Unit 12 Evolution and speciation
Key unit competence: Evaluate the relevance of theories of evolution that brought about speciation.
Key ideas
- Hypotheses on the origin of life: special creation (life created by a supernatural being); spontaneous generation (life from non-living matter — disproved by Redi and Pasteur's swan-necked flask experiment); cosmozoan/panspermia (life came from space); biochemical evolution (Oparin–Haldane): the early atmosphere had no free O₂ but contained methane, ammonia, hydrogen and water vapour; energy from lightning and UV formed simple organic molecules (the "primordial soup"). Miller–Urey (1953) passed sparks through these gases and produced amino acids. Polymers, membranes (coacervates) and self-replicating RNA may have followed, leading to the first prokaryotic cells.
- Evolution of multicellular organisms: first prokaryotes (about 3.5 billion years ago); photosynthetic cyanobacteria released oxygen; eukaryotes arose about 2 billion years ago, possibly by endosymbiosis (mitochondria and chloroplasts were once free-living bacteria — they have their own circular DNA, 70S ribosomes and double membranes); then multicellular organisms with cell specialisation; colonisation of land by plants and animals.
- Lamarckism: use and disuse (organs used often develop, unused ones degenerate) and inheritance of acquired characteristics (e.g. giraffes stretched their necks and passed longer necks to offspring). Rejected because acquired changes in body cells do not alter the DNA of gametes (Weismann cut mice tails for generations; tails did not shorten).
- Darwinism (Darwin and Wallace, 1859, On the Origin of Species): based on observations — overproduction, variation, constant population sizes, inheritance; conclusions — struggle for existence, survival of the fittest, natural selection, gradual change leading to new species. Darwin was influenced by Malthus (populations outgrow resources), Lyell (old Earth) and observations of Galápagos finches with beaks adapted to different foods. Neo-Darwinism (modern synthesis) combines Darwin's theory with Mendelian genetics and population genetics: variation comes from mutation and recombination; evolution is a change in allele frequencies in a gene pool. Creation theory is a religious belief, not testable by scientific methods.
- Evidence for evolution: fossils — preserved remains or traces in sedimentary rock; older layers are deeper; dated by radiometric dating (e.g. carbon-14, potassium-argon); show change over time and transitional forms (Archaeopteryx between reptiles and birds; horse evolution — increase in size, reduction in toes); limitations: incomplete record, soft bodies rarely fossilise. Comparative anatomy: homologous structures — same basic structure, different functions (pentadactyl limb in human arm, bat wing, whale flipper) → common ancestry, divergent evolution; analogous structures — different structures, same function (wings of insects and birds) → convergent evolution; vestigial organs (human appendix, coccyx, wisdom teeth). Comparative embryology: vertebrate embryos have gill slits (pharyngeal pouches) and tails at early stages.
- Molecular evidence: comparisons of DNA base sequences, amino acid sequences of proteins (cytochrome c, haemoglobin) and immunological reactions: the more similar the sequences, the more closely related the species (humans and chimpanzees share about 98–99% of DNA). The universal genetic code and shared biochemistry (ATP, ribosomes) suggest a common origin. Present-day (ongoing) evolution: antibiotic resistance in bacteria (e.g. MRSA), insecticide resistance in mosquitoes (DDT, pyrethroids), warfarin resistance in rats, industrial melanism, herbicide-resistant weeds, HIV drug resistance.
- Causes of evolutionary change: competition and environmental change (selection pressures), mutations, gene recombination (meiosis, fertilisation), migration (gene flow), genetic drift, industrialisation (pollution), chemical resistance, artificial selection.
- A species is a group of organisms with similar features that can interbreed to produce fertile offspring. Speciation is the formation of new species. It requires reproductive isolation so that gene flow stops and populations evolve separately. Allopatric speciation: populations separated by a geographical barrier (mountains, rivers, seas, continental drift); different mutations and selection pressures act; eventually they cannot interbreed. Sympatric speciation: in the same area, by behavioural isolation (different courtship, e.g. cichlid fish in Lake Victoria chose mates by colour), ecological/habitat isolation, temporal isolation (different breeding seasons), mechanical isolation, or polyploidy in plants (e.g. bread wheat, cord grass Spartina anglica).
- Isolating mechanisms: prezygotic (prevent mating or fertilisation: geographical, ecological, temporal, behavioural, mechanical, gametic) and postzygotic (hybrid inviability, hybrid sterility — e.g. a mule from horse × donkey is sterile because its chromosomes cannot pair in meiosis — hybrid breakdown). Patterns: adaptive radiation — one ancestral species evolves into many species filling different niches (Darwin's finches, cichlid fishes of the African Great Lakes, marsupials in Australia); divergent evolution — related species become different (homologous structures); convergent evolution — unrelated species become similar because of similar environments (dolphin and shark body shape; cactus and euphorbia); migration can bring new alleles or found new populations.
Common misconceptions
- "Humans evolved from chimpanzees." No: humans and chimpanzees share a common ancestor.
- "Evolution means individuals change during their lives." No: populations evolve as allele frequencies change over generations.
- "Lamarck's giraffes are an example of natural selection." No: Lamarck proposed inheritance of acquired traits; Darwin explained long necks by selection of existing variation.
- "The mule is a new species." No: it is sterile, so horses and donkeys remain separate species.
Worked examples
Example 1. Cytochrome c of a human differs from a chimpanzee's in 0 amino acids, a horse's in 12 and a yeast's in 44. What can be concluded?
Solution
The fewer the differences, the more recent the common ancestor. Humans and chimpanzees are most closely related; horses less so; yeast is the most distantly related. Molecular data support the evolutionary relationships shown by anatomy and fossils.
Example 2. Explain how two populations of a fish species could become separate species after a lake splits into two.
Solution
The populations are geographically isolated, so there is no gene flow. Different mutations arise, and different selection pressures (food, predators) and genetic drift act in each lake. Allele frequencies diverge; over many generations, changes in courtship, breeding time or chromosomes cause reproductive isolation. If they meet again and cannot produce fertile offspring, they are separate species (allopatric speciation).
Practice questions
Describe the Miller–Urey experiment and its significance.
Show answer
Gases methane, ammonia, hydrogen and water vapour circulated in a sealed apparatus; electric sparks as lightning; condensed liquid contained amino acids; shows organic molecules can form from inorganic ones under early-Earth conditions.
Common mistake: Saying it produced living cells.
Compare Lamarckism and Darwinism using giraffes.
Show answer
Lamarck: giraffes stretched necks, acquired longer necks, passed them on. Darwin: giraffes varied in neck length; those with longer necks reached more food, survived and reproduced, passing on alleles.
Common mistake: Mixing the two theories.
Explain how fossils provide evidence for evolution.
Show answer
Show organisms that lived in the past; deeper layers older; changes over time; transitional forms like Archaeopteryx; sequences like horse evolution; extinct species.
Common mistake: Saying all organisms are fossilised.
Distinguish homologous and analogous structures.
Show answer
Homologous: same basic structure, different functions, common ancestry, divergent evolution (pentadactyl limb). Analogous: different structure, same function, no close common ancestry, convergent evolution (insect and bird wings).
Common mistake: Swapping the definitions.
Explain molecular evidence for evolution.
Show answer
Compare DNA and protein sequences; similar sequences show close relationships; number of differences reflects time since divergence; universal genetic code; DNA hybridisation and immunological tests.
Common mistake: Not mentioning sequences.
Define species and explain allopatric speciation.
Show answer
Species: interbreed to give fertile offspring. Allopatric: geographical barrier separates populations; no gene flow; different mutations and selection; reproductive isolation develops; new species.
Common mistake: Leaving out reproductive isolation.
Describe four isolating mechanisms.
Show answer
Geographical; ecological (different habitats); temporal (different breeding times); behavioural (different courtship); mechanical (incompatible organs); gametic; hybrid sterility.
Common mistake: Listing only geographical.
Give four examples of present-day evolution.
Show answer
Antibiotic resistance in bacteria; insecticide resistance in mosquitoes; warfarin-resistant rats; peppered moth melanism; herbicide-resistant weeds; HIV drug resistance.
Common mistake: Giving extinct examples.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A mule cannot reproduce because it is: a. sterile 2. Darwin published On the Origin of Species in: b. 1859 3. The human arm, bat wing and whale flipper are: c. convergent evolution 4. Sharks and dolphins have similar shapes due to: d. homologous structures Show answer
1 → a (sterile) · 2 → b (1859) · 3 → d (homologous structures) · 4 → c (convergent evolution)
- Hybrid sterility.
- With Wallace's ideas.
- Pentadactyl limb.
- Same environment.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
endosymbiotic bacteria · radiometric dating · adaptive radiation · convergent evolution- Many species evolving from one ancestor to fill niches is .
- Fossils are dated using .
- Mitochondria and chloroplasts are thought to come from .
Show answer
1: adaptive radiation · 2: radiometric dating · 3: endosymbiotic bacteria
- Darwin's finches.
- Radioactive decay.
- They have their own DNA.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A fossil showing features of reptiles and birds is: a. amino acids from simple gases 2. Vertebrate embryos all show: b. Archaeopteryx 3. Pasteur's swan-necked flask experiment disproved: c. pharyngeal pouches 4. Miller and Urey produced: d. spontaneous generation Show answer
1 → b (Archaeopteryx) · 2 → c (pharyngeal pouches) · 3 → d (spontaneous generation) · 4 → a (amino acids from simple gases)
- Transitional form.
- Common ancestry.
- Microbes came from the air.
- Using electric sparks.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
species · evolution in action · spontaneous generation on islands · adaptive radiation from a common ancestor- Darwin's finches have beaks suited to different foods because of .
- Organisms that interbreed to give fertile offspring belong to the same .
- Mosquitoes resistant to insecticides show .
Show answer
1: adaptive radiation from a common ancestor · 2: species · 3: evolution in action
- Different islands, different niches.
- Biological species concept.
- Directional selection.