Natural selection is mathematics. Speciation is geography. Together they make new kinds of organism.
Every sub-topic below feeds at least one of these questions.
What is the evidence for evolution?
How do analogous and homologous structures exemplify commonality and diversity?
The required syllabus content for A4.1, in order. Each card is one lesson-sized checkpoint.
This definition helps to distinguish Darwinian evolution from Lamarckism. Acquired changes that are not genetic in origin are not regarded as evolution.
Sequence data gives powerful evidence of common ancestry.
Evidence for evolution from selective breeding of domesticated animals and crop plants
Evidence for evolution from homologous structures
Convergent evolution as the origin of analogous structures
Speciation by splitting of pre-existing species
Roles of reproductive isolation and differential selection in speciation
Evolution is a change in the heritable characteristics of a population over time — not a change in any one individual during its life.
Lamarck proposed that organisms acquire useful traits during their lifetime (giraffes stretch their necks, blacksmiths build big arms) and pass those traits to offspring. Not supported by genetics — acquired changes in muscle, learning, or behaviour are not inherited.
Variation already exists in a population (alleles). Nature selects individuals best adapted to survive and reproduce. Their alleles are passed on, changing the population over time. Supported by genetics — the mechanism for inheriting variation is now known.
Evolution is referred to as a theory because all scientific knowledge is provisional. The theory of evolution by natural selection predicts and explains an enormous range of observations and is unlikely ever to be falsified — but science can't formally prove it true. Theories in science are the strongest knowledge claims we have.
Sequence data gives the strongest possible evidence of common ancestry. Closely related species have nearly identical genes; distant relatives have many more differences.
"If there was any lingering doubt about the evidence from the fossil record, the study of DNA provides the strongest possible proof of our relatedness to all other living things." — Francis Collins, former director NHGRI.
The argument is straightforward: the same genes are present in organisms that have evolved from a common ancestor. Differences accumulate gradually through mutation at roughly constant rates. So the number of sequence differences between two species is a measure of how long ago they shared a common ancestor.
Humans and chimpanzees: ~1.2% of DNA differs. Humans and mice: ~15%. Humans and bananas: ~50%. The numbers fit the predicted pattern of common descent.
Selective breeding is observable evolution — humans choose desirable traits, those traits' alleles increase in frequency, and the population changes rapidly.
Canis lupus (grey wolf) was domesticated about 15,000 years ago. Through selective breeding humans have produced the modern variety of dog breeds — from Great Danes to dachshunds. All are still classified as Canis lupus familiaris, but morphologically and behaviourally they are wildly diverged.
Cabbage, broccoli, cauliflower, kale, Brussels sprouts and kohlrabi are all Brassica oleracea. Each was developed from the wild plant by selecting for different traits — terminal buds (cabbage), flowers (broccoli, cauliflower), leaves (kale), lateral buds (Brussels sprouts), stems (kohlrabi). Rapid evolutionary change visible in human history.
Homologous structures are similar in underlying design but adapted for different functions. They reveal common ancestry — and the pentadactyl limb is the canonical example.
The pentadactyl limb — five-fingered limb with a single upper bone (humerus/femur), two lower bones (radius+ulna or tibia+fibula), and a wrist/ankle of small bones leading to five digits — is shared by every tetrapod: mammals, birds, reptiles, amphibians.
If these limbs were designed from scratch for their functions, you wouldn't use the same underlying bone plan. The fact that they all share it argues that they were modified from a common ancestor's pentadactyl limb. A whale flipper has five digits because its ancestor walked.
Convergent evolution produces analogous structures — same function, different evolutionary origin. The reverse pattern of homology.
Wings are the canonical example. Bats, birds and insects all have wings — but they evolved independently:
No common winged ancestor exists for these three groups. The same selective pressure (the advantages of flight) produced superficially similar but structurally and developmentally different wings. Convergent evolution.
Speciation is the splitting of one species into two or more. It is the only way new species have ever appeared.
Speciation is gradual evolutionary change combined with the formation of a reproductive barrier — at some point in the process, the diverged populations can no longer interbreed and produce fertile offspring. They have become separate species.
Speciation needs two ingredients: reproductive isolation and differential selection. Geographical isolation is the classic route to both.
Roughly 2 million years ago, a single population of apes in central Africa was split by the formation and widening of the Congo River:
Chimpanzees and bonobos can no longer interbreed in nature. Two species from one, separated by water. A clean example of allopatric (geographic) speciation.
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Differences and similarities between sympatric and allopatric speciation
Adaptive radiation as a source of biodiversity
Barriers to hybridization and sterility of interspecific hybrids as mechanisms for preventing the mixing of alleles between species
Abrupt speciation in plants by hybridization and polyploidy
Both produce new species; the difference is whether the diverging populations are in the same place or in different places.
"Allo" = other; "patric" = homeland. Populations physically separated by a barrier — river, mountain range, ocean. The two populations can't interbreed because they can't meet. Over time, different selection pressures and accumulated mutations diverge them into separate species. Example: chimpanzees vs bonobos.
"Sym" = same. Populations occupy the same area but are reproductively isolated by behaviour, timing, ecology, or genetics. Harder to imagine, but well-documented in many groups — especially plants (polyploidy) and animals with strong mating preferences.
Eastern and Western meadowlarks — their geographic ranges overlap, and they're capable of interbreeding. But each uses a different song to attract mates. Songs determine breeding partners, and the two never reproduce. Two species, same place, behaviourally isolated.
Periodical cicadas — some species emerge to breed every 13 years; others every 17 years. They live in the same forests but rarely meet as adults. Their adult-emergence cycles are temporal barriers to interbreeding.
When a single ancestor enters a region with many empty niches, rapid diversification can produce a burst of new species — adaptive radiation.
Darwin's finches are the textbook example. A single ancestral seed-eating finch reached the Galápagos Islands. Each island offered different food sources and habitats with no existing finch competition. Populations adapted to seed-cracking, insect-eating, cactus-eating, tool use, blood-drinking. The result: at least 18 species of finch in the Galápagos archipelago today, all descended from one ancestor.
Adaptive radiation produces divergent evolution and dramatically increases biodiversity — fast — wherever vacant niches are available for a colonising lineage to fill.
Reproductive barriers between species can act before fertilisation (prezygotic) or after (postzygotic). Together, they keep species' gene pools separate.
Different songs, displays, pheromones. Individuals don't mate with the wrong species.
Different breeding seasons or different times of day for mating activity.
Species occupy different habitats within an area and rarely encounter each other.
Physical differences in reproductive structures prevent successful mating.
An embryo forms but doesn't survive to become a sexually mature adult.
Hybrid lives, but cannot produce functioning gametes. Mules (horse × donkey) — sterile because horses and donkeys have different chromosome numbers; meiosis fails.
F1 hybrids reproduce, but their offspring are inviable or infertile. Effective at preventing gene mixing after one generation.
In plants, polyploidy (extra sets of chromosomes) can produce instant speciation — a new organism with a different karyotype that can't interbreed with its parent species.
During meiosis, non-disjunction can produce gametes containing two full sets of chromosomes instead of one. If such a gamete fuses with a normal haploid gamete → triploid (3n). If two diploid gametes fuse → tetraploid (4n).
Example: the genus Persicaria (knotweeds/smartweeds) contains many polyploid species — P. maculosa is tetraploid (4n), P. nepalensis is octoploid (8n) — all descended through polyploid speciation events. Polyploid plants often produce bigger fruit, which is why many crop species (wheat, cotton, strawberry, banana) are polyploid.
If you can't define one of these in a sentence, that's where to revise next.
“How does the theory of evolution by natural selection predict and explain the unity and diversity of life on Earth?”
“What counts as strong evidence in biology?”