about 98–99% in some protein‑coding regions), supporting the idea that they share a recent common ancestor.
Proteins such as haemoglobin or cytochrome c are found across many species; the slow, gradual accumulation of sequence changes over time produces a pattern that matches independently constructed phylogenies based on fossils and morphology.

What this shows – the pattern of sequence similarities fits the prediction of evolution: all species share common ancestry, and their molecules diverge as lineages split and accumulate mutations over time. This makes base‑ and amino‑acid‑sequence comparisons powerful, quantifiable evidence for evolution.

Question 3

A4.1.3 Evidence for evolution from selective breeding of domesticated animals and crop plants

Answer 3

Selective breeding and visible evolution – humans selectively breed individuals with desired traits (e.g., size, milk yield, coat colour, seed size, disease resistance), so those traits become more frequent over generations.
This has led to extreme differences between modern domesticated varieties and their wild ancestors: Dog breeds (e.g., chihuahua vs. great dane) differ vastly in size, shape, and behaviour, yet all belong to Canis lupus familiaris; crop varieties (e.g., modern maize vs. its wild ancestor teosinte, or broccoli, cauliflower, and kale all from Brassica oleracea) show very different structures arising from the same wild species.

How this demonstrates evolution:
The variation among breeds and varieties reflects changes in allele frequencies in populations, driven by strong selection pressure (human choice of breeding stock).
Because these changes accumulate in a few generations, they show that evolutionary change can happen quickly when selection is intense, even though the underlying genetic mechanisms (mutation, recombination, inheritance) are the same as in natural evolution.

Question 4

A4.1.4 Evidence for evolution from homologous structures

Answer 4

What homologous structures are -
Homologous structures are body parts in different species that have a similar underlying anatomy and developmental origin, even if they look different or perform different functions. These similarities are best explained by descent from a common ancestor that had that basic structure, which was then modified by natural selection for different environments.

Example: the pentadactyl limb –
A classic example is the pentadactyl limb (a limb with five main digits in the hand or foot), found in many vertebrates such as humans, cats, bats, whales, and horses. Despite very different uses (walking, swimming, flying, running), these limbs share the same basic bone pattern: one long bone (humerus/femur), two long bones (radius and ulna / tibia and fibula), a series of small bones (carpals/tarsals), long bones in the digits (metacarpals/metatarsals and phalanges).

How this supports evolution –
The fact that such a complex and detailed layout is repeated across distantly related vertebrates is strong evidence that they all inherited it from a shared tetrapod ancestor.
Over time, natural selection reshaped the pentadactyl limb for different functions (e.g., bat wings for flight, whale flippers for swimming, horse hooves for running), showing adaptive evolution while preserving the underlying homologous pattern.

Question 5

A4.1.5 Convergent evolution as the origin of analogous structures

Answer 5

What convergent evolution is –
Convergent evolution occurs when unrelated or distantly related species evolve similar traits because they face similar environmental challenges or selection pressures, not because they inherited those traits from a recent common ancestor. The result is analogous structures: structures that have the same function but different evolutionary origins and often different underlying structures.

Analogous vs homologous –
Homologous structures share a common evolutionary origin but may have different functions (e.g., human hand vs bat wing).
Analogous structures share a similar function but evolved separately (e.g., bat wing vs insect wing).
Example of analogous features
A standard example is wings of bats and wings of insects:
Both are used for flight, but
Bat wings are modified mammalian forelimbs (bones covered with skin), whereas insect wings are outgrowths of the exoskeleton with no internal bones.
Another example is the streamlined body shape of sharks (fish) and dolphins (mammals), which both help efficient swimming but evolved independently in different lineages.

Question 6

A4.1.6 Speciation by splitting of pre-existing species

Answer 6

Speciation as splitting –
Speciation is the process in which one species splits into two or more new species, usually because populations become reproductively isolated and follow independent evolutionary paths.
The IB wording that this is the only way new species have appeared means that all species are ultimately descended from earlier ones; there is no evidence for entirely new species arising “from nothing” outside the framework of descent with modification.

Effect on species number –
Each speciation event increases the total number of species on Earth, because one lineage becomes two or more separate lineages. In contrast, extinction removes species, so it decreases the total number of species. The current global diversity reflects the balance between speciation and extinction over long periods of time.

Speciation vs gradual change –
Gradual evolutionary change within a single species (e.g., slight changes in size, colour, or physiology) does not count as speciation unless it leads to the formation of reproductively isolated lineages recognizable as distinct species. In other words, microevolutionary change within a species is not the same as speciation, which is the origin of new species‑level units.

Question 7

A4.1.7 Roles of reproductive isolation and differential selection in speciation

Answer 7

Reproductive isolation in speciation – reproductive isolation means that two groups no longer interbreed successfully or at all, so their gene pools stop mixing.
This allows each group to evolve independently, leading to divergence and eventually the formation of new species.

Geographical isolation –
One common mechanism is geographical (allopatric) isolation, where a physical barrier (such as a river, mountain range, or ocean) separates a population into two or more groups. Over time, mutations, genetic drift, and selection can make the separated groups genetically distinct, and if they later come back into contact they may no longer be able to interbreed effectively.

Differential selection –
Differential selection means that the two isolated groups face different selection pressures (e.g., different predators, food sources, habitats), so natural selection favours different traits in each group. This leads to divergent adaptation and further increases genetic and phenotypic differences between the groups, reinforcing reproductive isolation.

Example: bonobos and common chimpanzees
- Bonobos (Pan paniscus) and common chimpanzees (Pan troglodytes) are closely related species that diverged from a common ancestor. The Congo River is thought to have acted as a geographical barrier, separating the ancestral chimpanzee population into two groups.
On either side of the river, different environmental conditions (e.g., vegetation and food availability) led to differential selection, resulting in bonobos evolving a more gracile, less aggressive social system while common chimpanzees retained more aggressive, male‑dominant behaviours.