7.7Natural Selection
Common Ancestry — AP BIOLOGY Revision Notes & Topical Questions
5 min read 9 parts Lesson 8 of 13 1 quiz
Lesson summary
7.7 Common Ancestry — Notes — AP Biology notes: Describe structural and functional evidence at cellular and molecular levels for the common ancestry of all eukaryotes and all life.Common ancestry — all living organisms descend from a single ancestral population, LUCA (the last un…
- Endosymbiotic theory
- Conserved gene
- Genetic code
- Metabolism — core pathways such as glycolysis and amino-acid pathways are shared and all use ATP as the energy currency
- Introns — their presence supports a common eukaryotic lineage
- Origin — mitochondria and chloroplasts descended from engulfed prokaryotes
Section 1 of 9Common ancestry of life
1
Common ancestry — all living organisms descend from a single ancestral population, LUCA (the last universal common ancestor).
2
Genetic material — all use DNA (some viruses use RNA, consistent with an RNA-first ancestry) and share replication, transcription and translation.
3
Genetic code — nearly universal: the same codons specify the same amino acids in almost all organisms, which is highly unlikely by chance.
4
Cell basics — all cells have a plasma membrane (phospholipid bilayer), cytoplasm and ribosomes (rRNA plus protein) for protein synthesis.
5
Metabolism — core pathways such as glycolysis and amino-acid pathways are shared and all use ATP as the energy currency.
Section 2 of 9Common ancestry of eukaryotes
Common ancestry of eukaryotes
Shared features — membrane-bound organelles (nucleus, mitochondria, ER, Golgi), linear chromosomes and genes containing introns,
- which are not found in prokaryotes.
Introns — their presence supports a common eukaryotic lineage.
Section 3 of 9Endosymbiotic theory
Endosymbiotic theory
Origin — mitochondria and chloroplasts descended from engulfed prokaryotes.
Evidence — double membrane, circular DNA, 70S-type ribosomes and division by binary fission.
Section 4 of 9Conserved genes and domains
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Slow change — highly conserved genes (ribosomal RNA genes, Hox genes) change slowly because mutations in them are usually harmful.
2
Deep relationships — comparing them reveals ancient relationships, and archaea are more closely related to eukaryotes than bacteria are.
3
Agreement — structural (homologous anatomy) and molecular (sequence similarity) evidence usually agree, strengthening the conclusion.
4
Redrawing the tree — molecular evidence rather than appearance is used to redraw the tree of life.
5
Bacteria and archaea are separate domains of prokaryotes, both with no nucleus; archaea (methanogens, extremophiles) share more genes with eukaryotes than bacteria (E. coli, MRSA) do.
Section 5 of 9Origin-of-life link (LUCA)
- Likely features — anaerobic, using hydrogen as energy source, converting CO2 to organic compounds,
- and thermophilic, possibly in hydrothermal vents using chemosynthesis.
Section 6 of 9Evidence of common ancestry compared
| Evidence | Found in | Suggests | Level |
|---|---|---|---|
| DNA and RNA as genetic material | All life | Single origin | Molecular |
| Universal genetic code | Almost all life | Common ancestor | Molecular |
| Glycolysis | All life | Ancient pathway | Metabolic |
| Membrane-bound organelles | Eukaryotes | Shared eukaryotic ancestor | Cellular |
| Linear chromosomes and introns | Eukaryotes | Shared ancestry | Genetic |
Section 7 of 9More from the specification
1Cladistic branch analysis on phylogenetic trees
- Cladistic branch analysis on phylogenetic trees tracking shared derived characteristics on phylogenetic trees illustrates evolutionary relationships between ancestral and modern species.
2Punctuated equilibrium
- Speciation in rapid bursts between long stable periods.
3Environmental change
- A change in abiotic or biotic conditions, often caused by humans.
4Sympatric speciation
- Speciation in the same area without a geographical barrier.
5Cladogram
- Diagram showing branching pattern of relationships with no timescale.
6Habitat
- The place where an organism lives.
7Genetic diversity
- The range of alleles within a population.
8Selection pressure
- A factor, such as an antibiotic, that favours survival of some individuals.
9Phylogenetic tree
- Diagram showing evolutionary relationships between species.
10Allopatric speciation
- Speciation caused by geographical isolation.
11Genetic drift
- Change in allele frequency due to chance, most important in small populations.
12Fossil record
- The collected evidence of past life from fossils.
13Speciation
- The formation of new species.
14Evolution
- Change in adaptive features of a population over time through natural selection.
15Extinction
- Loss of every member of a species.
16Fossil
- Preserved remains of an organism in rock.
17Isolation
- Separation of populations so that they cannot interbreed.
18Barrier
- A structure that prevents pathogens from entering.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| LUCA | last universal common ancestor |
| Endosymbiotic theory | mitochondria and chloroplasts arose from engulfed prokaryotes |
| Conserved gene | gene that changes very slowly over evolutionary time |
| Intron | non-coding region within a eukaryotic gene |
| Genetic code | rules linking codons to amino acids |
| Archaea | domain more closely related to eukaryotes than bacteria |
Section 9 of 9Exam technique
1
Saying the code is universal without exceptions — it is nearly universal.
2
Saying mitochondria arose within the nucleus — they came from engulfed prokaryotes.
3
Forgetting that conserved genes change slowly because mutations are usually harmful.
4
Calling bacteria and archaea the same domain — they are separate domains.
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